This standard gives various measurement methods for black-and-white and color television transmissions in simulated radio-relay systems. These measurement methods are a supplement to the measurement methods for telephone and television given in Section 2 of the third general value of this series of standards, "Baseband Measurements". The relevant CCIR recommendations and reports are listed in the reference documents in Appendix B. These documents specify test waveforms suitable for various common television formats. GB/T 4958.11-1988 Measurement methods for equipment used in terrestrial radio-relay systems Part 3: Measurements on simulated systems Section 3: Measurements on black-and-white and color television transmissions GB/T4958.11-1988 Standard download decompression password: www.bzxz.net

National Standard of the People's Republic of China
GB/T4958.11—1988
idtIEC487—3—3:1981
Methods of measurement for equipment used in terrestrial Radio-relay systems
Section 3
Part 3: Measurement for simulated systems
Measurement for monochrome and colour television transmission
Promulgated on March 28, 1988
Implemented on February 1, 1989
Ministry of Posts and Telecommunications of the People's Republic of China
National Standard of the People's Republic of China
Methods of measurement for equipment used in terrestrial radio-relay systems Part 3: Measurement for simulated systems
Section 3 Methods of measurement for equipment used in monochrome and colour television transmission Terrestrial Radio-relay systems Part 3: Simulated systems Section Three: Measurement for monochrome and colour Television
621.317.08
GB/T4958.11—1988
IEC487—3—3(1981)
This standard is one of the national standards "Measurement methods for equipment used in terrestrial radio-relay systems" series. This standard is equivalent to the International Electrotechnical Commission (IEC) standard 487-—3—3(1981) "Measurement methods for equipment used in terrestrial radio-relay systems Part 3: Measurement of simulated systems Section 3 1 Scope
Measurement of black-and-white and colour television transmissions". This standard gives various measurement methods for black-and-white and colour television transmissions in simulated radio-relay systems. These measurement methods are supplementary to the measurement methods commonly used for telephone and television given in Section 2 of Part 3 of this series of standards "Baseband measurements". The reference documents in Appendix B list relevant CCIR recommendations and reports, which specify test waveforms applicable to various common television formats.
Note: The actual circuit system dedicated to transmitting black-and-white and color television program signals shall be implemented in accordance with GB3659-83 "Test Methods for Television Video Channels". 2 Introduction
Generally, suitable commercial measuring instruments can be selected, but their performance must be ensured to be competent for the specified measurements. For example, the oscilloscope is required to have a flat frequency response and good return loss (for example, 30dB) at least within the nominal upper frequency range of the video band. When measuring the amplitude of the waveform displayed on the oscilloscope screen, time calibration, voltage calibration and display linearity are all important factors, and sometimes it is difficult to achieve the required accuracy. When 0.1dB accuracy is required, the scale on the screen may not be able to achieve the required accuracy, but such accuracy is often required, for example, when measuring synchronization pulse distortion. Using the calibrator introduced in Appendix A can ease this contradiction. This solution can also save time when there are many measurements to be performed. When the various test waveforms covered by this standard are used to superimpose on the standard horizontal synchronization pulse, the commercial waveform generators generally used to provide these waveforms are sufficient to ignore their internal distortion and can be used directly for measurement without calibration. If this is not the case, or when the accuracy range required for the measurement is comparable to the accuracy range of the measuring instrument itself, the measurement results should be appropriately corrected according to the distortion of the test instrument. 3 Test signal level
Unless otherwise specified, this standard requires that the test signal be applied to the system input at the nominal level. The nominal input level of the system refers to the level required to produce an 8MHz peak-to-peak frequency deviation without pre-emphasis (i.e., 1V peak-to-peak value, see Appendix BB.1) 4 Noise
For measurement purposes, noise in television systems is divided into three categories: Approved by the Ministry of Posts and Telecommunications of the People's Republic of China on March 28, 1988 and implemented on February 1, 1989
Periodic noise
b. Continuous random noise
c. Pulse noise
GB/T4958.11—1988
The following noise measurements should be performed without input signal. 4.1 Measurement method of periodic noise
The measurement of periodic noise is performed in two frequency bands, the first from 10kHz to the upper frequency limit of the video band, and the second below 10kHz. The nature of periodic noise depends on its source. To ensure that the measured noise has the appropriate meaning, it is necessary to measure it in both the time domain and the frequency domain. Time domain measurement requires a broadband oscilloscope and a suitable band-limited filter. Frequency domain measurement requires a frequency-selective level tester with a tuning range that can cover the required frequency band. This type of instrument is usually calibrated with power, and adding 9B to the measured level value can convert it to a peak-to-peak voltage level with sufficient accuracy.
