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Measurement of TV Camera Noise
Measurement of TV Camera Noise A. S. J E N S E N and J. M. FAWCETT Westinghouse Defense and Space Center, Baltimore, Maryland, U . S . A .
INTRODUCTION The measurement of noise a t the output of a TV camera, or its camera tube, has always been a problem.1-5 The TV system requires a scanning waveform that has a maximum useful scan time and a minimum return time. Signals including noise usually appear different when read from the storage surface during return scan from what they do during the forward scan. This is particularly true if there is significant dark current, for then the average signal level is different. Hence, noise measurements cannot be made in the usual manner with a simple thermal-power meter. In fact, the video signal appearing on a kinescope is not a valid signal so that conventionally the return signal is blanked t o a constant black level. For a physical measurement of noise by a thermal-power meter these blanking pulses and all spurious signals during return must be gated out, but by a method which introduces a switching signal power which is insignificant compared with the noise power to be measured. Two methods of gating have been used t o date, both of which have been somewhat unsatisfactory. Ordinarily the video signal is displayed on an oscilloscope (A-scope); the experimenter estimates by eye the double amplitude of the visual envelope of the noise, excluding in his judgment shading, blemishes and other disturbance signals which form a fixed pattern stationary with time, and hence are not noise, unless it is his intention to include these. He may compare this measurement with the same type of measurement using a noise generator as ti source or he may simply divide his voltage measurement by six and call the quotient the r.m.s. noise voltage. The IEEE standarde for measuring camera noise is similar, but the eye of the experimenter is replaced by a spot photometer which measures the amplitude density distribution, including high frequency disturbances, in the small region selected by the eye of the experimenter. The r.m.s. noise voltage is calculated from this distribution. 288
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Though probably reliable and repeatable, these measurements are not as convincing or as satisfying as a thermal-power meter-measurement. The IEEE standard measurement is too lengthy and complex to be generally used in the laboratory. To date satisfactory video gating circuits have not been possible since the input to output capacitance of available tubes and transistors has been so high that undesirable spikes on pedestals were injected that contained too much power compared with the noise to be measured. Satisfactory transistors are now available so that camera noise measurements can be made by direct means. It is advisable that the standards for camera noise measurements be revised accordingly.
VIDEOGATINUCIRCUIT The gating circuit performs synchronous blanking of the video signal.
This blanking can be controlled so that noise from small intervals of +V
t
Video in
Mixed blank
Gated video out
-
Multivibrator (delay width)
1
Multivibrator (sate width) I
FIG.1. Gating circuit for noise measurement.
the forward scan can be selected for measurement during different portions of the active scan time. These intervals can be rather small, and arbitrarily located within the raster, whilst still giving accurate information of the true r.m.8. noise of the system. However, to explain the technique used, the noise present in a vertical strip of the raster will be selected. Therefore during each horizontal line, a portion of the noise will be passed while all other signals are attenuated. The circuit for selecting and passing this strip of noise is illustrated in Pig. 1. The synchronizing signals of the camera system are used to trigger variable delay, one-shot multivibrators. These in turn drive a transistor switch. The transistor is normally saturated, therefore it attenuates all
MEASUREMENT O F NOISE I N TELEVISION CAMERAS
291
signals passing through the resistor R. The horizontal sync pulse triggers the delay multivibrator which drives the gate multivibrator which turns the transistor switch off, thus passing the video noise for the portion of horizontal time selected by the multivibrators. If the video signal at the time the transistor is turned off is a t the same d.c. level as the level when the transistor was turned on, then no stray signals would be introduced to the video by the blanking circuit. However, to achieve this same level, the video signal would have to be capacitance coupled, and a d.c. potentiometer used to adjust the d.c. level. This technique would require adjustment each time a new system was measured, and would be critical at low signal levels. Since this adjustment is undesirable the technique shown in Fig. 1 was used. The circuit is essentially a clamping or d.c. restoration circuit common in many television systems. The purpose of the circuit is to develop
FIG.2. Wave forms in gating circuit.
