ITU-R BT 1204-1995 Measuring Methods for Digital Video Equipment with Analogue Input Output《带有模拟输入 输出的数字视频设备的测量方法》.pdf

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1、226 Rec. ITU-R BT.1204 RECOMMENDATION ITU-R BT. 1204 MEASURING METHODS FOR DIGITAL VIDEO EQUIPMENT WITH ANALOGUE INPUT/OUTPUT (Question i73J-R 63/1 i) (1995) The ITU Radiocommunication Assembly, considering 4 that analogue/digital conversion processes will continue even in “totally-digitai studio sy

2、stems; b) with analogue input/output; that there is the problem of instability in measuring the analogue characteristics of digital video equipment c) analogue input/output; that stable measuring methods are essential for improving the performance of digital video equipment with d) that it is desira

3、ble that such measuring methods should make use of existing instruments as much as possible; e) that the dithering method is effective for randomizing the quantization errors; 0 that a shallow-ramp signai is a suitable test signal to measure signal-to-noise ratio, recommends 1 input/output based on

4、525-line or 625-line systems. that the measuring method described in Annex 1 should be used for digital video equipment with analogue 1 Introduction This Recommendation is intended to assist in measuring the characteristics of studio digital video equipment with analogue input/output. When measuring

5、 analogue characteristics such as signal-to-noise ratio (SIN), differential gaidphase (DGDP), and the luminance non-linearity of the equipment, the quantization error of the coding process causes variations in the results of the measurement. Considering the SIN of 8-bit video equipment, for example,

6、 the measured SIN shows some number within 50 dE3 and 70 dB when using a conventional method with a noise meter. To solve these problems, the Broadcasting Technology Association of Japan (BTA), has organized a Working Group to study new measurement methods for digital video equipment with analogue i

7、nput/output. Similar work has been carried out in the United Kingdom. The approach described in the Recommendation provides stable measuring results using the following methods to avoid quantization errors: - superimposing a dither signai on a test signal, and removing the dither signal prior to the

8、 measurement with frequency band limitation filters; and - using a shallow-ramp signal instead of a flat signal for an SIN measurement to make the test signal cross the quantizing levels as much as possible. Measurement items described in the Recommendation are SlN, DGDP, pulse characteristics and l

9、uminance non-linearity (see Note 1). Although the Recommendation addresses measuring methods for equipment of 525-line and 625-line systems, the concept of the methods is applicable to other systems such as HDTV and EDTV. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicens

10、ed by Information Handling ServicesRec. ITU-R BT.1204 227 NOTE 1 - Gairfrequency characteristics are important in specifying the performance of television equipment. However, a sped measuring instrument with an averaging function is required to remove the dither signai, so the measuring method for g

11、airfrequency characteristics is described in Appendix 1. 2 General definitions 2.1 Video equipment and test signais The video equipment and test signals are based on 525-line systems or 625-line systems. 2.2 DUT DUT stands for a device under test, namely, a piece of video equipment being measured. T

12、he AID or D/A conversion in DUTs to be measured using the recommended methods should have a resolution of 8 bits or more. 2.3 IREunits An IRE unit is a linear scale for measuring the relative amplitudes of a television signal with a zero reference at blanking level. 100 IRE units correspond to the r

13、eference white level, and -40 IRE units correspond to the bottom of the synchronization pulses. 525-line systems are often operated with 100 IRE units = 0.714 V p-p, while 625-line systems are often operated with 100 IRE units = 0.7 V p-p. 2.4 Video analyser In this Recommendation, the video analyse

14、r is defined as an automatic video signal measuring instrument which has the functions to average and interpolate the signais to reduce the noise components of the measured signals. 3 The defiiition of dither signal 3.1 Dither signai A dither signal is used to stabilize measured values by causing ve

15、ry srnail random changes to the level of test signals and thus dispersing quantization errors evenly in relation to the quantization levels. Since it is necessary to remove the dither signai easily before measuring, in this Recommendation a sine-wave signal is used as a dither signai, which can be r

16、emoved with a simple filter. The sine-wave signal should be asynchronous (non-locked) with the test signal in order to work as a dither signal. The frequency of the sine-wave signal for this purpose should preferably be outside the nominal cut-off frequency of the system, but within the bandwidth of

