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The digital spectrum-compatible HDTV system
Signal Processing: Image Communication 4 (1992) 293-305 Elsevier
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The digital spectrum-compatible HDTV system Pieter Fockens Zenith Electronics Corporation, 1000 Milwaukee Avenue, Glenview, IL 60025, USA
A r u n Netravali A T & T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ07974, USA
Abstract. The system describes a digital high-definition television (HDTV) system designed for US terrestrial broadcasting but friendly to alternate delivery means. The new system, Digital Spectrum Compatible (DSC-)HDTV, is to be simulcast with NTSC during a multi-year transition period, using all existing television bands. DSC-HDTV uses progressively scanned video at three times the current horizontal line rate, compressed to a data rate that fits in a 6 MHz channel using a mix of two- and four-level symbols. Keywords. Video compression (video redundancy reduction), VSB (vestigial sideband), television transmission, data transmission, interference rejection, automatic channel equalization.
I. Introduction
The Digital Spectrum Compatible (DSC)H D T V system is designed to occupy a 6 M H z channel in the existing TV bands o f US terrestrial broadcasting. The system requires a new receiver but simulcast transmission o f N T S C and D S C H D T V makes both kinds o f signal available to the viewing public. The channels needed for the new service are the ones not currently in use or currently ' t a b o o ' . ( ' T a b o o ' in this context means not-usable under the current channel assignment grid, mainly for interference reasons.) Simulcasting implies independence o f N T S C and D S C - H D T V signals which will eventually free all currently used N T S C spectrum for additional television or other c o m m u nication services. The D S C - H D T V system uses a video compression system which is optimized for the terrestrial broadcast environment. A very high compression ratio is achieved in such a way that robust transmission is possible without sacrificing
image quality. The system is designed for practical implementation and results in a decoder which is realizable in a small n u m b e r o f V L S I integrated circuits. The D S C - H D T V receiver has a picture free o f the c o m m o n transmission-induced imperfections t h r o u g h o u t its service area. This is due to the digital transmission format which has an imperfection threshold that is m u c h higher than for analog signals. The sudden drop-off in service often associated with data transmission is avoided by a bi-rate transmission system under which more important data are transmitted with greater immunity against noise, interference, ghosts and other imperfections. The D S C - H D T V system can operate within the existing N T S C assignment grid without unacceptable mutual interference. C o m p a r e d to an N T S C station with the same service area, the D S C - H D T V station radiates at least 12 dB less power. This is possible for two reasons: the D S C - H D T V transmission signal is m o r e efficient and, secondly, a digital receiver operates with a lower carrier-to-
0923-5965/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved
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noise ratio. As a result, the D S C - H D T V transmitter can operate closer to a cochannel N T S C station than another N T S C station without increasing the interference into receivers tuned to the cochannel N T S C station and yet achieve an H D T V service area comparable to NTSC. The decreased cochannel spacing does not disturb the D S C - H D T V receiver for five reasons. First, there is a newly devised interference rejection system; secondly, the receiver operates with a lower acceptable carrier-to-interference ratio compared to an N T S C receiver; thirdly, a low-level pilot signal is transmitted to aid the receiver demodulator in channel acquisition even under severe interference conditions; fourthly, a precision carrier frequency offset can be applied to the new transmitter with respect to the interfering N T S C transmitter, if necessary; and, finally, a directional antenna can be used in the most severe potential cochannel interference regions. The D S C - H D T V transmission signal has low and evenly distributed power. This makes it ideal for cable distribution. The data rate is low enough for satellite transmission over existing transponders either of DBS or fixed characteristics. The digital format is convenient for optical fiber transmission. The system's home recorder (VCR) is of a mechanical complexity comparable to a current S-VHS unit. Recording and playback are in the compressed digital format only, which simplifies the electronic circuits. The receiver's input buffer acts as an inherent time base corrector. Thus, although primarily designed for terrestrial broadcasting, all television delivery means are compatible with the new signal. The D S C - H D T V signal includes four digital audio channels of compact-disk quality. Digital data space also has been set aside for teletext, captioning for the deaf, cryptography, box address and other ancillary services. 2. The video source
The video source is an R, G, B signal of 787/ 788 lines per frame, 59.94 frames per second, that Signal Processing: Image Communication
is progressively scanned at 47 203 Hz which is three times the N T S C scan rate. The bandwidth is 34 MHz. These standards imply a display of 1575 lines every 1/29.97 s. There are several reasons why these particular choices were made. The benefits of progressive scan (versus the artifacts of interlaced scan) are well known. The 787/788 rate (3/2 of 525 equals 787.5) was chosen to avoid the less than high-definition of 525 progressive scan and the difficulty of producing real hardware at a 1050 progressive rate. C h r o m a resolution is one half of the luminance resolution. The aspect ratio is 16:9 and the display contains 1280× 720 active pixels per frame; the pixels are square. Using linear R G B source signals, the color matrix produces chroma and true constant luminance signals. The simple ratios to N T S C horizontal and vertical rates and the progressive scan result in easy conversion between formats. This is important for the broadcaster when he wants to transmit NTSC source material on the H D transmitter. (A certainty in the early stages of the new service.) Easy conversion also enables the transmission of H D T V source material on the N T S C transmitter. As H D T V receivers will have to receive N T S C transmissions as well as H D T V , and as it is most economical to have a single sweep rate, it is important to choose a scan rate for H D T V (47.2 kHz) that is a simple multiple of the N T S C rate, 15.7 kHz. Similarly, the 59.94 Hz vertical rate of N T S C is retained. The video compression (source coding) is described in Section 3. If all active pixels of an R G B video source were encoded at 9 bits, the total video bit-rate would be 1.49 Gigabit/s. The video compression reduces this to a variable bit-rate between 8.6 and 17.1 Mbit/s including overhead for motion vectors and video control but not including forward error correction. The total data rate after adding forward error correction, audio and all other ancillary bits varies between 11.1 and 21.0 Mbit/s. The data are transmitted at a constant symbol rate as either 2-level signals (1 bit/symbol) or 4-level signals. When transmitted at equal
P. Foekens, A. Netravali / The digital spectrum-compatible HDTV system
average power, the 2-level data have an approximately 6 dB greater carrier-to-noise ratio threshold than 4-level data. By assigning the most important data to 2-level transmission and the remainder to 4level transmission (bi-rate transmission), the noiselimited fringe-area service fades gradually rather than suddenly.
3. Video encoding 3.1. Introduction
The DSC-HDTV video encoding system [5, 6] uses a color television source signal of 994 Mbit/s bit-rate which is compressed to a variable bit-rate of between 8.6 and 17.1 Mbit/s without sacrificing image quality. (Reduced chroma resolution yields 2/3 x 1.49 = 0.994 Gbit/s). Preprocessing includes A/D conversion of RGB camera signals, degamma processing, matrixing to the YUV format and, optionally, processing in accordance with CIE L*u*v* equations. Video compression exploits three basic types of redundancy [4]. Motion compensation removes temporal redundancy, spatial frequency transformation removes spatial redundancy, and perceptual weighting removes amplitude redundancy by putting quantization noise in less visible areas. Temporal processing occurs in two stages. The motion of blocks of pixels from frame to frame is estimated using hierarchical block matching. Using the motion vectors, a displaced frame difference (DFD) is computed. The DFD generally contains a small fraction of the information in the original frame. The DFD is transformed using a twodimensional discrete cosine transform (DCT) prior to removal of the spatial redundancy. Each new frame of DFD is analyzed prior to coding to determine its rate versus perceptual distortion characteristics and the dynamic range of each coefficient. Quantization of the DCT coefficients is performed based on the perceptual importance of each coefficient, the precomputed dynamic range of the coefficients, and the rate versus distortion characteristics.