a d.c. charge across the capacitor shortly before the transistor switch is opened. Once the switch is opened the impedance seen by the capacitor is kept very high so that the d.c. potential is maintained across the capacitor. The effect of this procedure is illustrated in Fig. 2. The RC product has to be kept small so that the capacitor will reach full charge shortly after any transient signals. However, the time constant should not be too short. If the RC time constant is comparable with the reciprocal of the noise bandwidth, then the probability that the noise will have some d.c. component before the switch is turned off is high. This effect is sometimes referred t o as clamping noise, i.e. the d.c. level of the gated noise is not equal t o that of the ungated video signal, but varies randomly from gate to gate depending on the noise charge stored on the capacitor before the switch is opened. Therefore, the RC time constant has to be long enough to ensure that the noisecharge fluctuation on the capacitor averages to some small value of the original r.m.8. value. This time constant compromise was empirically
292
A. 5. JENSEN AND
J. M.
FAWCETT
determined to be about 10psec (T= 2.2RG) for 5 MHz noise and standard television scan rates. The transistor is selected for fast switching and low input to output capacitance, so that switching transients are small. Figure 3 is a photograph of a dual-trace oscillograph showing one line of video Aignal taken from 3 successive TV frames. The lower trace (b) is the output signal from a TV camera with the lens capped, showing noise and horizontal blanking pulses. The upper trace (a) is the same, but gated so that the output signal is attenuated except for a selected time duration in the middle of the line. These are the signals that appeared at the terminal labelled “gated video out” in Fig. 1. At this point the horizontal blanking pulse amplitude is 0.35V, and the noise
FIa. 3. Video signals from three successive frames with lens capped. (a) After gating. (b) Before gating.
FICA4. Video signals between two successive vertical blanking pulses. (a) After gating. (b) Before gating.
is 7 mV r.m.s. This corresponds to a noise current at the input of the pre-amplifier of 2 nAr.m.s. Figure 4 shows the same signals on the dual-trace oscilloscope displayed between two successive vertical blanking pulses. I n the lower trace the horizontal sync and blanking pulses run together to give the effect of the lower two apparently continuous lines with small dips during the vertical blanking. Note that in both figures switching transients and d.c. shifts are imperceptible.
MEASUREMENTS The noise power indicated on the meter must be corrected for the duty cycle of the gate. This can be done quite accurittely if the gate width is determined by a high speed counter that operates at a frequency near the top of the video passband. There are, of course, some small but finite switching spikes and low
MEASUREMENT O F NOISE I N TELEVISION CAMERAS
293
level leakage signals so that the most accurate measurements are made a t the output of a linear amplifier of high gain. To verify the accuracy of the measurements made using the gating circuit, white noise having a bandwidth of 5MHz from a General Radio Type 1390B random noise generator was gated with various duty cycles and measured. The results are plotted in Fig. 5 and constitute a set of calibration curves. For a large duty cycle and large noise input, the expected accuracy is achieved, but for a small duty cycle (less than 1%) a high amplifier gain is required, but this is limited by the dynamic range of the gate to 0.32 V r.m.8. of noise signal at the input.
FIQ.6. Calibration curves. Noise output as a function of noise input and duty cycle.
A thermal-power meter measures the total signal power so that it does not distinguish between noise and disturbance, i.e. the fixed pattern, stationary in time, at the output of the camera. This latter, the disturbance, includes shading, blemishes and other variations in the signal from the camera when its lens is capped. If the experimenter wishes to exclude the disturbance from his measurement and confine it entirely to time-varying noise, he must reduce the duty cycle to a small amount, probably with the gate operated to select a small area by being gated in both horizontal and vertical directions. He then locates the gate in an area of the raster that he has determined from a kinescope to be reasonably free from low frequency disturbance. Alternatively, he could search the raster with the gate to determine the lowest output power. This general procedure is not unlike that now
294
A.
