HDTV transmission system in an ATM-based network

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Signal Processing : Image Communication 3 (1991) 111-122 Elsevier

HDTV transmission system in an ATM-based network Ryozo Kishimoto and Kazunari Irie NTT Transmission Systems Laboratories, 1-2356 Take, Yokosuka-shi, Kanagawa-ken 23X-03, Japan

Abstract . This paper proposes an ATM-based nationwide HDTV transmission and distribution network architecture and a picture coding algorithm, which are of key importance to high quality HDTV transmission systems . This HDTV coding algorithm is based on sub-band coding, and its ability to maintain the high picture quality of the original is continued . Computer simulation results are presented in terms of bit-per-pel and the quality of the reconstructed picture . This paper also proposes a cell loss compensation method for video signals using random error correcting codes and an interleave structure in an ATM-based network . Analysis results show that the proposed cell loss compensation method reduced cell loss probability to the point that the decoded sequence can be regarded as cell loss free . Keywords . HDTV, ATM, subband, DCT, cell loss compensation.

1 . Introduction Our industrial society is currently moving into the information age and high-definition television (HDTV) [4, 6, 9] will be the coming standard for television in the future . HDTV will be one of the driving market forces for introducing fiber optic cable into homes . Therefore, a nationwide HDTV transmission network between broadcasting stations must be developed in order to economically offer this HDTV service to the customers . On the other hand, the Broadband Integrated Services Digital Network (B-ISDN), which is based on lightwave technology, is expected to become the prime multi-media broadband network in the next generation . Many studies on B-ISDNs which handle multi-media (voice, data and video) are being conducted . Among them, an asynchronous transfer mode (ATM) network [10] is attracting the attention of engineers as a solution to provide B-ISDN services, since the ATM-based network provides a high degree of flexibility at the usernetwork interface . Based on the prior alscussion, the ATM-based network can also serve as an HDTV transmission 0923-5965/91/$03 .50 © 1991 - Elsevier Science Publishers B .V .

network, and its requirements as well as architecture must be studied in detail . In developing this network, key components such as cross-connect nodes [2, 7], picture coders and decoders should also be researched . This paper first describes network architecture for high quality HDTV transmission using an ATM-based network [3] . Next, it proposes a picture coding algorithm based on a sub-band discrete cosine transform (DCT) algorithm . Finally, a cell loss compensation method for video signals is proposed .

2 . Nationwide HDTV transmission and distribution network The demands for transmission of large capacity information such as HDTV signals are expected to increase in the near future. Our research goal is a nationwide HDTV transmission and distribution network's development . This nationwide HDTV network will be composed of HDTV transport and HDTV management networks, as shown



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in Fig . 1 . The HDTV transport network will be composed of nationwide HDTV backbone and HDTV subscriber networks . In this HDTV transport network, analog HDTV signals are converted to digital HDTV signals using an HDTV coder in a broadcasting studio and (or) a video theater . Digital HDTV signals are efficiently handled by the cross-connect nodes and transmitted nationwide . The digital cross-connect nodes have such cross-connect functions as switching, add/drop, and insertion of digital signals in order to select the local areas to which the HDTV signals are distributed according to the studio user's demands . General requirements for the HDTV transmission system, which is shown in Fig . 1, are as follows : (1) transmission and distribution of the original high quality picture signal without degradation .

(2) highly reliable network architecture . Considering that most users of the HDTV transmission system will be broadcasting stations, video theaters, CATV suppliers and industry-application users, picture quality must be high enough to enable special effect processing such as chromakey . In some cases, the HDTV signal is carried on transmission lines connected in tandem and goes through several coding and decoding processes . When the coders and decoders are connected in tandem, the network must also guarantee high picture quality . To fulfill this requirement, the transmission system must handle signals of from several hundred Mbps to several Gbps . Also, the network must provide a digital interface to eliminate additional noise generated by the analog-todigital (A/D) converter . The ATM-based network is split into constant bit-rate (CBR) and variable bit-rate (VBR) ser-