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The perceptual criterion uses a model of the human visual system to determine a human observer's sensitivity to color, brightness, spatial frequency and spatial-temporal masking. This information is used to minimize the perception of coding artifacts throughout the picture. Parameters of the encoder are optimized to handle the scene changes that occur frequently in entertainment and sport events, and in channel changes made by the viewer. 3.2. Video encoder
The video encoder is shown in Fig. 1. Its main parts are motion estimator, forward analyzer, encoder loop and buffer. The motion estimator produces motion vectors which are compressed and sent to the buffer for transmission. The forward analyzer analyses each frame before it is processed in the encoder loop. The motion vectors and control parameters resulting from forward analysis are input to the encoder loop which outputs the compressed prediction error to the buffer. The encoder loop parameters are weighted by the buffer state which is fed back from the buffer. In the predictive encoder loop, the generally sparse differences between the new image data and the motion-compensation predicted image data are encoded using adaptive transform coding. The coding parameters are controlled in part by forward analysis. The encoder output data consists of some global parameters of the video frame computed by the forward analyzer and of DCT coefficients that have been selected and quantized according to a perceptual criterion. The chrominance bit-rate is generally a small fraction of the total bit-rate without perceptible chrominance distortion. 3.3. Motion estimation
Motion is estimated in stages on a block by block basis using the luminance frames only. A block consists of 8 x 8 pixels. At each stage the best block match is defined to be that which has the least Vol. 4, Nos. 4 5, August 1992
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absolute difference between blocks. The results from one stage are used as a starting point for the next stage to minimize the number of block matches per image. The motion estimator is capable of handling large frame-to-frame displacements typical of entertainment and sport scenes. Finally, the block size of motion estimation is adapted spatially to those places in the picture which could have the most benefit within the limit of the compressed motion vector bit-rate. The motion estimator compares a block of pels in the current frame with a block in the previous frame by forming the sum of the absolute differences between the pels, known as the prediction error. Each block in the current image is compared to displaced blocks at different locations in the previous image and the displacement vector that gives the minimum prediction error is chosen as being the best representation of the motion of that block. This is the motion vector for that block. The end result of motion estimation is to associate a motion vector with every block of pels in the image. The motion estimator is shown in Fig. 2. To reduce the complexity of the search, hierarchical motion estimation is used in which a first stage of coarse estimation is refined by a second, finer estimation. The first stage matching is performed on the images after they have been decimated by a factor of two both vertically and horizontally. This reduces both the block size and the search area and greatly reduces the size of the motion estimator. A coarse block size of 16H × 8V pixels in the decimated image is used with 1 pixel accuracy. The motion vectors that are generated are passed to Signal Processing: ImageCommunication
the second stage which performs a search centered around this coarse estimate. The motion vectors generated from the first stage are used by the fine motion estimator that can estimate the motion of 8 x 8 pixel blocks to within sub-pixel accuracy. The total search area is 96H x 80V pixels. The second motion estimator stage generates the prediction errors of the 8 x 8 pixel blocks for each location within the search area. The prediction errors of the coarse blocks are derived from the sums of the appropriate small block prediction errors. The final stage of the motion estimator uses the prediction errors to generate the motion vectors by finding the minimum prediction error for all blocks in every location. The resulting motion vectors are then passed to the motion vector selector. Given the motion vectors from the coarse and fine motion estimator stages, the motion vector selector must select the set of motion vectors that will give the best prediction of the next frame while limiting the bit-rate of the compressed motion vector data to be in a range. This is achieved by sending two resolutions of motion vectors. The first set represents the motion vectors of the coarse blocks which are unconditionally transmitted, and the second set represents the motion vectors for 8 x 8 pixel blocks. However, all of the 8 x 8 pixel block motion vectors may not be transmitted, only those which can be sent within the bit budget remaining after the coarse motion vectors have been sent. The motion vectors are encoded for transmission to improve efficiency. For example, five of the six motion vectors of a coarse block are sent as the difference between itself and an adjacent motion
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vector. The coding of a coarse block can be refined by sending the difference between the motion vector of the larger block and those of eight 8 × 8 block motion vectors.
3.4. Transform coding
The encoder loop in Fig. 1 generates a transformed and quantized displaced frame difference ( D F D ) using the motion vectors, perceptual thresholds and loop control parameters. The motion vectors are applied to the predicted frame which is stored in a separate buffer. The displaced frame (DF) is scaled by a DF-factor and subtracted from the input frame. This yields the D F D which contains only a fraction of the original image.
The D F D is spatially transformed using the DCT and the coefficients are then adaptively quantized. The coarseness of quantization of individual coefficients is adjusted in local regions of the image. This represents adaptation to the limitations of human vision while minimizing the amount of transmitted information. Fast recovery from channel errors and channel changes is facilitated by D F scaling. The mean of each frame is calculated in the forward analyzer and subtracted from each frame before differencing with the zero-mean DF. This results in a zero-mean D F D for greater D F D efficiency. The quantized coefficients, control parameters and coded selection vectors are passed to the channel buffer and formatter. Vol. 4, Nos. 4 5, August 1992
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3.5. Vector quantization Rather than quantizing the 64 coefficients of a block according to a fixed scheme, a set of nonuniform quantizers is used. A quantizer selection vector (QSV) is a 64 element vector in which each element represents one out of 3 quantizers or a drop command. Less than 2000 QSVs are used and are sufficient to quantize all luminance coefficients; they constitute the luminance QSV code book. Similarly, there are two 500 QSV chrominance code books. The QSVs of the luminance code book are successively applied to a luminance super block consisting of tbur 8 × 8 blocks. One quantizer (or drop command) of the set of 64 of one QSV is applied to the four coefficients of the superblock that are in corresponding locations in each of the four 8 × 8 blocks. Chrominance QSVs are similarly applied to chrominance super-blocks of six 8 × 8 blocks (16H × 24V). Quantization errors are computed and are compared to a perceptual error threshold; the result is summed to produce the selection error for the vector. The optimal vector is selected by considering both selection error and bit-rate.
Temporal masking refers to the increase of the perceptual threshold for high frequencies when there is motion in the scene. The perceptual criterion is implemented by the calculation of perceptual thresholds to be used in the quantizers. Separate luminance and chrominance perceptual thresholds are generated. They are not transmitted but are used to optimize the transmitted information and minimize perceptible artifacts.
3.7. Channel buffer and formatter The buffer (Fig. 1) regulates the variable input bit-rate into an output bit-rate for transmission of between 8.6 and 17.1 Mbit/s. Errors in variablelength coded data cause errors long after the error is past. To avoid error propagation, the data are packed into slices with header information. A slice corresponds to a fixed region in the original image. Motion vectors, quantizer selection vectors and quantized coefficients are variable length encoded. The formatter arranges these data intermixed with various coding parameters of fixed length. The state of the buffer is calculated periodically and relayed back to the forward analyzer. Here the perceptual thresholds are altered to prevent overflow or underflow of the buffer.