For color television, it must be ensured that periodic noise components above the upper frequency limit of the video band do not produce beat components exceeding the specified level with the color subcarrier, the continuous pilot, or both in the video band. This beat effect can be checked by adding a sinusoidal signal with a peak-to-peak amplitude equal to the nominal peak-to-peak amplitude and a frequency equal to the color subcarrier frequency at the system input, and searching the entire video band (except for a small section around the color subcarrier) with a narrowband frequency selection table at the system output. To avoid overloading the test instrument, a narrowband bandstop filter tuned to the color subcarrier frequency is inserted between the system under test and the test instrument. In this case, the test results should be appropriately corrected according to the insertion loss of the filter. In order to verify whether the periodic noise components searched are intermodulation products, the color subcarrier or the continuous pilot, or both, can be temporarily removed. If they are intermodulation products, they should disappear. The level of each intermodulation component searched in the video band should not exceed the allowable value of the detailed equipment specification. Note: ① The level of the continuous baseband component appearing outside the video band can be much higher than the allowable value of the in-band component, unless otherwise specified. The out-of-band signal may be a useful signal, such as a sound subcarrier. In this case, when measuring periodic noise, all subcarriers to be transmitted in the system should exist simultaneously at the correct level.
② When the amplitude of the periodic noise is comparable to the amplitude of the random noise, pay attention during the measurement process. From a measurement perspective, in order to distinguish these low-level noises, an oscilloscope with a time base that can be locked to the low-level signal is required. 4.2 Measurement method of continuous random noise
Continuous random noise should be measured at a known brightness signal level point using a suitable band-limited filter. The band-limited filter is used to filter out noise above the upper limit of the video band (Figure 1) and below about 10kHz (Figure 2). However, if the frequency of the periodic noise generated by the power supply exceeds 10kHz, a filter with a higher cutoff frequency can be used. Considering that the subjective effect of noise varies with the frequency distribution of noise, and the human eye is less sensitive to noise in the upper part of the video band, a noise weighting network is always used for measurement. The weighting network shown in Figure 3 is suitable for all TV formats, and the network shown in Figure 4 can be used for some additional measurements when necessary. When measuring noise in the frequency band above 10kHz, a broadband RMS meter should be used, provided that only continuous random noise exists. If periodic noise or pulse noise exists at the same time as random noise, the results measured by the RMS response meter cannot represent the true value of continuous random noise. An oscilloscope can be used to check whether there are other components. If necessary, the levels of other noise components should be appropriately reduced before measuring random noise.
It is not necessary to measure continuous random noise in the frequency band below 10kHz, because the noise in this frequency domain is generally periodic noise generated by the power supply.
4.3 Measurement method of pulse noise
Pulse noise is measured with an oscilloscope, and no weighting network is used for measurement. 4.4 Expression of results
The measurement results shall be expressed in a table and the test conditions for each measurement result shall be listed. 4.4.1 Periodic noise
The measurement results of periodic noise shall be expressed in decibels as the ratio of the peak-to-peak amplitude of the brightness signal to the peak-to-peak amplitude of the periodic noise.
When there is identifiable periodic noise, its level, frequency or repetition period shall be filled in Table 1: 2
Frequency or repetition period
4.4.2 Continuous random noise
GB/T4958.11—1988
Level relative to luminance signal
The measurement results of continuous random noise in various cases listed in Table 2 should be expressed in decibels as the ratio of the peak-to-peak value of the luminance signal to the weighted root mean square value of the continuous random noise.