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vices . CBR maintains high transmission quality and can be controlled by simple mechanisms . Therefore, CBR will be applied to the HDTV transmission system in the first stage . On the other hand, VBR is more suitable for multimedia transmission service than CBR . Therefore, application of VBR may possibly be the second step for integration of HDTV with other services in the future, such as voice and data. This paper discusses HDTV transmission in the VBR mode of the ATM-based network .

amount of voice and video information is small . However, when the information amount is large, a quantizer whose quantization steps are rough is selected, so that the codec's bit-rate is constant . Therefore, the picture quality of codecs changes from hour to hour, and, in the worst case, there is degradation of picture quality to below the performance objective . On the contrary, an ATM-based network will, in principle, provide the user with whatever bit-rate is required within the constraints of the interface and the network, due to the dynamic allocation of transmission and switching resources and the absence of a physical channel structure of the user-network interface in an ATM-based network . Therefore, an ATM-based network has many advantages for many TV transmission applications containing broadcast TV transmission, as follows . (1) The user-network interface in an ATM-based network has no physical channel structure . A great flexibility is offered for the selection of bit-rates for specific services : a service can be coded at precisely the bit-rate that offers a good compromise between coding quality,

3 . Television transmission in an ATM-based network

3 .1 . Advantages of TV transmission in an ATM-based network

In a synchronous transfer mode (STM) network, the fixed rate video coding is used, as shown in Fig . 2(a) . Surplus bits are inserted into the buffer memory of the fixed rate video codecs when the

Rate control

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(b) Variable rate coding Fig . 2 . Video coding model . Vol . 3 . No, . 2 3 . June 1991



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coding complexity and coding efficiency . Broadcasters, for example, could select the optimum bit-rate in order to fulfill the picture quality for contribution, distribution and ENG applications using single ATM user-network interface . (2) Variable rate video coding [5] shown in Fig . 2(b) can be adopted, in which video signals are efficiently transmitted with constant picture quality by varying the bit-rate of a video codec according to the input video characteristics . (3) Packet switching naturally provides the user with exactly the bandwidth required . As new services are developed with different bandwidth requirements, packet switching systems can adapt to the changing conditions easily . For example, new television services at different bit-rates could be transmitted over the network through the same user-network interface . With continuing improvements in picture coding algorithms, and with advances in technology allowing more complex algorithms to be implemented, service providers could, in the future, offer either an improved quality of service at the same average bit-rate, or the same quality of service at a lower average bit-rate . Especially, variable rate video coding is one of the most attractive concepts in an ATM-based network. Several advantages of variable rate video coding are listed below . (1) Constant quality of video signals, (2) Simple implementation of video codec, and (3) Adaptability to changing traffic which provides the user the transmission capacity with exactly the bandwidth required . This flexibility opens new vistas for the implementations of TV services in greatest demand . 3 .2. Disadvantages of TV transmission in an ATM-based network When video signals are transmitted in packet mode through an ATM network, there are several

degradation factors ; (1) video signal time delay, (2) bit errors in the transmission link, and (3) cell losses and cell jitter . Therefore, these factors should be defined as network performance in an ATM-based network. Among degradation factors, cell losses inherent in the ATM-based network put specific requirements on the video coding . Furthermore, video signal cell losses have not yet been studied, and their characteristics are unknown . There are three factors contributing to cell loss : (1) transmission bit errors, (2) cell buffer overflows in the switch fabric, and (3) excessive time delay . For voice and video cells, all three factors contribute to cell loss . Data cells are only subject to the first two types . Cell losses and cell jitter inherent to the ATMbased network put specific requirements on the coding of these services, whether the rate is fixed or variable . Television signals are deeply compressed in the case of a still picture scene, which means that even slight information losses effect the picture quality of video signals . Therefore, cell loss compensation for video signals is very important . In data transmission using X .25 protocol, when digital data cell losses occur, the data is re-transmitted . However, this X .25 protocol does not guarantee time-transparent transmission for voice and video signals . Therefore, a cell loss compensation method has to be developed for voice and video with simple communication protocol, in order to realize simple communication protocol for multi-media services . For example, we assume that a cell 40 bytes long is lost and this cell contains the top scanning line of the 20-th video frame of HDTV used in a computer simulation . The coding algorithm used in this simulation is a variable rate video coding by motion-compensated adaptive intra-interframe prediction [5] . The SNR of HDTV signals at the receiver is shown in Fig . 3 for this loss . As shown in Fig . 3, SNR significantly degrades . Therefore, it is very important to compensate for the cell loss in video signals . This paper proposes new cell loss compensation method using random error correcting codes and an interleave structure, as described below in detail .