3.6. Perceptual criterion This refers to matching the coding algorithm to the characteristics of the human visual system (HVS). The following properties are used: frequency sensitivity, contrast sensitivity, spatial masking and temporal masking. Frequency sensitivity refers to the property of the HVS that tolerates more quantization errors at high frequencies than at low frequencies. Contrast sensitivity refers to flat field stimuli. This sensitivity varies with the brightness of the flat field and perceptual thresholds are adjusted accordingly. Coding includes a model of spatial masking which adjusts the perceptual threshold based on the amount of local texture present at each location in the input. Texture refers to the amount by which the input deviates from a flat field. Signal Processing: ImageCommunication
4. The transmission system
4.1. Data processing The transmitter block diagram is shown in Fig. 3. The variable video bit-rate supplied by the video encoder is converted to a constant symbol rate. The addition of audio and ancillary data, forward error control data and sync data results in a symbol rate of 10.76 Msymbol/s. The transmission bytes are arranged in a 'data frame' shown in Fig. 4, timed identical to an NTSC frame. One data frame contains two 'data fields' and one data frame is divided into 525 'data segments' all in correspondence with NTSC 'frame', 'field' and horizontal line, respectively. (These new
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equalizer in the receiver and also in the decision process of using the receiver's cochannel interference rejecting comb filter. The remainder of a data field is occupied by video, audio and ancillary data to which are added Reed-Solomon (RS) bytes for forward error correction. All data are transmitted in accordance with a birate scheme. Sync signals, one pair of stereo audio signals, and a fixed portion of the ancillary signals are always transmitted at 1 bit/symbol. The other Vol.4, Nos.4-5, August1992
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audio pair and the remaining ancillary signals are always transmitted at 2 bits/symbol. Video data are transmitted at 1 or 2 bits/symbol as determined by the video encoder. DC and low frequency coefficients, quantizer scale factors, DF-factor and buffer fullness constitute the most important data and are always transmitted at 1 bit/symbol. The remaining video data are transmitted at 1 bit/symbol up to the attainment of a global target distortion (related to buffer fullness) corresponding to a desired picture quality. The rest of the video data is transmitted at 2 bits/symbol. Side information for receiver interpretation of the data is provided by the video encoder and is encoded in the transmission bit map, transmitted twice at 1 bit/symbol during the first four data segments of each field. Syncs are not protected by RS parity bits but they have their own protective redundancy. To protect against impulse noise, the symbols are interleaved on an intra-segment basis and also inter-segment to a depth of approximately one half field.
4.2. Interference rejection The decreased cochannel spacing between NTSC and DSC-HDTV transmitters requires special measures in the DSC-HDTV receiver as mentioned. The system uses a receiver comb-filter that has nulls at or very near the interfering NTSC cochannel's visual, aural and chroma carriers and thus rejects most of the interference. The system is most easily explained referring to the familiar duobinary or correlative data transmission coding techniques [3]. One of those techniques uses a comb-filter in order to place a null in the transmitted spectrum at the Nyquist frequency. The correlation between comb-filter output symbols can be removed by digital pre- and postcoding. Both precoder and comb filter are placed at the transmitter. In the present system the comb-filter is placed at the receiver and it has the multiple nulls described and shown in Fig. 5. The interference-rejecting nulls obviously would not serve 'their purpose at Signal Processing: Image Communication
the transmitter. Only the digital pre-coding that removes the comb filter output correlation is placed at the transmitter. Due to the digital operation on an essentially random sequence the transmitted spectrum remains flat and noise-like so that the property of minimum interference into an NTSC cochannel is retained. These advantages are offset by a loss of 3 dB in signal-to-noise ratio. This can, however, be prevented when no significant NTSC cochannel interference is present. In that case, the comb-filter is switched out and a digital post-coder is used instead. The latter device does not have the 3 dB SNR ratio loss. A measured error rate in both modes using the pseudo-random field sync signal is used to decide which of the two circuits shall be active. This arrangement allows receiver simplification when simulcasting comes to an end. The precoder following next in the transmitter block diagram performs the function described in connection with the interference rejection system. DC offset will be described in the next section. The digital processing in the transmitter described so far has proceeded on 4 parallel bus systems. The mapper accepts test data (for data segment ~525, Fig. 4), sync data and pilot data to be described below. The output is converted to a single bus system. The pre-equalizer is a digital filter that precorrects for IF, RF, transmission line and antenna deviations from nominal response. The sin x / x compensation for the subsequent D / A converter is also performed here.
4.3. Modulation The DSC-HDTV receiver has to operate reliably under severe cochannel interference conditions, as described. The interference rejection system only works after the receiver is properly tuned to the desired DSC-HDTV station. Tuning is greatly facilitated by a pilot signal. CW signals are potential sources of interference into NTSC receivers. There is, however, a region in the 6 MHz N T S C television channel where this potential is significantly
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reduced. This is at the low end of the channel where the NTSC receiver has considerable IF attenuation. Therefore, the data signal modulation chosen is vestigial side band (VSB) with the carrier (suppressed) low in the band at the same frequency as the pilot. The modulated signal is designated '4VSB' when the modulating signal consists of 4-level symbols and '2-VSB' when modulated by 2-level symbols. The nominally occupied spectrum is shown in Fig. 6 together with an NTSC cochannel. Note the dashed-line NTSC receiver Nyquist slope in relation to the DSC-HDTV pilot placement. The pilot signal could be added in analog form in the modulator by adding a slight amount of unmodulated carrier. Control over the level is maintained more accurately, however, by adding a
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(digital) DC level at baseband. The nominal 4-level transmission signals can be represented by -3, -1, +1, +3 and of the 2-level signals by -2, +2. Pilot is added by changing these levels to -2, 0, +2, +4 (4-VSB) and to -1, +3 (2-VSB). There is some risk involved in adding slight amounts of pilot. The very long-time average of the data signal is zero, but over shorter time spans the average may be negative which can decrease or even cancel the pilot. This can be avoided by subjecting groups of data to a modulo-4 bit addition until the average is positive. The addition is encoded in the 12 symbols following data segment sync. The low-level modulator is followed by a SAW filter that includes dispersion in the form of constant-slope varying group delay. The only
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transmitted data that occur so regularly that they would cause a noticeable, non-random interference pattern on an NTSC receiver, are the data segment syncs. To reduce this effect, even though minor, the data dispersion operation is introduced. As a result, the random level of the data adjacent to the sync spreads into the sync data. This reduces the visibility of the described interference. The transmitter is operated with a maximum effective radiated power (ERP) which is a minimum of 12 dB less than current NTSC transmitters. At U H F , a portion of this power reduction may be used to reduce antenna gain in order to reduce beam-steering effects [1] which can cause excessive level tilts over the 6 MHz channel in some locations. An advantage of the one-channel-only format is that no aural transmitter and no notchdiplexer are needed. R F carrier frequency offset of the DSC-HDTV transmitter with respect to the NTSC cochannel is not expected to noticeably reduce the visibility of interference at an NTSC receiver due to the noiselike character of the DSC-HDTV transmission signal. The DSC-HDTV receiver, however, in extreme cases of cochannel interference, will be aided in sync detection by such an offset.