Number of relay sections
Receiver RF"
Relative input level dB
Signal-to-noise ratio
Video band weighting
Chroma band weighting**
The data given are for example only, 0dB represents the nominal RF input level of the receiver on each relay section in the corresponding performance specification. In general, this measurement is only required when the clutter power per unit bandwidth at 5MHz exceeds the clutter power per unit bandwidth at 1MHz by 11dB (see Appendix BB.3). Figure 4 is an example of a weighting network suitable for this measurement. 4.4.3 Pulse clutter
It should be stated whether this type of clutter is observed. If observed, its duration, level and approximate waveform should be recorded. 4.5 Details to be specified
In the detailed equipment The following items shall be included in the specification as needed: a.
Bandwidth used for noise measurement;
Weighting characteristics adopted;
Allowable continuous random noise level;
Allowable periodic noise level;
Allowable pulse noise level.
5 Linear waveform distortionwww.bzxz.net
For an ideal system, within the normal operating level range, linear waveform distortion is independent of the level of the input signal. The waveform distortion of the video signal and its impact on the displayed image can be divided into four different time scales according to the damage caused, which correspond to the duration of multiple fields, one field, one line and pixels respectively. When this measurement method considers one of the time scales, the damage corresponding to the other three is not considered. Linear distortion is caused by many factors. In order to fully evaluate its impact on the system , a variety of controlled tests are required. 3
5.1 Long-term waveform distortion
5.1.1 Definition and general considerations
GB/T4958.11—1988
Long-term waveform distortion is mainly caused by baseband AC coupling. It is a measure of the difference between the linear response of the simulated radio system and the linear response of a single resistor-capacitor circuit with a similar time constant and multi-field duration (see Appendix BB.4). A television test signal is added to the input of the simulation system. This signal simulates the image signal changing from a low average image level to a high average image level, or from a high average image level to a low average image level. The blanking level in the output signal of the simulation system cannot accurately follow the blanking level in the input signal, that is, long-term waveform distortion occurs. The result is either exponential or It is a damped oscillation form with very low frequency superimposed on the signal.
The following items should be measured for this oscillation:
The peak amplitude of the signal overshoot;
The time required for the oscillation to decay to a certain specified value. 5.1.2 Measurement method
The measurement method is: Connect a television test signal to the input of the simulation system, whose average picture level can be changed between 12.5% ​​and 87.5%. The interval between changes should be long enough so that the damped oscillation can decay to a negligible value before the next change. At the output, use a DC-coupled oscilloscope that does not have this type of distortion to measure the maximum overshoot exceeding the final steady state of the signal envelope (X value in Figure 5) and the damping time t for the signal to reach a certain specified value X: and remain below this value. This measurement can be made on a photograph of the displayed waveform or with a memory oscilloscope.
Usually, the overshoot caused by the change from a low average picture level to a high average picture level is the main one because it exceeds the steady-state amplitude range. The overshoot caused by the change from high average picture level to low average picture level is mostly within the steady-state range, so the measurement result should give the maximum value of the overshoot exceeding the steady-state range. 5.1.3 Presentation of results
The measurement result should be presented in narrative form, i.e. the ratio of the maximum overshoot to the luminance signal amplitude is y (expressed as a percentage) and the damping time to reach the value specified in the detailed equipment specification is t seconds. It is best to have a photograph of the waveform displayed on an oscilloscope. 5.1.4 Details to be specified
In the detailed equipment specification, include the following items as required: a. The maximum overshoot percentage allowed (e.g. X = 20%) b. The damping time t to reach and remain below a given percentage X of the luminance signal amplitude (e.g. t = 5 seconds when reaching 3%) Note: Long-term waveform distortion is related to the number of modulators and demodulators in cascade, and has nothing to do with the number of relay stations that do not demodulate. 5.2 Field Time Waveform Distortion
5.2.1 Definition and General Considerations
Field Time Waveform Distortion is defined as the change in the top shape of the square wave at the output when a square wave signal with a duration equal to the field duration and an amplitude equal to the nominal brightness amplitude is added to the input of the simulation system. This measurement does not take into account the distortion at the beginning and end of the square wave within a period of a few lines.