R . Kishimoto, K. Irie / HDTV transmission system in an ATM-based network

shown in Fig . 4 . In this algorithm, the quadrature mirror filters (QMF) [1] in the first stage decompose the input signal into two bands in a horizontal direction and the second stage filters decompose the two bands into four bands (LL, LH, HL, HH) in a vertical direction . For example, the LH band means the lower band in the horizontal direction and the higher band signal in the vertical direction.

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Frame number Fig . 3 . Degradation of SNR by cell losses .

4 . High-quality picture coding algorithm for HDTV signals 4.1 . Proposed adaptive sub-band DCT coding The advantages of a sub-band coding scheme [1, 8] are (1) each band can select the optimum coding algorithm, (2) quantization noise generated in a particular band is not allowed to spread to other bands, (3) parallel processing can be applied to each band . Considering these advantages, this paper proposes the adaptive sub-band DCT coding scheme

of

The computer simulation experiments were conducted using HDTV signals, whose sampling frequency is 74 .25 MHz . Picture coding simulator configuration is shown in Fig . 5 . The picture memory contains 1150 MByte semiconductor memory chips and can store 5 seconds of an HDTV component signal . The signal is put into the picture memory from a camera or a digital HDTV VTR, coded and decoded by a general purpose computer (Cl), and displayed on an HDTV monitor . 'Fashion Show' is used as HDTV original sample picture as shown in Fig. 6 . Each decomposed signal is shown in Fig . 7 for the sample picture, 'Fashion Show', where all elements for picture values, except for LL, are shifted to a certain level for the convenience of the observer . Computer simulation also shows that the difference between LL signal

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Lp: Low-pass filter and down sampling Hp : High-pass filter and down sampling MC : Motion Copensation DCT : Discrete Cosine Transform Fig . 4 . Adaptive sub-band DCT coding (Coder) . Vol . 3, No, . 2-3, June 1991



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Fig . 6 . HDTV original picture.

Fig. 7 . Decomposed signals (Fashion Show) . Signal Processing : Image Communwation



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Next, the coding algorithms for other bands are discussed . To clarify the effect of DPCM coding on LH signals, information bit-rate and SNR were obtained for various coding algorithms for the LH, HL and HH signals . Table 1 summarizes computer simulation results of quantization characteristics for LH, HL and HH signals . The sample picture contains a scene of a girl just about to stand up, which has almost the same characteristics as the sample HDTV picture used previously . The SNR is the ratio of peak-to-peak signal to RMS noise, and it is obtained by SNR=20log[256/(E e 2 /n) 112 ],

(1)

where en is the quantization error and n is the number of pixels, where the peak-to-peak signal is 256 for the 8 bit analog-to-digital (A/D) converter . A comparison of methods A and B shows that DPCM coding for LH band can reduce information bit-rate without degradation in SNR . The results for methods B and D show that applying a deadzone for LH band signal can further reduce information bit-rate with a little SNR degradation .