5. DSC-HDTV reception A receiver block diagram is shown in Fig. 7. The functions are the inverse of those of the transmitter in reverse order with a few additions. The additions include sync and clock recovery, interference rejection, ghost-cancellation/channel-equalization and error correction. The receiver front-end is basically a conventional analog device except that channel acquisition may have to take place under extremely strong cochannel interference conditions. The pilot signal aids in channel acquisition. This is further helped by the choice of a frequency-and-phase-locked loop (FPLL) [2] for carrier regeneration. The FPLL's wide pull-in range, narrow hold-in range Signal Processing: Image Communication
and stable operating points at 90 and 270 ° assure reliable acquisition and hold-in. The receiver SAW filter has a group delay characteristic that provides dispersion which is complementary to that in the transmitter. Data segment sync is detected in a correlator circuit that uses the sync's repetitive and periodic character. Data field sync detection correlates the input with a look-up table that contains the transmitted pseudo-random sequence information. The NTSC interference filter acts in conjunction with the transmitter's pre-coder to effectively reject NTSC cochannel interference. As described, either the comb-filter or the digital post-coder is activated. It should be noted, that the DSC-HDTV transmitted spectrum being flat and noise-like - is not only optimal from a point-of-view of interference into NTSC but also for minimizing DSC-HDTV into DSC-HDTV cochannel interference. If, eventually, all NTSC is permanently off-the-air, receivers can delete the comb-filter and only the simple digital post-coder is retained. In addition, optimum cochannel interference conditions are gained. The ghost-canceler/channel-equalizer uses the data field sync signal as a training signal. Multiple ghosts can be handled. The amount of cancellation required for data signals is much less than for analog signals due to the much higher threshold for imperfections which can be tolerated for data signals.
6. Coverage and interference issues FCC studies have shown that the current minimum distance of 155 miles between U H F cochannel stations has to be reduced to approximately 110 miles between NTSC and H D T V cochannel stations, in order to be able to allot a second channel for simulcast to more than 99% of all current TV stations. In the introduction, Section 1, two reasons were given why DSC-HDTV can obtain a service area
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c o m p a r a b l e to N T S C with less power, a n d five reasons w h y a D S C - H D T V receiver can s t a n d m u c h m o r e interference f r o m N T S C t h a n a n N T S C receiver can. It was there c o u c h e d in q u a l i t a tive t e r m s ; here results o f q u a n t i t a t i v e studies will be described. T h e results are b a s e d on the p u b l i s h e d F C C U H F p r o p a g a t i o n curves a n d on the a s s u m p t i o n s o f T a b l e 1. T h e following three situations were studied: 1. N T S C into N T S C c o c h a n n e l interference; 2. D S C - H D T V into NTSC cochannel interference; 3. N T S C into D S C - H D T V c o c h a n n e l interference (4-VSB a n d 2-VSB). S i t u a t i o n 1 serves as the reference. T h e study has shown that at 155 miles distance the desired D / U r a t i o o f 28 dB is n o t reached in a segment o f the G r a d e B service c o n t o u r o f the desired station which segment occupies 14.2% o f the t o t a l desired G r a d e B service a r e a (see Fig. 8). T h e S i t u a t i o n 2 analysis s h o w e d t h a t at 112 miles distance between a desired N T S C station a n d an
interfering D S C - H D T V cochannel, the p o w e r o f the D S C - H D T V s t a t i o n has to be r e d u c e d b y 14.5 dB (to 22.5 dB) to o b t a i n a segment o f 12.7% o f the N T S C G r a d e B a r e a where the D / U r a t i o o f 30 dB is n o t reached. T h e noise-limited D S C - H D T V service a r e a is f o u n d as follows. A 22.5 d B K D S C - H D T V t r a n s m i t t e r reaches the c a r r i e r - t o - n o i s e ( C / N ) t h r e s h o l d o f 16 dB (4-VSB) at 53 miles distance a n d the C / N t h r e s h o l d o f 10 dB (2-VSB) at 59 miles distance. B o t h are c o m p a r a b l e to the 57 m i l e s for the N T S C G r a d e B service a r e a c o n t o u r r a d i u s thus yielding c o m p a r able noise-limited service areas. N o t e in T a b l e 1 t h a t the D S C - H D T V m e a s u r e d D~ U r a t i o equals 0 dB (4-VSB) or - 6 dB (2-VSB). W i t h these n u m b e r s , in S i t u a t i o n 3 at 112 miles distance between a D S C - H D T V desired station a n d an interfering N T S C cochannel, the a r e a where 0 dB D~ U ratio is n o t reached is 5.9% o f the 4-VSB noise-limited service area. T h e a r e a where - 6 d B D / U r a t i o is n o t reached is 9.9% o f the 2-VSB noise-limited service area.
Table 1 Coverage and interference calculation assumptions Transmitter NTSC effective radiated power (UHF), ERP = 37 dBK (Desired) ERP = 37 dBK (Undesired) Antenna height = 1250 ft above average terrain (all cases) NTSC service area radius = 57 miles (Grade B) Receiver Antenna Gain = 10 dB Front/back ratio = 6 dB (NTSC) =0-15 dB (DSC-HDTV) Downlead loss = 4 dB Noise figure = 10 dB Desired field strength (FS)=F(50, 50)a (NTSC) =F(50, 90) (DSC-HDTV) Undesired field strength= F(50, 10) Desired/undesired FS ratio, D~ U = 28 dBb (NTSC into NTSC) = 30 dBb (DSC-HDTV into NTSC) Measured D/U ratio = 0 dB (NTSC into 4-VSB) = - 6 dB (NTSC into 2-VSB) C/N threshold= 16 dB (4-VSB) = 10 dB (2-VSB) a F(L, T) refers to field strength, exceeded at L% of the locations, T% of the time (FCC propagation curves). b Causing equal measured subjective interference. Signal Processing: Image Communication
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conditions. Reduced and evenly distributed power (compared to NTSC stations of the same service area) and the absence of substantial CW signal portions cooperate to reduce cochannel interference into NTSC in spite of reduced cochannel transmitter distance.
7. Conclusion References
The D S C - H D T V system is a digital high-definition television simulcast system designed for US terrestrial broadcasting on available and on taboo channels. The system is friendly to all other television delivery means: cable, satellite, VCR and fiber. The system uses progressively scanned source signals and is characterized by an effective, high performance video compression system. Compression includes motion compensation with hierarchical block matching, and block transform coding with adaptive quantization according to perceptual criteria. Video compression is designed to simplify the receiver decoding; only a few VLSI chips and only one full frame memory are required. The transmission signal uses NTSC timing and is robust under severe cochannel interference
[1] R.D. Bogner, "TV signal degradation : Is your antenna the culprit?", B M / E Magazine, Vol. 24, No. 3, March 1988, pp. 164 169. [2] R. Citta, "Frequency and phase lock loop", 1EEE Trans. Consumer Electron., Vol. CE-23, No. 3, August 1977, pp. 358-365. [3] A. Lender, "Correlative (partial response) techniques and applications to digital radio systems", in: K. Feher, ed., Digital Communications, Microwave Applications, Prentice Hall, Englewood Cliffs, N J, Chaper 7. [4] A. Netravali and B. Haskell, "Digital pictures, representation and compression", Plenum, New York, 1988. [5] A. Netravali, E. Petajan, S. Knauer, K. Matthews, R. Safranek and P. Westerink, "A high performance digital HDTV codec", National Association of Broadcasters ( NA B) Technical Conf., Las Vegas, NV, USA, 18 April 1991. [6] Digital spectrum-compatible HDTV: Technical details, Zenith Electronics Corporation and AT&T Bell Laboratories, 23 September 1991.