S. JENSEN AND J. M. FAWUETT
followed by other methods. It does not exclude high frequency disturbance; probably only successive frame cancellation methods can succeed in doing this. Recent experiments with magnetic disks hold out hope of success in this respect. If they are successful, so that a separation of noise and disturbance may be achieved, then noise and camera-tube disturbance-pattern may both be measured objectively. This would, indeed, be a step forward in the art.
CONCLUSIONS Since there are available today transistors with sufficiently low input to output capacitance, it is quite possible to build a video gate which gates out the blanking pulses at the output of a TV camera or camera tube. This makes possible the direct measurement of noise by means of a thermal-power meter in the conventional manner. This is sufficiently simple to be standard equipment for every TV laboratory so that there should no longer be any need of estimating noise by eye on an A-scope. The various industrial standards for the measurement of camera noise should now be revised to make them similar to the standards for the measurement of noise in other circuits. REFERENCES 1. Holder, J. E., Television Engineering. Report of the International Conference of the Institution of Electrical Engineers 1962, p. 20 (1963). 2. Weaver, L. E., BBC Engineering Monograph No. 24 (1959). 3. Edwardson, J. M., BBC Engineering Monograph No. 37 (1961). 4. Broderick, P., Marconi Imtrumentation, 10, 18 (1965). 5. Rye, J. H., Electronic Equipment News (May 1967). 6. I.R.E., Standards in Electron Tube Methods of Testing, 62 IRE, 7.51, p. 128 (1962).
DISCUSSION M. ROME: Have you, using your method, been able to determine whether the conventional method of measuring S/N ratios (observation of peak-to-peaknoise) is most accurate using the usual factor of 6 or perhaps a smaller factor? Since your method is not subjective, does comparison with subjective determinations show much variation in the latter. A. s. JENSEN: We did make a comparison using two subjects who were skilled and experienced in the measurement of noise from cameras and camera tubes. Their measurements and those of the meter agreed within the rated accuracy of the thermal voltmeter used. We do not claim that the gated meter is necessarily any more reliable than a skilled observer, but it is certainly more satisfying than a measurement that depends upon human acuity. Our observers find the factor of 6.0 good; other observers report factors as low as 5.6. This variation depends upon the observer’s contrast acuity. P. H. BATEY: We have examined the output of one of the commercial TV noise measuring equipments which uses valve circuits to switch out the blanking
MEASUREMENT OF NOISE IN TELEVISION CAMERAS
295
pulses from the video signal. Although there were no obvious switching spikes in the noise waveform when it was viewed on an oscilloscopein the manner mentioned in the paper-a search through the spectrum from line frequency to 5MHz revealed power peaks at harmonics of the switching frequency up to 300 kHz. The equipment used for the search was of the “Weaver” type.2 This is a more sensitive test for switching transients than visual examination of the waveform with an oscilloscope. A. s. ZENSEN: In this paper we do not claim to be the first to measure TV camera noise by gating out the blanking. Rather we mean to emphasize that with today’s transistors any skilled electronic circuit engineer can design and build a highly satisfactory, simple, gating circuit that permits an accurate measurement by a thermal voltmeter. Hence our TV camera and camera-tube standards should be changed to exploit this, and laboratories should cease using the method that depends upon human acuity. The existence of prior literature only serves to support this view. We are indebted to Mr. P. H. Batey for these references to prior work which we now gladly acknowledge. 11. Q. FREEMAN: A technique based on this principle was described in a BBC monograph some 4 years ago. By using the thermal-power meter as a null device in conjunction with a calibrated osciIlator, it is possible to obtain p.t.p. signal/ r.m.8. noise ratios directly. How do your results compare with those of the BBC device? A. s. JENSEN: We were not aware of the BBC device so we have no comparison. We are indebted to you and Mr. Batey for calling our attention to these prior works.