Fig . 8 . Power in each sub-band signal .

power and the others is 20 to 40 dB, as shown in Fig . 8 . As shown in Fig . 8, among the four bands, the LL band has the largest amount of signal power, the highest pixel-to-pixel (pel-to-pel) correlation, and can be compressed . Each band signal is further encoded using adaptive discrete cosine transform (DCT), pulse code modulation (PCM) and differential PCM (DPCM) . These processes will be discussed later in detail . We first discuss the coding algorithm for the LL band signal . Since this LL band signal has about 7 MHz bandwidth and is similarto a conventional TV signal, one of the optimum algorithms for the LL signal is DCT coding, which gives satisfactory results for 6 MHz TV signal coding .

4 .3 . Coding algorithm of proposed adaptive sub-band DCT 4.3.1 . LL hand signal coding

To maximize bit-rate reduction efficiency in LL band signal, this paper proposes an adaptive selection of the input signal mode to DCT coding from the intra-field, inter-field and motion compensated

Table I Characteristics of quantization schemes for LH, HL and HH signals Quantization scheme

A B C D E

PCM LH : DPCM, other signal : PCM LH, HL : DPCM, HH : PCM Scheme B+deadzone Scheme C+deadzone

Entropy [bit-per-pel]

SNR [dB]

LL

LH

HL

14H

Average

0 .63 0,63 0 .63 0 .63 0 .63

0 .84 0 .61 0 .61 0 .46 0 .46

0 .50 0 .50 0 .50 0 .32 0.30

0 .13 0 .13 0 .13 0 .10 0 .10

0 .53 0 .47 0 .47 0 .38 0 .37

40 .8 40 .8 40 .8 39 .9 39 .9

v01 . 3 . No, .2 3 . June 1991



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Kishimoto, K. Irie /HDTV transmission system in an ATM-based network.

(MC) inter-frame signals . To further reduce the information bit-rate, non-uniform length coding and run-length coding are applied to the quantized signal . In the non-uniform length coding, a higher level signal is assigned a longer length code . In addition, the run-length coding reduces the length of consecutive zero-level signals . In the above algorithm, the MC algorithm is applied to the LL signal only and the motion vector searching area can be restricted to 25% of the full band signal . Therefore, it can reduce the amount of processing steps by more than 90% for the MC algorithm, which is usually a burden to the coding equipment.

40 30

a

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20

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entropy .

4.4. Performance of proposed coding algorithm 4 .3.2 . LH band signal coding

Since the LH signal contains the lower band signal in the horizontal, its pel-to-pel correlation is high as can be seen in Fig . 7 . Therefore, the proposed algorithm applies DPCM coding to signals on the same horizontal line of the LH band . The quantizers for LH and LL signals are nonlinear quantizers . Non-uniform length coding algorithm for the LH signal is the same as that for the LL signal . In addition to quantization, picture noise elimination and information bit-rate compression are carried out using a deadzone . The deadzone ranges from -3 to +3, which is determined by picture quality and information bit-rate . After quantizing the signal, the algorithm applies the same run-length coding to the quantized signal as the LL signal .

4 .3.3. Coding of HL and HH signals

Since the HL and HH signals show little correlation among pixels, they are quantized pixel by pixel . The quantizers for HL and HH signals are non-linear quantizers . Non-uniform length coding algorithm for those signals is the same as that for the LL signal . The quantizer has the same deadzone as the LH signal quantizer. After quantizing the signal, the algorithm applies the same run-length coding to the quantized signal as the LL signal .

The performance of the proposed coding algorithm was evaluated in terms of SNR and bit-per-pel ratio, using the picture coding simulator shown in Fig . 5 and the HDTV sample picture, `Fashion Show', which is shown in Fig . 6 . SNR and bit-to-pel results for the luminance signal are shown in Fig . 9 . The average SNR is 37 .87 dB, and the average entropy is 1 .08 bit-per-pel, which corresponds to 150 Mbps of information bit-rate . If the sample picture contains rapidly-moving-objects or fine patterns, the information bit-rate for HDTV signals will be 300 to 600 Mbps considering experimental results for a conventional 6 MHz TV signal.