Vol. 4, Nos. 4 5, August 1992
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The digital spectrum-compatible HDTV system Pieter Fockens Zenith Electronics Corporation, 1000 Milwaukee Avenue, Glenview, IL 60025, USA
A r u n Netravali A T & T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ07974, USA
Abstract. The system describes a digital high-definition television (HDTV) system designed for US terrestrial broadcasting but friendly to alternate delivery means. The new system, Digital Spectrum Compatible (DSC-)HDTV, is to be simulcast with NTSC during a multi-year transition period, using all existing television bands. DSC-HDTV uses progressively scanned video at three times the current horizontal line rate, compressed to a data rate that fits in a 6 MHz channel using a mix of two- and four-level symbols. Keywords. Video compression (video redundancy reduction), VSB (vestigial sideband), television transmission, data transmission, interference rejection, automatic channel equalization.
I. Introduction
The Digital Spectrum Compatible (DSC)H D T V system is designed to occupy a 6 M H z channel in the existing TV bands o f US terrestrial broadcasting. The system requires a new receiver but simulcast transmission o f N T S C and D S C H D T V makes both kinds o f signal available to the viewing public. The channels needed for the new service are the ones not currently in use or currently ' t a b o o ' . ( ' T a b o o ' in this context means not-usable under the current channel assignment grid, mainly for interference reasons.) Simulcasting implies independence o f N T S C and D S C - H D T V signals which will eventually free all currently used N T S C spectrum for additional television or other c o m m u nication services. The D S C - H D T V system uses a video compression system which is optimized for the terrestrial broadcast environment. A very high compression ratio is achieved in such a way that robust transmission is possible without sacrificing
image quality. The system is designed for practical implementation and results in a decoder which is realizable in a small n u m b e r o f V L S I integrated circuits. The D S C - H D T V receiver has a picture free o f the c o m m o n transmission-induced imperfections t h r o u g h o u t its service area. This is due to the digital transmission format which has an imperfection threshold that is m u c h higher than for analog signals. The sudden drop-off in service often associated with data transmission is avoided by a bi-rate transmission system under which more important data are transmitted with greater immunity against noise, interference, ghosts and other imperfections. The D S C - H D T V system can operate within the existing N T S C assignment grid without unacceptable mutual interference. C o m p a r e d to an N T S C station with the same service area, the D S C - H D T V station radiates at least 12 dB less power. This is possible for two reasons: the D S C - H D T V transmission signal is m o r e efficient and, secondly, a digital receiver operates with a lower carrier-to-
0923-5965/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved
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noise ratio. As a result, the D S C - H D T V transmitter can operate closer to a cochannel N T S C station than another N T S C station without increasing the interference into receivers tuned to the cochannel N T S C station and yet achieve an H D T V service area comparable to NTSC. The decreased cochannel spacing does not disturb the D S C - H D T V receiver for five reasons. First, there is a newly devised interference rejection system; secondly, the receiver operates with a lower acceptable carrier-to-interference ratio compared to an N T S C receiver; thirdly, a low-level pilot signal is transmitted to aid the receiver demodulator in channel acquisition even under severe interference conditions; fourthly, a precision carrier frequency offset can be applied to the new transmitter with respect to the interfering N T S C transmitter, if necessary; and, finally, a directional antenna can be used in the most severe potential cochannel interference regions. The D S C - H D T V transmission signal has low and evenly distributed power. This makes it ideal for cable distribution. The data rate is low enough for satellite transmission over existing transponders either of DBS or fixed characteristics. The digital format is convenient for optical fiber transmission. The system's home recorder (VCR) is of a mechanical complexity comparable to a current S-VHS unit. Recording and playback are in the compressed digital format only, which simplifies the electronic circuits. The receiver's input buffer acts as an inherent time base corrector. Thus, although primarily designed for terrestrial broadcasting, all television delivery means are compatible with the new signal. The D S C - H D T V signal includes four digital audio channels of compact-disk quality. Digital data space also has been set aside for teletext, captioning for the deaf, cryptography, box address and other ancillary services. 2. The video source
The video source is an R, G, B signal of 787/ 788 lines per frame, 59.94 frames per second, that Signal Processing: Image Communication
is progressively scanned at 47 203 Hz which is three times the N T S C scan rate. The bandwidth is 34 MHz. These standards imply a display of 1575 lines every 1/29.97 s. There are several reasons why these particular choices were made. The benefits of progressive scan (versus the artifacts of interlaced scan) are well known. The 787/788 rate (3/2 of 525 equals 787.5) was chosen to avoid the less than high-definition of 525 progressive scan and the difficulty of producing real hardware at a 1050 progressive rate. C h r o m a resolution is one half of the luminance resolution. The aspect ratio is 16:9 and the display contains 1280× 720 active pixels per frame; the pixels are square. Using linear R G B source signals, the color matrix produces chroma and true constant luminance signals. The simple ratios to N T S C horizontal and vertical rates and the progressive scan result in easy conversion between formats. This is important for the broadcaster when he wants to transmit NTSC source material on the H D transmitter. (A certainty in the early stages of the new service.) Easy conversion also enables the transmission of H D T V source material on the N T S C transmitter. As H D T V receivers will have to receive N T S C transmissions as well as H D T V , and as it is most economical to have a single sweep rate, it is important to choose a scan rate for H D T V (47.2 kHz) that is a simple multiple of the N T S C rate, 15.7 kHz. Similarly, the 59.94 Hz vertical rate of N T S C is retained. The video compression (source coding) is described in Section 3. If all active pixels of an R G B video source were encoded at 9 bits, the total video bit-rate would be 1.49 Gigabit/s. The video compression reduces this to a variable bit-rate between 8.6 and 17.1 Mbit/s including overhead for motion vectors and video control but not including forward error correction. The total data rate after adding forward error correction, audio and all other ancillary bits varies between 11.1 and 21.0 Mbit/s. The data are transmitted at a constant symbol rate as either 2-level signals (1 bit/symbol) or 4-level signals. When transmitted at equal
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average power, the 2-level data have an approximately 6 dB greater carrier-to-noise ratio threshold than 4-level data. By assigning the most important data to 2-level transmission and the remainder to 4level transmission (bi-rate transmission), the noiselimited fringe-area service fades gradually rather than suddenly.
3. Video encoding 3.1. Introduction
The DSC-HDTV video encoding system [5, 6] uses a color television source signal of 994 Mbit/s bit-rate which is compressed to a variable bit-rate of between 8.6 and 17.1 Mbit/s without sacrificing image quality. (Reduced chroma resolution yields 2/3 x 1.49 = 0.994 Gbit/s). Preprocessing includes A/D conversion of RGB camera signals, degamma processing, matrixing to the YUV format and, optionally, processing in accordance with CIE L*u*v* equations. Video compression exploits three basic types of redundancy [4]. Motion compensation removes temporal redundancy, spatial frequency transformation removes spatial redundancy, and perceptual weighting removes amplitude redundancy by putting quantization noise in less visible areas. Temporal processing occurs in two stages. The motion of blocks of pixels from frame to frame is estimated using hierarchical block matching. Using the motion vectors, a displaced frame difference (DFD) is computed. The DFD generally contains a small fraction of the information in the original frame. The DFD is transformed using a twodimensional discrete cosine transform (DCT) prior to removal of the spatial redundancy. Each new frame of DFD is analyzed prior to coding to determine its rate versus perceptual distortion characteristics and the dynamic range of each coefficient. Quantization of the DCT coefficients is performed based on the perceptual importance of each coefficient, the precomputed dynamic range of the coefficients, and the rate versus distortion characteristics.