INTRODUCTION The measurement of noise a t the output of a TV camera, or its camera tube, has always been a problem.1-5 The TV system requires a scanning waveform that has a maximum useful scan time and a minimum return time. Signals including noise usually appear different when read from the storage surface during return scan from what they do during the forward scan. This is particularly true if there is significant dark current, for then the average signal level is different. Hence, noise measurements cannot be made in the usual manner with a simple thermal-power meter. In fact, the video signal appearing on a kinescope is not a valid signal so that conventionally the return signal is blanked t o a constant black level. For a physical measurement of noise by a thermal-power meter these blanking pulses and all spurious signals during return must be gated out, but by a method which introduces a switching signal power which is insignificant compared with the noise power to be measured. Two methods of gating have been used t o date, both of which have been somewhat unsatisfactory. Ordinarily the video signal is displayed on an oscilloscope (A-scope); the experimenter estimates by eye the double amplitude of the visual envelope of the noise, excluding in his judgment shading, blemishes and other disturbance signals which form a fixed pattern stationary with time, and hence are not noise, unless it is his intention to include these. He may compare this measurement with the same type of measurement using a noise generator as ti source or he may simply divide his voltage measurement by six and call the quotient the r.m.s. noise voltage. The IEEE standarde for measuring camera noise is similar, but the eye of the experimenter is replaced by a spot photometer which measures the amplitude density distribution, including high frequency disturbances, in the small region selected by the eye of the experimenter. The r.m.s. noise voltage is calculated from this distribution. 288
290
A. S. JENSEN AND J. M. FAWCETT
Though probably reliable and repeatable, these measurements are not as convincing or as satisfying as a thermal-power meter-measurement. The IEEE standard measurement is too lengthy and complex to be generally used in the laboratory. To date satisfactory video gating circuits have not been possible since the input to output capacitance of available tubes and transistors has been so high that undesirable spikes on pedestals were injected that contained too much power compared with the noise to be measured. Satisfactory transistors are now available so that camera noise measurements can be made by direct means. It is advisable that the standards for camera noise measurements be revised accordingly.
VIDEOGATINUCIRCUIT The gating circuit performs synchronous blanking of the video signal.
This blanking can be controlled so that noise from small intervals of +V
t
Video in
Mixed blank
Gated video out
-
Multivibrator (delay width)
1
Multivibrator (sate width) I
FIG.1. Gating circuit for noise measurement.
the forward scan can be selected for measurement during different portions of the active scan time. These intervals can be rather small, and arbitrarily located within the raster, whilst still giving accurate information of the true r.m.8. noise of the system. However, to explain the technique used, the noise present in a vertical strip of the raster will be selected. Therefore during each horizontal line, a portion of the noise will be passed while all other signals are attenuated. The circuit for selecting and passing this strip of noise is illustrated in Pig. 1. The synchronizing signals of the camera system are used to trigger variable delay, one-shot multivibrators. These in turn drive a transistor switch. The transistor is normally saturated, therefore it attenuates all
MEASUREMENT O F NOISE I N TELEVISION CAMERAS
291
signals passing through the resistor R. The horizontal sync pulse triggers the delay multivibrator which drives the gate multivibrator which turns the transistor switch off, thus passing the video noise for the portion of horizontal time selected by the multivibrators. If the video signal at the time the transistor is turned off is a t the same d.c. level as the level when the transistor was turned on, then no stray signals would be introduced to the video by the blanking circuit. However, to achieve this same level, the video signal would have to be capacitance coupled, and a d.c. potentiometer used to adjust the d.c. level. This technique would require adjustment each time a new system was measured, and would be critical at low signal levels. Since this adjustment is undesirable the technique shown in Fig. 1 was used. The circuit is essentially a clamping or d.c. restoration circuit common in many television systems. The purpose of the circuit is to develop
FIG.2. Wave forms in gating circuit.