5.2.2 Measurement Method
The measurement method is: add a square wave signal in accordance with Figure 6 to the input of the simulation system and use a DC-coupled oscilloscope to detect the waveform at the output of the system. Measure the maximum deviation of the top level of the square wave signal from its center point level and express it as a percentage of the amplitude of the square wave signal. This measurement does not take into account the distortion within 250μs (about four lines) at the beginning and end. 5.2.3 Presentation of Results
The measurement results shall be expressed in narrative form, that is, the field time distortion does not exceed % of the amplitude measured at the midpoint of the square wave. The measurement results shall include a photograph of the received waveform.
5.2.4 Details to be specified
In the detailed equipment specification, include the following items as required: a. Repetition frequency of the square wave signal (e.g. 50 Hz); 4
b. Permissible distortion percentage.
5.3 Line time waveform distortion
5.3.1 Definition and general considerations
GB/T4958.11—1988
When a square wave signal with a duration of the same order as the line duration and an amplitude of the nominal brightness amplitude is added to the input of the simulation system, the line time waveform distortion is defined as the change in the shape of the top of the square wave at the output. This measurement does not take into account the distortion at the beginning and end of the square wave equivalent to a few pixels.
5.3.2 Measurement method
The measurement method is similar to that in 5.2.2, but the waveform used must comply with the B3 square wave signal in Figure 7. The principles and precautions used are also the same. This measurement does not take into account the distortion within 1us at the beginning and end of the square wave. 5.3.3 Presentation of results
The method of presenting results is the same as 5.2.3.
5.3.4 Details to be specified
In the detailed equipment specification, the following items shall be included as required: a. Duration of the square wave signal;
b. Permissible distortion percentage.
Note: The magnitude of the line time waveform distortion is related to the number of cascaded modulators and demodulators, and has nothing to do with the number of relay stations that do not demodulate. 5.4 Short-time waveform distortion
5.4.1 Definition and general considerations
When a short pulse or fast step signal with a certain amplitude and shape is added to the input of the simulation system, the short-time waveform distortion is defined as the deviation of the output pulse (or step) from its original shape. 5.4.2 Measurement method
The test signal used for the measurement consists of B1 and B2 in Figure 7. Distortion measurement with these two waveforms is carried out in two steps: first, the B1 pulse amplitude is expressed as a percentage of the B2 center amplitude, and then the amplitude of the raised part ("ringing") that lags behind or leads the pulse or bar signal is expressed as a percentage of the amplitude of the received pulse or bar signal. The half-amplitude duration of pulse B1 in Figure 7 is 2T. This pulse used for bar measurement can be replaced by a pulse with a half-amplitude duration of T.
5.4.3 Representation of results
The following should be stated in the measurement results:
2T pulse amplitude expressed as a percentage of the midpoint amplitude of the bar signal; a.
The amplitude of the damped oscillation and the time position of each peak relative to the instant of maximum amplitude of the 2T pulse; b.
T pulse expressed as a percentage of the bar signal amplitude The amplitude of the overshoot. c.
Also provide a photo showing the appearance of the above waveform. The measurement results of the step signal should reflect whether it is consistent with the tolerance diagram, and also state: a. Measured rise time;
b. Maximum amplitude of overshoot (one or more), c. Frequency of "ringing";
d. Time interval during which the "ringing" amplitude exceeds the tolerance value. 5.4.4 Details to be specified
The detailed equipment specification shall include the following items as needed: a. Ratio of 2T pulse and T pulse to bar signal, b. Minimum frequency of 2T pulse ringing;
c. Amplitude of the raised part of the 2T pulse.
When testing step signals, the following items shall be specified: 5
a. Rise time of the output pulse,
b.Allowable overshoot amplitude;
c. Allowable minimum ringing frequency.