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The perceptual criterion uses a model of the human visual system to determine a human observer's sensitivity to color, brightness, spatial frequency and spatial-temporal masking. This information is used to minimize the perception of coding artifacts throughout the picture. Parameters of the encoder are optimized to handle the scene changes that occur frequently in entertainment and sport events, and in channel changes made by the viewer. 3.2. Video encoder
The video encoder is shown in Fig. 1. Its main parts are motion estimator, forward analyzer, encoder loop and buffer. The motion estimator produces motion vectors which are compressed and sent to the buffer for transmission. The forward analyzer analyses each frame before it is processed in the encoder loop. The motion vectors and control parameters resulting from forward analysis are input to the encoder loop which outputs the compressed prediction error to the buffer. The encoder loop parameters are weighted by the buffer state which is fed back from the buffer. In the predictive encoder loop, the generally sparse differences between the new image data and the motion-compensation predicted image data are encoded using adaptive transform coding. The coding parameters are controlled in part by forward analysis. The encoder output data consists of some global parameters of the video frame computed by the forward analyzer and of DCT coefficients that have been selected and quantized according to a perceptual criterion. The chrominance bit-rate is generally a small fraction of the total bit-rate without perceptible chrominance distortion. 3.3. Motion estimation
Motion is estimated in stages on a block by block basis using the luminance frames only. A block consists of 8 x 8 pixels. At each stage the best block match is defined to be that which has the least Vol. 4, Nos. 4 5, August 1992
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absolute difference between blocks. The results from one stage are used as a starting point for the next stage to minimize the number of block matches per image. The motion estimator is capable of handling large frame-to-frame displacements typical of entertainment and sport scenes. Finally, the block size of motion estimation is adapted spatially to those places in the picture which could have the most benefit within the limit of the compressed motion vector bit-rate. The motion estimator compares a block of pels in the current frame with a block in the previous frame by forming the sum of the absolute differences between the pels, known as the prediction error. Each block in the current image is compared to displaced blocks at different locations in the previous image and the displacement vector that gives the minimum prediction error is chosen as being the best representation of the motion of that block. This is the motion vector for that block. The end result of motion estimation is to associate a motion vector with every block of pels in the image. The motion estimator is shown in Fig. 2. To reduce the complexity of the search, hierarchical motion estimation is used in which a first stage of coarse estimation is refined by a second, finer estimation. The first stage matching is performed on the images after they have been decimated by a factor of two both vertically and horizontally. This reduces both the block size and the search area and greatly reduces the size of the motion estimator. A coarse block size of 16H × 8V pixels in the decimated image is used with 1 pixel accuracy. The motion vectors that are generated are passed to Signal Processing: ImageCommunication
the second stage which performs a search centered around this coarse estimate. The motion vectors generated from the first stage are used by the fine motion estimator that can estimate the motion of 8 x 8 pixel blocks to within sub-pixel accuracy. The total search area is 96H x 80V pixels. The second motion estimator stage generates the prediction errors of the 8 x 8 pixel blocks for each location within the search area. The prediction errors of the coarse blocks are derived from the sums of the appropriate small block prediction errors. The final stage of the motion estimator uses the prediction errors to generate the motion vectors by finding the minimum prediction error for all blocks in every location. The resulting motion vectors are then passed to the motion vector selector. Given the motion vectors from the coarse and fine motion estimator stages, the motion vector selector must select the set of motion vectors that will give the best prediction of the next frame while limiting the bit-rate of the compressed motion vector data to be in a range. This is achieved by sending two resolutions of motion vectors. The first set represents the motion vectors of the coarse blocks which are unconditionally transmitted, and the second set represents the motion vectors for 8 x 8 pixel blocks. However, all of the 8 x 8 pixel block motion vectors may not be transmitted, only those which can be sent within the bit budget remaining after the coarse motion vectors have been sent. The motion vectors are encoded for transmission to improve efficiency. For example, five of the six motion vectors of a coarse block are sent as the difference between itself and an adjacent motion
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vector. The coding of a coarse block can be refined by sending the difference between the motion vector of the larger block and those of eight 8 × 8 block motion vectors.
3.4. Transform coding
The encoder loop in Fig. 1 generates a transformed and quantized displaced frame difference ( D F D ) using the motion vectors, perceptual thresholds and loop control parameters. The motion vectors are applied to the predicted frame which is stored in a separate buffer. The displaced frame (DF) is scaled by a DF-factor and subtracted from the input frame. This yields the D F D which contains only a fraction of the original image.
The D F D is spatially transformed using the DCT and the coefficients are then adaptively quantized. The coarseness of quantization of individual coefficients is adjusted in local regions of the image. This represents adaptation to the limitations of human vision while minimizing the amount of transmitted information. Fast recovery from channel errors and channel changes is facilitated by D F scaling. The mean of each frame is calculated in the forward analyzer and subtracted from each frame before differencing with the zero-mean DF. This results in a zero-mean D F D for greater D F D efficiency. The quantized coefficients, control parameters and coded selection vectors are passed to the channel buffer and formatter. Vol. 4, Nos. 4 5, August 1992
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3.5. Vector quantization Rather than quantizing the 64 coefficients of a block according to a fixed scheme, a set of nonuniform quantizers is used. A quantizer selection vector (QSV) is a 64 element vector in which each element represents one out of 3 quantizers or a drop command. Less than 2000 QSVs are used and are sufficient to quantize all luminance coefficients; they constitute the luminance QSV code book. Similarly, there are two 500 QSV chrominance code books. The QSVs of the luminance code book are successively applied to a luminance super block consisting of tbur 8 × 8 blocks. One quantizer (or drop command) of the set of 64 of one QSV is applied to the four coefficients of the superblock that are in corresponding locations in each of the four 8 × 8 blocks. Chrominance QSVs are similarly applied to chrominance super-blocks of six 8 × 8 blocks (16H × 24V). Quantization errors are computed and are compared to a perceptual error threshold; the result is summed to produce the selection error for the vector. The optimal vector is selected by considering both selection error and bit-rate.
Temporal masking refers to the increase of the perceptual threshold for high frequencies when there is motion in the scene. The perceptual criterion is implemented by the calculation of perceptual thresholds to be used in the quantizers. Separate luminance and chrominance perceptual thresholds are generated. They are not transmitted but are used to optimize the transmitted information and minimize perceptible artifacts.
3.7. Channel buffer and formatter The buffer (Fig. 1) regulates the variable input bit-rate into an output bit-rate for transmission of between 8.6 and 17.1 Mbit/s. Errors in variablelength coded data cause errors long after the error is past. To avoid error propagation, the data are packed into slices with header information. A slice corresponds to a fixed region in the original image. Motion vectors, quantizer selection vectors and quantized coefficients are variable length encoded. The formatter arranges these data intermixed with various coding parameters of fixed length. The state of the buffer is calculated periodically and relayed back to the forward analyzer. Here the perceptual thresholds are altered to prevent overflow or underflow of the buffer.