a d.c. charge across the capacitor shortly before the transistor switch is opened. Once the switch is opened the impedance seen by the capacitor is kept very high so that the d.c. potential is maintained across the capacitor. The effect of this procedure is illustrated in Fig. 2. The RC product has to be kept small so that the capacitor will reach full charge shortly after any transient signals. However, the time constant should not be too short. If the RC time constant is comparable with the reciprocal of the noise bandwidth, then the probability that the noise will have some d.c. component before the switch is turned off is high. This effect is sometimes referred t o as clamping noise, i.e. the d.c. level of the gated noise is not equal t o that of the ungated video signal, but varies randomly from gate to gate depending on the noise charge stored on the capacitor before the switch is opened. Therefore, the RC time constant has to be long enough to ensure that the noisecharge fluctuation on the capacitor averages to some small value of the original r.m.8. value. This time constant compromise was empirically
292
A. 5. JENSEN AND
J. M.
FAWCETT
determined to be about 10psec (T= 2.2RG) for 5 MHz noise and standard television scan rates. The transistor is selected for fast switching and low input to output capacitance, so that switching transients are small. Figure 3 is a photograph of a dual-trace oscillograph showing one line of video Aignal taken from 3 successive TV frames. The lower trace (b) is the output signal from a TV camera with the lens capped, showing noise and horizontal blanking pulses. The upper trace (a) is the same, but gated so that the output signal is attenuated except for a selected time duration in the middle of the line. These are the signals that appeared at the terminal labelled “gated video out” in Fig. 1. At this point the horizontal blanking pulse amplitude is 0.35V, and the noise
FIa. 3. Video signals from three successive frames with lens capped. (a) After gating. (b) Before gating.
FICA4. Video signals between two successive vertical blanking pulses. (a) After gating. (b) Before gating.
is 7 mV r.m.s. This corresponds to a noise current at the input of the pre-amplifier of 2 nAr.m.s. Figure 4 shows the same signals on the dual-trace oscilloscope displayed between two successive vertical blanking pulses. I n the lower trace the horizontal sync and blanking pulses run together to give the effect of the lower two apparently continuous lines with small dips during the vertical blanking. Note that in both figures switching transients and d.c. shifts are imperceptible.
MEASUREMENTS The noise power indicated on the meter must be corrected for the duty cycle of the gate. This can be done quite accurittely if the gate width is determined by a high speed counter that operates at a frequency near the top of the video passband. There are, of course, some small but finite switching spikes and low
MEASUREMENT O F NOISE I N TELEVISION CAMERAS
293
level leakage signals so that the most accurate measurements are made a t the output of a linear amplifier of high gain. To verify the accuracy of the measurements made using the gating circuit, white noise having a bandwidth of 5MHz from a General Radio Type 1390B random noise generator was gated with various duty cycles and measured. The results are plotted in Fig. 5 and constitute a set of calibration curves. For a large duty cycle and large noise input, the expected accuracy is achieved, but for a small duty cycle (less than 1%) a high amplifier gain is required, but this is limited by the dynamic range of the gate to 0.32 V r.m.8. of noise signal at the input.
FIQ.6. Calibration curves. Noise output as a function of noise input and duty cycle.
A thermal-power meter measures the total signal power so that it does not distinguish between noise and disturbance, i.e. the fixed pattern, stationary in time, at the output of the camera. This latter, the disturbance, includes shading, blemishes and other variations in the signal from the camera when its lens is capped. If the experimenter wishes to exclude the disturbance from his measurement and confine it entirely to time-varying noise, he must reduce the duty cycle to a small amount, probably with the gate operated to select a small area by being gated in both horizontal and vertical directions. He then locates the gate in an area of the raster that he has determined from a kinescope to be reasonably free from low frequency disturbance. Alternatively, he could search the raster with the gate to determine the lowest output power. This general procedure is not unlike that now
294
A.