5.5 Luminance/chrominance difference
5.5.1 Definition and general considerations
GB/T4958.11—1988
The luminance/chrominance difference is caused by the uneven gain/frequency response and the uneven envelope delay response measured in the baseband of the television transmission system. In the internationally standardized color television system, part of the luminance band is shared with the chrominance band, so it is necessary to specify the amplitude and delay of the chrominance signal relative to the luminance signal. For measurement purposes, the test signal should have luminance band components and chrominance band components. 5.5.2 Measurement method
Add the composite waveform shown in Figure 9 to the input of the simulation system. The composite waveform is composed of two signal parts, one is a sine square pulse with an amplitude of half the nominal luminance signal amplitude, and the other is a color subcarrier, which is 100% modulated by the sine square pulse. The peak-to-peak amplitude of the composite signal is equal to the nominal luminance signal amplitude. The luminance/chrominance gain difference appears on the displayed graph as a bend in the baseline upward or downward relative to the blanking level. The delay difference appears as a sinusoidal distortion of the baseline, and its peak-to-peak amplitude is proportional to the luminance/chrominance delay difference (see Figure 10). Using an oscilloscope with a graticule that is vertically marked with -10 to +100 units, adjust the oscilloscope so that the displayed pulse is exactly within the range of 0 to 100, and measure the peak-to-peak amplitude of the concave and convex parts observed above and below the blanking level, as shown in Figure 10. and Y, use these two measurements to calculate the gain difference and delay difference. If Y. and Y. Expressed in linear units relative to a specified reference level (e.g. blanking level), when the oscilloscope gain is adjusted to make the amplitude of the image signal (brightness plus chrominance) output by the system exactly 100 units relative to the same reference level, the gain difference is: 2(Y.-Y)
100+(Y,-Y)×100%
N100×100
Wu Zhong: T-
2(for 5MHz bandwidth, T=100ns)
-proportional to the half-amplitude duration of the chrominance pulse (for 10T pulse, n=10)n
Note: ① The delay difference formula ② is an approximate formula, which is applicable regardless of whether n is an integer. (1)
② Any commercial instrument that can offset inherent distortion by adjusting the calibration equalizer can be used to measure gain difference and delay difference. The oscilloscope is only used as a zero point indicator.
③ In the presence of crosstalk (see 6.3), the additional distortion presented on the test waveform needs to be considered when measuring the luminance/chrominance difference. The gain difference can also be measured by the comparison method, comparing the peak-to-peak amplitude of the line bar B2 shown in Figure 7 with the peak-to-peak amplitude of the test signal unit G1 in Figure 16, or with the peak-to-peak amplitude of the last step of G2. For the 525-line format, the relative amplitudes of the B2 and G signals in the original signal should be considered.
5.5.3 Representation of results
The luminance/chrominance gain difference should be expressed as a percentage of the luminance peak amplitude, and a positive sign is taken when the chrominance signal exceeds the luminance signal. The luminance/chrominance delay difference is measured in nanoseconds, and a positive sign is taken when the chrominance signal lags behind the luminance signal. 5.5.4 Details to be specified
In the detailed equipment specification, the following items shall be included as required: half-amplitude duration of the pulse used;
Permissible gain difference (±X%);
c. Permissible delay difference (±yns).
6 Non-linear distortion
The transmission characteristics of long-distance television circuits are not completely linear. The magnitude of the non-linear distortion generated depends on the following factors: 6
Average picture level:
Instantaneous amplitude of the luminance signal.
Amplitude of the chrominance signal.