5. Cell loss compensation for HDTV signals 5.1 . Proposed cell loss compensation for video signals

In this section, a cell loss compensation method for HDTV signals is proposed using random error correcting codes and interleave structures . Transmission cell for video signals is shown in Fig . 10 . First, a continuous video signal is divided into blocks with a constant length of X bytes, as shown in Fig . 10(a) . Next, the address of this video signal's destination terminal and cell sequence number are added to each block of the continuous



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video signal, as shown in Fig . 10(b) . This is called the original information, which consists of the continuous video signal, destination addresses and cell sequence numbers . Finally, check symbols of the random error correcting code are added to the original information shown in Fig . 10(c), in order to construct the original cell . For example, it is assumed that the Reed Solomon code RS(40, 36) is used as the random error correcting code . This random error correcting code can correct the errors when one or two byte random errors occur in one code word 40 bytes long . The total length of the

transmission cell using RS(40, 36) is 40 bytes . This original cell is interleaved . Interleaved cell structure for one-cell loss compensation is shown in Fig . 11 and twenty cells are interleaved . Two bytes are selected from each cell, and interleaved transmission cells are constructed so that the length of the interleaved cell to be transmitted is equal to the length of the original cell, as shown in Fig . 11 . By doing this, even if one transmission cell is lost in the transmission, the lost cell can be reconstructed at the receiver, because the lost bytes correspond to the random error bytes in the decoded Vol. 3, Nos.2-3, June 1991



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t

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cell and the random errors can be corrected by the random error correcting codes . In an ATM network, a cell sequence number is given to each cell, as shown in Fig . 10 . Therefore, the existence of cell losses can be detected at the receiver . The network's cell loss ratio performance for video signals can be determined from its random error correcting capability and the interleave bit number .

Ncm is the maximum number of lost cells which can be compensated for and reconstructed during an interleaved period . For example, the interleaved cell number Ni necessary for two-cell loss com-

1.0

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It is assumed that the cell loss compensation method is carried out using random error correcting codes and an interleave structure as an end-toend protocol at the customers' terminals . In this section, analysis results for cell loss compensation characteristics are described in detail . BCH and Reed-Solomon codes are applied as the random error correcting codes . Transmission efficiency for the original information versus the interleaved cell number, which is normalized by cell loss compensation ability, is shown in Fig . 12 . Cell loss compensation ability

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Fig . 12 . Transmission efficiency versus normalized interleaved cell number.



R. Kishimoto, K. [tie / HDTV transmission system in an

pensation using RS(40, 36) is 40, because the normalized interleaved cell number (Ni/Ncm) is twenty and Ncm is two, as shown in Fig . 12 . The normalized interleaved cell numbers using ReedSolomon codes are smaller than those using BCH codes, with the same transmission efficiency as the original information. Next, we consider a case where one cell is lost . If the transmission efficiency is 0 .9, the number of cells to be interleaved for cell loss compensation using RS(40, 36) is twenty . When Ncm cells are lost during the total interleaved period, Ncm times the normalized number of interleaved cells must be interleaved . For example, when forty cells are interleaved and the RS(40, 36) code is applied, two lost cells can be reconstructed . Normalized interleave delay versus information bit-rate is shown in Fig . 13 . The RS(40, 36) code is used . The error correcting coded cell lengths are 40 and 80 bytes for original information lengths of 36 and 72 bytes. The interleaved delay is inversely proportional to the information bit-rate, as shown in Fig . 11 . When the information bit-rate is about 384 kbit/s for a TV conference bit-rate and Ncm is one, the interleaved delay for a coded cell length of 40 bytes is about 40 msec . Since the