S. JENSEN AND J. M. FAWUETT
followed by other methods. It does not exclude high frequency disturbance; probably only successive frame cancellation methods can succeed in doing this. Recent experiments with magnetic disks hold out hope of success in this respect. If they are successful, so that a separation of noise and disturbance may be achieved, then noise and camera-tube disturbance-pattern may both be measured objectively. This would, indeed, be a step forward in the art.
CONCLUSIONS Since there are available today transistors with sufficiently low input to output capacitance, it is quite possible to build a video gate which gates out the blanking pulses at the output of a TV camera or camera tube. This makes possible the direct measurement of noise by means of a thermal-power meter in the conventional manner. This is sufficiently simple to be standard equipment for every TV laboratory so that there should no longer be any need of estimating noise by eye on an A-scope. The various industrial standards for the measurement of camera noise should now be revised to make them similar to the standards for the measurement of noise in other circuits. REFERENCES 1. Holder, J. E., Television Engineering. Report of the International Conference of the Institution of Electrical Engineers 1962, p. 20 (1963). 2. Weaver, L. E., BBC Engineering Monograph No. 24 (1959). 3. Edwardson, J. M., BBC Engineering Monograph No. 37 (1961). 4. Broderick, P., Marconi Imtrumentation, 10, 18 (1965). 5. Rye, J. H., Electronic Equipment News (May 1967). 6. I.R.E., Standards in Electron Tube Methods of Testing, 62 IRE, 7.51, p. 128 (1962).
DISCUSSION M. ROME: Have you, using your method, been able to determine whether the conventional method of measuring S/N ratios (observation of peak-to-peaknoise) is most accurate using the usual factor of 6 or perhaps a smaller factor? Since your method is not subjective, does comparison with subjective determinations show much variation in the latter. A. s. JENSEN: We did make a comparison using two subjects who were skilled and experienced in the measurement of noise from cameras and camera tubes. Their measurements and those of the meter agreed within the rated accuracy of the thermal voltmeter used. We do not claim that the gated meter is necessarily any more reliable than a skilled observer, but it is certainly more satisfying than a measurement that depends upon human acuity. Our observers find the factor of 6.0 good; other observers report factors as low as 5.6. This variation depends upon the observer’s contrast acuity. P. H. BATEY: We have examined the output of one of the commercial TV noise measuring equipments which uses valve circuits to switch out the blanking
MEASUREMENT OF NOISE IN TELEVISION CAMERAS
295
pulses from the video signal. Although there were no obvious switching spikes in the noise waveform when it was viewed on an oscilloscopein the manner mentioned in the paper-a search through the spectrum from line frequency to 5MHz revealed power peaks at harmonics of the switching frequency up to 300 kHz. The equipment used for the search was of the “Weaver” type.2 This is a more sensitive test for switching transients than visual examination of the waveform with an oscilloscope. A. s. ZENSEN: In this paper we do not claim to be the first to measure TV camera noise by gating out the blanking. Rather we mean to emphasize that with today’s transistors any skilled electronic circuit engineer can design and build a highly satisfactory, simple, gating circuit that permits an accurate measurement by a thermal voltmeter. Hence our TV camera and camera-tube standards should be changed to exploit this, and laboratories should cease using the method that depends upon human acuity. The existence of prior literature only serves to support this view. We are indebted to Mr. P. H. Batey for these references to prior work which we now gladly acknowledge. 11. Q. FREEMAN: A technique based on this principle was described in a BBC monograph some 4 years ago. By using the thermal-power meter as a null device in conjunction with a calibrated osciIlator, it is possible to obtain p.t.p. signal/ r.m.8. noise ratios directly. How do your results compare with those of the BBC device? A. s. JENSEN: We were not aware of the BBC device so we have no comparison. We are indebted to you and Mr. Batey for calling our attention to these prior works.