3.6. Perceptual criterion This refers to matching the coding algorithm to the characteristics of the human visual system (HVS). The following properties are used: frequency sensitivity, contrast sensitivity, spatial masking and temporal masking. Frequency sensitivity refers to the property of the HVS that tolerates more quantization errors at high frequencies than at low frequencies. Contrast sensitivity refers to flat field stimuli. This sensitivity varies with the brightness of the flat field and perceptual thresholds are adjusted accordingly. Coding includes a model of spatial masking which adjusts the perceptual threshold based on the amount of local texture present at each location in the input. Texture refers to the amount by which the input deviates from a flat field. Signal Processing: ImageCommunication
4. The transmission system
4.1. Data processing The transmitter block diagram is shown in Fig. 3. The variable video bit-rate supplied by the video encoder is converted to a constant symbol rate. The addition of audio and ancillary data, forward error control data and sync data results in a symbol rate of 10.76 Msymbol/s. The transmission bytes are arranged in a 'data frame' shown in Fig. 4, timed identical to an NTSC frame. One data frame contains two 'data fields' and one data frame is divided into 525 'data segments' all in correspondence with NTSC 'frame', 'field' and horizontal line, respectively. (These new
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Fig. 4. DSC-HDTV one data frame. transmission signal terms avoid confusion with 'frame', 'field' and 'line' pertaining to source and display signal.) One data segment contains 684 symbols, four of which are intended to synchronize the receiver video data clock. Each data field is preceded by a data field sync signal of one data segment duration consisting of pseudo-random data sequences. This signal is used for field synchronization and as a training signal for a ghost-canceler/channel-
equalizer in the receiver and also in the decision process of using the receiver's cochannel interference rejecting comb filter. The remainder of a data field is occupied by video, audio and ancillary data to which are added Reed-Solomon (RS) bytes for forward error correction. All data are transmitted in accordance with a birate scheme. Sync signals, one pair of stereo audio signals, and a fixed portion of the ancillary signals are always transmitted at 1 bit/symbol. The other Vol.4, Nos.4-5, August1992
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audio pair and the remaining ancillary signals are always transmitted at 2 bits/symbol. Video data are transmitted at 1 or 2 bits/symbol as determined by the video encoder. DC and low frequency coefficients, quantizer scale factors, DF-factor and buffer fullness constitute the most important data and are always transmitted at 1 bit/symbol. The remaining video data are transmitted at 1 bit/symbol up to the attainment of a global target distortion (related to buffer fullness) corresponding to a desired picture quality. The rest of the video data is transmitted at 2 bits/symbol. Side information for receiver interpretation of the data is provided by the video encoder and is encoded in the transmission bit map, transmitted twice at 1 bit/symbol during the first four data segments of each field. Syncs are not protected by RS parity bits but they have their own protective redundancy. To protect against impulse noise, the symbols are interleaved on an intra-segment basis and also inter-segment to a depth of approximately one half field.
4.2. Interference rejection The decreased cochannel spacing between NTSC and DSC-HDTV transmitters requires special measures in the DSC-HDTV receiver as mentioned. The system uses a receiver comb-filter that has nulls at or very near the interfering NTSC cochannel's visual, aural and chroma carriers and thus rejects most of the interference. The system is most easily explained referring to the familiar duobinary or correlative data transmission coding techniques [3]. One of those techniques uses a comb-filter in order to place a null in the transmitted spectrum at the Nyquist frequency. The correlation between comb-filter output symbols can be removed by digital pre- and postcoding. Both precoder and comb filter are placed at the transmitter. In the present system the comb-filter is placed at the receiver and it has the multiple nulls described and shown in Fig. 5. The interference-rejecting nulls obviously would not serve 'their purpose at Signal Processing: Image Communication
the transmitter. Only the digital pre-coding that removes the comb filter output correlation is placed at the transmitter. Due to the digital operation on an essentially random sequence the transmitted spectrum remains flat and noise-like so that the property of minimum interference into an NTSC cochannel is retained. These advantages are offset by a loss of 3 dB in signal-to-noise ratio. This can, however, be prevented when no significant NTSC cochannel interference is present. In that case, the comb-filter is switched out and a digital post-coder is used instead. The latter device does not have the 3 dB SNR ratio loss. A measured error rate in both modes using the pseudo-random field sync signal is used to decide which of the two circuits shall be active. This arrangement allows receiver simplification when simulcasting comes to an end. The precoder following next in the transmitter block diagram performs the function described in connection with the interference rejection system. DC offset will be described in the next section. The digital processing in the transmitter described so far has proceeded on 4 parallel bus systems. The mapper accepts test data (for data segment ~525, Fig. 4), sync data and pilot data to be described below. The output is converted to a single bus system. The pre-equalizer is a digital filter that precorrects for IF, RF, transmission line and antenna deviations from nominal response. The sin x / x compensation for the subsequent D / A converter is also performed here.
4.3. Modulation The DSC-HDTV receiver has to operate reliably under severe cochannel interference conditions, as described. The interference rejection system only works after the receiver is properly tuned to the desired DSC-HDTV station. Tuning is greatly facilitated by a pilot signal. CW signals are potential sources of interference into NTSC receivers. There is, however, a region in the 6 MHz N T S C television channel where this potential is significantly
P. Fockens, A. Netravali / The digital spectrum-compatible H D T V system NTSC
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reduced. This is at the low end of the channel where the NTSC receiver has considerable IF attenuation. Therefore, the data signal modulation chosen is vestigial side band (VSB) with the carrier (suppressed) low in the band at the same frequency as the pilot. The modulated signal is designated '4VSB' when the modulating signal consists of 4-level symbols and '2-VSB' when modulated by 2-level symbols. The nominally occupied spectrum is shown in Fig. 6 together with an NTSC cochannel. Note the dashed-line NTSC receiver Nyquist slope in relation to the DSC-HDTV pilot placement. The pilot signal could be added in analog form in the modulator by adding a slight amount of unmodulated carrier. Control over the level is maintained more accurately, however, by adding a
1.- . . . .
(digital) DC level at baseband. The nominal 4-level transmission signals can be represented by -3, -1, +1, +3 and of the 2-level signals by -2, +2. Pilot is added by changing these levels to -2, 0, +2, +4 (4-VSB) and to -1, +3 (2-VSB). There is some risk involved in adding slight amounts of pilot. The very long-time average of the data signal is zero, but over shorter time spans the average may be negative which can decrease or even cancel the pilot. This can be avoided by subjecting groups of data to a modulo-4 bit addition until the average is positive. The addition is encoded in the 12 symbols following data segment sync. The low-level modulator is followed by a SAW filter that includes dispersion in the form of constant-slope varying group delay. The only
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transmitted data that occur so regularly that they would cause a noticeable, non-random interference pattern on an NTSC receiver, are the data segment syncs. To reduce this effect, even though minor, the data dispersion operation is introduced. As a result, the random level of the data adjacent to the sync spreads into the sync data. This reduces the visibility of the described interference. The transmitter is operated with a maximum effective radiated power (ERP) which is a minimum of 12 dB less than current NTSC transmitters. At U H F , a portion of this power reduction may be used to reduce antenna gain in order to reduce beam-steering effects [1] which can cause excessive level tilts over the 6 MHz channel in some locations. An advantage of the one-channel-only format is that no aural transmitter and no notchdiplexer are needed. R F carrier frequency offset of the DSC-HDTV transmitter with respect to the NTSC cochannel is not expected to noticeably reduce the visibility of interference at an NTSC receiver due to the noiselike character of the DSC-HDTV transmission signal. The DSC-HDTV receiver, however, in extreme cases of cochannel interference, will be aided in sync detection by such an offset.