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interleaved delay shown in Fig . 13 is less than conventional video coding delay, it does not substantially affect end-to-end delay performance and the proposed cell loss compensation is proven effective . Cell loss probability after compensation using RS(40, 36) versus cell loss probability before compensation is shown in Fig . 14 . When the number of lost cells is not over the maximum compensation number, lost cells can be perfectly compensated, as shown in Fig. 11 . However, the number of lost cells might be larger than the maximum compensation number in practice during the interleaved period. The cell loss probability shown in Fig . 14 represents the ratio of uncompensated cells to total cells at the decoded terminal, provided that the interleave structure is long enough to randomize errors caused by cell losses over the maximum compensation number, in the interleaved transmission sequence . Since the RS(40, 36) code corrects 2 symbol errors in each deinterleaved code word, the cell loss probability after compensation is approximately proportional to the third power of the cell loss probability before compensation . When the cell loss probability before compensation is 10-6 , the cell loss probability after compensation at the end-to-end terminals is 10 -14 . This result shows that cell loss probability is

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Fig. 14 . Cell loss probability after compensation versus cell loss probability before compensation . Vnl .1, No, . 2-1, June 1991



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successfully reduced using the proposed cell loss compensation method . Furthermore, when the cell loss probability before compensation is 10 - ', the decoded sequence can be regarded as free of cell loss .

6 . Conclusion In this paper, we presented a high-quality HDTV transmission system architecture and a highquality coding algorithm, which are of key importance in deploying the network . Also, a high-quality HDTV picture coding algorithm is proposed using sub-band coding and a DCT-based coding algorithm . Computer simulation results show that the SNR is about 38 dB when the average entropy is I bit-per-pel . The required SNR will be determined by subjective picture observation experiments conducted by TV engineers, and still remains for further study . This coding algorithm can be applied to HDTV transmission between broadcasting stations by optimizing coding parameters . This paper also proposes a cell loss compensation method for video signals using random error correcting codes and an interleave structure in an ATM network. Analysis results show that the proposed cell loss compensation method reduces cell loss probability to the point that the decoded sequence can be regarded as free of cell loss .

Acknowledgments The authors would like to thank Dr . Masaki Koyama and Dr . Haruo Yamaguchi of NTT Transmission Systems Laboratories for their participation in fruitful discussions on broadband transmission, and for their guidance .

Signal Processing: Image Communkatfnn

References [1] H . Gharavi and A . Tabatahai, "Sub-band coding of monochrome and color images", IEEE Trans . Circuits and Systems, Vol . 35, February 1988, pp . 207-214. [2] R . Kishimoto and M . Ikeda, "Optical self-routing switch using integrated laser diode optical switch", IEEE JSAC, Vol . 6, No . 7, August 1988 . [3] R . Kishimoto and K. Irie, "HDTV transmission system and coding method in an ATM network", 3rd Infernal. Workshop on HDTV, Torino, August 1989. [4] R . Kishimoto and N . Sakurai, "High-efficiency TCM bandwidth reduction for high-definition TV", 2nd Internot. Workshop on Signal Processing of HDTV, L'Aquila, February 1988 . [5] R . Kishimoto, Y . Ogata and F . Inumaru, "Generation interval distribution characteristics of packetized variable rate video coding data streams in an ATM network", IEEE JSAC, Vol . 7, No . 5, June 1989 . [6] R . Kishimoto, N . Sakurai and A. Ishikura, "Bit-rate reduction in the transmission of high-definition television signals", SMPTE1, Vol . 96, No . 2, February 1987 . [7] R . Kishimoto, K . Yoshino and M . Ikeda, "Fiber-optic digital video distribution system for high-definition television signals using laser diode optical switch", IEEE JSAC, Vol . 6, No . 7, August 1988 . [8] D . LeGall, H . Gaggioni and C . Tie Chen, "Transmission of HDTV signals under 140Mbit/s using a sub-band decomposition and Discrete Cosine Transform Coding", 2nd Internal. Workshop on Signal Processing of HDTV, L'Aquila, February 1988 . [9] Y . Ninomiya, Y. Ohtsuka and Y . Izumi, "A single channel HDTV broadcasting system - MUSE", NHK Laboratories Note, 304, 1984. [10] 7 .S . Turner, "New directions in communications", Zurich Seminar, 1986 .