GB/T4958.11—1988
Generally speaking, it is meaningless to try to specify all non-linearities in the transmission circuit. Therefore, it is necessary to limit the number of parameters to be measured and only measure items that are considered to be directly related to image quality. In addition, a systematic classification method should be used to define the items to be measured and limit their test conditions. The waveform characteristics of video signals are: the impact of circuit nonlinear distortion on synchronization signals is different from that on image signals. Moreover, nonlinearity may affect brightness signals and chrominance signals separately, or cause them to affect each other. This leads to the following systematic classification of non-linear distortions (see Appendix BB.2):
Non-slow distortion
Aspect ratio
Brightness distortion
Strangeness distortion
Main height signal
Caused by
Caused by chrominance signal scaling
(intermodulation)
Amplitude distortion
When the image signal
Chroma signal
Current distortion
Caused by the envelope signal
Caused by the chrominance signal
Caused by the chrominance signal
Caused by the phase difference
Caused by the amplitude of the bright imaginary signal
Conservative gain
The above classification applies to steady-state conditions, where the time interval is long relative to the picture period. In such conditions, the average picture level has a definite meaning. If these conditions are not met, for example if there is a sudden change in the DC component, additional non-linear effects may occur, the extent of which depends on the transient response of the line to very low frequency signals. 6.1 Luminance signal distortion
6.1.1 Definitions and general considerations
Luminance signal non-linear distortion is defined as the relative deviation between the amplitude of a small unit step at the input of the circuit and the corresponding amplitude at the output when the level of the step signal changes from blanking level to white level, for a given average picture level. 6.1.2 Measurement Method
Luminance nonlinear distortion is measured using the staircase waveform D1 of Figures 11 and 12, which is sent to the input of the system under test at 0 dB and +3 dB (relative to the nominal input level of the system) with average picture levels of 12.5% ​​and 87.5%. These two average picture levels are formed by combining the signal D1 with the signal that provides the maximum and minimum brightness shown in Figure 13. The received waveform passes through a differential shaping network, which converts the staircase signal into a five-pulse sequence that approximates a sine square (see Figure 14). The amplitudes of these pulses are compared, and the difference between their maximum and minimum values ​​is expressed as a percentage of the maximum value, thereby obtaining the distortion value. This measurement is performed at average picture levels of 12.5% ​​and 87.5%. Figure 14a shows a suitable filter, with component values ​​for a pulse half-width duration of 1us. Using such a filter helps reduce noise when measuring long-distance simulated radio systems. 6.1.3 Representation of results
The measurement results should be expressed in a narrative manner, that is, the non-linear distortion is X%, and the X% value at 12.5% ​​and 87.5% average picture levels should be given for both input levels of the system. 7
GB/T4958.11—1988
If necessary, a photograph of the graph displayed by the oscilloscope may be attached. 6.1.4 Details to be specified
In the detailed equipment specification, include the following items as required: 12.5% ​​and 87.5% average picture levels and relative to the system The allowable distortion values ​​under various conditions of the nominal input level of 0dB and +3dB.
6.2 Synchronization signal distortion
There are two forms of synchronization signal distortion. The first type appears with the change of the picture level and lasts until the picture level changes again. This distortion is called static distortion of the synchronization signal. The second type also appears with the change of the picture level, but only lasts for a very short time. It is called transient distortion of the synchronization signal. Both distortions should be measured under the conditions of 87.5% and 12.5% ​​average picture levels and system input levels relative to the nominal input level of the system of 0dB and +3dB. 6.2.1 Static distortion
6.2.1.1 Definition and general considerations
The definition of static distortion of the synchronization signal is: for a given average picture Image level, the deviation of the midpoint amplitude of the synchronization pulse from the nominal value. The insertion loss and insertion gain of the circuit should be taken into account. The definition of insertion gain is: the ratio of the peak-to-peak amplitude of the brightness signal (from blanking level to white level) of the video signal at the output of the simulation system to the amplitude of the signal at the input end, expressed in decibels. 6.2.1.2 Measurement method
The static distortion of the synchronization signal uses the measurement signal shown in Figure 13, or a similar signal that can obtain the required 12.5% ​​and 87.5% average image level. The total level at the input of the simulation system is 0dB and +3B relative to the nominal input level of the system. Measure the amplitude between the center of the synchronization pulse and the average blanking level (see Appendix BB.2). The difference between the measured level and the nominal level represents the distortion generated. 6.2.1.3 Expression of results
The measurement results shall be expressed in narrative form, i.e. the static distortion of the sync pulse is ×%, where X is the difference between the measured value and the nominal value, expressed as a percentage of the nominal value. The values ​​of × shall also be given for the 12.5% ​​and 87.5% average picture levels and for the two system input levels. When the measured value is less than the nominal value (i.e. compression), the value of X is negative, and when the measured value is greater than the nominal value (i.e. expansion), the value of X is positive. 6.2.1.4 Details to be specified
In the detailed equipment specification, the following items shall be included as required: a. The maximum permissible compression channel for each system input level; b. The maximum permissible expansion value for each system input level; c. Test signals used
6.2.2 Transient distortion
6.2.2.1 Definition and general considerations