5. DSC-HDTV reception A receiver block diagram is shown in Fig. 7. The functions are the inverse of those of the transmitter in reverse order with a few additions. The additions include sync and clock recovery, interference rejection, ghost-cancellation/channel-equalization and error correction. The receiver front-end is basically a conventional analog device except that channel acquisition may have to take place under extremely strong cochannel interference conditions. The pilot signal aids in channel acquisition. This is further helped by the choice of a frequency-and-phase-locked loop (FPLL) [2] for carrier regeneration. The FPLL's wide pull-in range, narrow hold-in range Signal Processing: Image Communication
and stable operating points at 90 and 270 ° assure reliable acquisition and hold-in. The receiver SAW filter has a group delay characteristic that provides dispersion which is complementary to that in the transmitter. Data segment sync is detected in a correlator circuit that uses the sync's repetitive and periodic character. Data field sync detection correlates the input with a look-up table that contains the transmitted pseudo-random sequence information. The NTSC interference filter acts in conjunction with the transmitter's pre-coder to effectively reject NTSC cochannel interference. As described, either the comb-filter or the digital post-coder is activated. It should be noted, that the DSC-HDTV transmitted spectrum being flat and noise-like - is not only optimal from a point-of-view of interference into NTSC but also for minimizing DSC-HDTV into DSC-HDTV cochannel interference. If, eventually, all NTSC is permanently off-the-air, receivers can delete the comb-filter and only the simple digital post-coder is retained. In addition, optimum cochannel interference conditions are gained. The ghost-canceler/channel-equalizer uses the data field sync signal as a training signal. Multiple ghosts can be handled. The amount of cancellation required for data signals is much less than for analog signals due to the much higher threshold for imperfections which can be tolerated for data signals.
6. Coverage and interference issues FCC studies have shown that the current minimum distance of 155 miles between U H F cochannel stations has to be reduced to approximately 110 miles between NTSC and H D T V cochannel stations, in order to be able to allot a second channel for simulcast to more than 99% of all current TV stations. In the introduction, Section 1, two reasons were given why DSC-HDTV can obtain a service area
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c o m p a r a b l e to N T S C with less power, a n d five reasons w h y a D S C - H D T V receiver can s t a n d m u c h m o r e interference f r o m N T S C t h a n a n N T S C receiver can. It was there c o u c h e d in q u a l i t a tive t e r m s ; here results o f q u a n t i t a t i v e studies will be described. T h e results are b a s e d on the p u b l i s h e d F C C U H F p r o p a g a t i o n curves a n d on the a s s u m p t i o n s o f T a b l e 1. T h e following three situations were studied: 1. N T S C into N T S C c o c h a n n e l interference; 2. D S C - H D T V into NTSC cochannel interference; 3. N T S C into D S C - H D T V c o c h a n n e l interference (4-VSB a n d 2-VSB). S i t u a t i o n 1 serves as the reference. T h e study has shown that at 155 miles distance the desired D / U r a t i o o f 28 dB is n o t reached in a segment o f the G r a d e B service c o n t o u r o f the desired station which segment occupies 14.2% o f the t o t a l desired G r a d e B service a r e a (see Fig. 8). T h e S i t u a t i o n 2 analysis s h o w e d t h a t at 112 miles distance between a desired N T S C station a n d an
interfering D S C - H D T V cochannel, the p o w e r o f the D S C - H D T V s t a t i o n has to be r e d u c e d b y 14.5 dB (to 22.5 dB) to o b t a i n a segment o f 12.7% o f the N T S C G r a d e B a r e a where the D / U r a t i o o f 30 dB is n o t reached. T h e noise-limited D S C - H D T V service a r e a is f o u n d as follows. A 22.5 d B K D S C - H D T V t r a n s m i t t e r reaches the c a r r i e r - t o - n o i s e ( C / N ) t h r e s h o l d o f 16 dB (4-VSB) at 53 miles distance a n d the C / N t h r e s h o l d o f 10 dB (2-VSB) at 59 miles distance. B o t h are c o m p a r a b l e to the 57 m i l e s for the N T S C G r a d e B service a r e a c o n t o u r r a d i u s thus yielding c o m p a r able noise-limited service areas. N o t e in T a b l e 1 t h a t the D S C - H D T V m e a s u r e d D~ U r a t i o equals 0 dB (4-VSB) or - 6 dB (2-VSB). W i t h these n u m b e r s , in S i t u a t i o n 3 at 112 miles distance between a D S C - H D T V desired station a n d an interfering N T S C cochannel, the a r e a where 0 dB D~ U ratio is n o t reached is 5.9% o f the 4-VSB noise-limited service area. T h e a r e a where - 6 d B D / U r a t i o is n o t reached is 9.9% o f the 2-VSB noise-limited service area.
Table 1 Coverage and interference calculation assumptions Transmitter NTSC effective radiated power (UHF), ERP = 37 dBK (Desired) ERP = 37 dBK (Undesired) Antenna height = 1250 ft above average terrain (all cases) NTSC service area radius = 57 miles (Grade B) Receiver Antenna Gain = 10 dB Front/back ratio = 6 dB (NTSC) =0-15 dB (DSC-HDTV) Downlead loss = 4 dB Noise figure = 10 dB Desired field strength (FS)=F(50, 50)a (NTSC) =F(50, 90) (DSC-HDTV) Undesired field strength= F(50, 10) Desired/undesired FS ratio, D~ U = 28 dBb (NTSC into NTSC) = 30 dBb (DSC-HDTV into NTSC) Measured D/U ratio = 0 dB (NTSC into 4-VSB) = - 6 dB (NTSC into 2-VSB) C/N threshold= 16 dB (4-VSB) = 10 dB (2-VSB) a F(L, T) refers to field strength, exceeded at L% of the locations, T% of the time (FCC propagation curves). b Causing equal measured subjective interference. Signal Processing: Image Communication
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conditions. Reduced and evenly distributed power (compared to NTSC stations of the same service area) and the absence of substantial CW signal portions cooperate to reduce cochannel interference into NTSC in spite of reduced cochannel transmitter distance.
7. Conclusion References
The D S C - H D T V system is a digital high-definition television simulcast system designed for US terrestrial broadcasting on available and on taboo channels. The system is friendly to all other television delivery means: cable, satellite, VCR and fiber. The system uses progressively scanned source signals and is characterized by an effective, high performance video compression system. Compression includes motion compensation with hierarchical block matching, and block transform coding with adaptive quantization according to perceptual criteria. Video compression is designed to simplify the receiver decoding; only a few VLSI chips and only one full frame memory are required. The transmission signal uses NTSC timing and is robust under severe cochannel interference
[1] R.D. Bogner, "TV signal degradation : Is your antenna the culprit?", B M / E Magazine, Vol. 24, No. 3, March 1988, pp. 164 169. [2] R. Citta, "Frequency and phase lock loop", 1EEE Trans. Consumer Electron., Vol. CE-23, No. 3, August 1977, pp. 358-365. [3] A. Lender, "Correlative (partial response) techniques and applications to digital radio systems", in: K. Feher, ed., Digital Communications, Microwave Applications, Prentice Hall, Englewood Cliffs, N J, Chaper 7. [4] A. Netravali and B. Haskell, "Digital pictures, representation and compression", Plenum, New York, 1988. [5] A. Netravali, E. Petajan, S. Knauer, K. Matthews, R. Safranek and P. Westerink, "A high performance digital HDTV codec", National Association of Broadcasters ( NA B) Technical Conf., Las Vegas, NV, USA, 18 April 1991. [6] Digital spectrum-compatible HDTV: Technical details, Zenith Electronics Corporation and AT&T Bell Laboratories, 23 September 1991.
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