Transient distortion of synchronizing signals is defined as the maximum instantaneous deviation of the synchronizing pulses at the output of the system under test from the nominal midpoint amplitude when the mean picture level at the input is varied between a specified range. If the mean picture level is changed abruptly, the resulting dc step applied to the system under test may produce damped oscillations due to various factors, such as various baseband couplings and possibly due to the time constant of any automatic frequency control circuit (see 5.1), so that the signal may enter the non-linear region of the transfer characteristic for a short time (e.g. a few fields). This distortion is transient and generally has no significant effect on the picture area, however, compression of the synchronizing pulses may cause the picture to lose synchronism. 6.2.2.2 Measurement method
This distortion is measured using the test signal shown in Figure 13: or a similar signal where the mean picture level can be varied from 12.5% ​​to 87.5%. The maximum transient change in the amplitude of the sync pulse can be measured by changing the average picture level from 12.5% ​​to 87.5% and back again after a few seconds. The transition time should be less than one line period. Due to the transient nature of this distortion, it can be measured using a graphic photograph or a storage oscilloscope. Regardless of which method is used, a filter (see Figure 14a) is used to differentiate the waveform before it is applied to the oscilloscope. In this way, the position of the blanking level displayed on the display screen is independent of the brightness level. The oscilloscope time base is set to a slow speed (for example, 0.1cm/s), and the Y-axis gain is adjusted so that the deflection of the pulse envelope formed by the differential sync pulse is the same as the deflection of the 1V peak-to-peak video signal before differentiation (for example, 100 units). The 8
GB/T4958.11—1988
response pulse that appears after the image signal is differentiated is ignored. When the average picture level changes, take a picture of the displayed graphic and measure the maximum change in envelope height from the photographed picture (see Figure 15).
This measurement is made at both 0 dB and +3 dB total level relative to the nominal system input level. 6.2.2.3 Presentation of results
The results shall be presented in narrative form as the transient distortion of the sync pulses, where × is the difference between the nominal value and the measured extreme value varying with the average picture level at each system input level, expressed as a percentage of the nominal value. Where necessary, photographs of the results shall be provided. 6.2.2.4 Details to be specified
The following items shall be included in the detailed equipment specification as required: a. The maximum allowable transient compression at each system input level; b. The maximum allowable transient expansion at each system input level. 6.3 Chroma/luminance crosstalk
6.3.1 Definitions and general considerations
The chroma/luminance crosstalk is measured with signal element B2 (Figure 7) as the luminance signal. Chroma/luminance crosstalk is defined as the change in the luminance signal level when the chroma signal is removed for each specified average picture level. 6.3.2 Measurement method
When measuring chroma/luminance crosstalk, signal unit B2 (Figure 7) is used as the luminance signal. Signal G, G1 or G2 is superimposed on the 50% reference luminance signal (Figure 16). After transmission through the system, when the chroma subcarrier has the maximum amplitude, the incremental change in the amplitude of the reference signal is measured and expressed as a percentage of the amplitude of the line signal B2. For ease of measurement, a filter is used at the receiving point to remove the subcarrier before sending it to the oscilloscope.
When the reference level changes towards the white level, the crosstalk is positive, and when it changes in the opposite direction, the crosstalk is negative. This measurement should be performed under conditions of 0 dB and +3 dB relative to the nominal system input level. 6.3.3 Expression of results
The measurement results shall be expressed in a narrative form, that is, the chroma/luminance crosstalk does not exceed X% for each system input level, where X is the change in the reference voltage amplitude relative to the amplitude of the line signal B2. The measurement results can be positive or negative. 6.3.4 Details to be specified
In the detailed equipment specification, include the following items as required: The permissible crosstalk value for each system input level condition. 6.4 Differential gain distortion
6.4.1 Definitions and general considerations
The differential gain distortion of a television system is the variation of the amplitude of the chrominance signal with the luminance signal. The differential gain is defined as the variation of the amplitude of the chrominance subcarrier at the system output when the luminance signal changes from blanking level to white level when a constant amplitude colour subcarrier is superimposed on the luminance signal and the average picture level is maintained at a specified value. 6.4.2 Measurement method
For this measurement, waveform D2 with five ascending steps is used (Figures 11, 12 and 13). The differential gain distortion is measured at average picture levels of 12.5% ​​and 87.5% and total levels of 0 dB and +3 dB relative to the nominal system input level. In order to separate the color subcarrier signal, a bandpass filter should be connected between the measuring oscilloscope and the output of the simulated system under test. In this way, the subcarrier envelope displayed by the oscilloscope can easily measure the subcarrier amplitude of each step. The bandwidth of the filter should be selected in such a way that it can ensure sufficient signal-to-noise ratio and avoid transients during step conversion, so that both can be optimally attenuated. Both factors may affect the measurement accuracy. The 625-line system usually uses a filter with a bandwidth of 1MHz. The amplitude of the color subcarrier at the blanking level and at steps 1 to 5 where the brightness signal gradually increases is measured respectively, and the differential gain distortion X% and Y% are calculated by formula (3). These two values ​​represent the extreme values ​​(maximum and minimum values) of the subcarrier amplitude measured at each point relative to the subcarrier amplitude at the blanking level point.
The subcarrier amplitude at the blanking level point is represented by A. Indicated, the subcarrier amplitude at each step is represented by A19
GB/T4958.11—1988
A2, A3, A4, As in the ascending order of the brightness signal level. Compare the amplitudes from A. to As, the maximum amplitude exceeding A. is expressed as a percentage of A., that is: [(Ao,,A6)max - 1] × 100
A. When it is the maximum value, X=0.
If any of these amplitudes is less than A., the minimum value is expressed as a percentage of A., that is: Y = [(Ao,,As)min
-1×100
A. When it is the minimum value, Y=0.
The peak-to-peak differential gain is expressed as:
[x + ] [,A)max(,.)min] ×100Ao
Note: If higher resolution is required, the waveform at the filter output can be envelope detected so that the oscilloscope can use a higher gain. Otherwise, the oscilloscope gain is limited by the relatively high subcarrier amplitude. 6.4.3 Representation of Results
The measurement results are expressed as a percentage of the subcarrier amplitude at the blanking level (X and Y) and are entered in Table 3. Table 3
System input level relative to nominal value
6.4.4 Details to be specified
In the detailed equipment specification, include the following items as required: Gain distortion
Differential gain
87.5% average picture level
12.5% ​​average picture level
The system input level is the nominal value. For each average picture level, the allowable differential gain distortion; a.
b. The system input level is relative to the nominal value + 3 dB. For each average picture level, the allowable differential gain distortion. 6.5 Differential phase distortion
6.5.1 Definition and general considerations
The differential phase distortion of a television system is the amount by which the phase of the chrominance signal changes with that of the luminance signal. The definition of differential phase is: a color subcarrier with a constant amplitude is superimposed on the luminance signal, the average image level is maintained at a specific value, and the phase of the color subcarrier at the system output changes when the luminance signal changes from the blanking level to the white level. 6.5.2 Measurement method
The measurement method and the waveform used are similar to those in 6.4.2. The difference is that the differential phase measures the extreme value (positive or negative) of the color subcarrier phase relative to the blanking level point, rather than the amplitude extreme value. These two values ​​are represented by X° and Y° respectively: X=(o,,9s)maxqo(degrees)
Y=(qo,,q)min—o(degrees)
Where: 9,98-
GB/T4958.11—1988
The phase of the subcarrier measured at each step from the blanking level to the maximum brightness signal level. Both X and Y may be zero.
The peak-to-peak differential phase distortion is expressed as:
x+Y=(qoq)max-(qo,,)min(degrees) Note: A special instrument is required to measure the phase of the subcarrier, which can be purchased on the market. 6.5.3 Expression of results
The measured values ​​are expressed in terms of phase (degrees) X° and Y° of the subcarrier relative to the blanking level point and are entered in Table 4: Table 4
|System input level relative to nominal value
6.5.4 Details to be specified
In the detailed equipment specification, include the following items as required: a.
System input level is the nominal value, for each differential phase distortion
12.5% ​​average picture level
87.5% average picture level
Average picture level, allowable differential phase distortion, Yo
System input level is relative to the nominal value + 3 dB, for each average picture level, allowable differential phase distortion. (8)
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