17、 the AID and D/A converter of the DUT. For the 525-line system, it is recommended to use a frequency of 5.162 MHz which is the second-null point of a 4.2 MHz low pass filter (LPF) in a video noise meter, and for the 625-line system, to use a frequency of 6.145 MHz which is the null-point of 5.0 MHz

18、LPF. This assumes that DUTs are operating at a sampling frequency significantly greater than twice the highest frequency of the noise measurement filters; for example, at a sampling frequency of four times colour subcarrier frequency as in SMPTE 244 format for composite signals and 13.5 MHz as in Re

19、commendation ITU-R BT.601 for component signals. The amplitude of the sine-wave signai should be 30 mV p-p f 10%. It is desirable that the sine-wave signal is superimposed during the active period of the test video signal. 3.2 Notes 3.2.1 Superimposing the dither signal Superimposing the dither sign

20、al onto a test signal can be realized by the circuits shown in Fig. 1. 1TU-R RECMN*BT. 1204 75 m 4855212 O526101 205 m COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services228 Rec. ITU-R BT.1204 FIGURE 1 Example ciraiis for superimposing th

21、e dither signal 5.162-6.145 MHz sine wave generator gate input A i Blanking signai a) Normal supairriposer 3.64 V pp 5.162-6.145 MHz 4.5 ksL sine wave generator gate input I Blanking signal I Test signai generator El- b) Simple superimposer Note I -The amplitude of test signais may decrease after pa

22、ssing through this circuit. mr ; 3.2.2 When measuring a DUT using the dither signal, it is necessary to observe and measure the DUTs output waveform after removing the dither signal. The dither signal using a non-locked sine-wave signal can be removed by the following methods: - by using band-limiti

23、ng filters; - - The latter two methods are effective but require special measuring instruments. a) Video signal measuring instruments incorporate various types of filters for easy and accurate measurements. These filters are efficient in removing the dither signal if it has a relatively high frequen

24、cy such as 5.162 MHz for 525-line systems and 6.145 MHz for 625-line systems. Removing the dither signai before measurements by means of averaging with the video analyser; by cancelling with an operational amplifier. Dither removal using band-limiting filters ITU-R RECMN*BT= 1204 95 D 4855212 052bl1

25、02 141 - COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesRec. ITU-R BT.1204 229 a Test signal generator A vector scope, for example, employs a band-pass filter which passes only chrominance signals. This means that the dither signal is

26、 outside the range of the band-pass filter. Accordingly, the vector scope can observe the output signal from the DUT without any dither influence when measuring DG/DP. Waveform monitors and noise meters incorporate the following filters for measurements. All these filters are able to reduce the dith

27、er signal. - A Osciiioscope _f Dither superimposer DUT rB - Low-pass filter: Observes the low-frequency components of a video signal, such as filters for measuring signal-to-noise ratio, IRE units filters, and luminance filters which have a notch at the frequency of the chrominance signal. sine wave

28、 generator - Band-pass filter: A chrominance filter for observing chrominance signals and a staircase differentiating network defined in Annex II to Part C, 0 4.3 of ITU-T Recommendation 5.61, are used to measure luminance linearity using a staircase signal. b) Dither removal using the video analyse

29、r As described in 0 2 of this Annex, the video analyser has a data averaging capability to increase the stability and consistency of measured results. This function removes the dither signal efficiently. c) Dither removal using a cancellation circuit As shown in Fig. 2 the output signai from the DUT

30、 being measured and a sine-wave signal with the same phase and amplitude as those of the dither signal components of the DUT output are put into a subtractor to cancel the dither component. Note that this method requires a subtractor with superior performance. FiGW 2 Block diagramof dither cancellat

31、ion procedum I I PhaseAevelshifter 3.2.3 Amplitude of the dither signal From the experiments, it has been proved that the minimum amplitude of the dither signal is 20 to 30 mV p-p for an 8-bit DUT. The value of 30 mV p-p is equivalent to 4 to 6 LSB for an 8-bit DUT. For DUTs with a resolution of 9 b

32、its or over, amplitudes of 30 mV are acceptable. Therefore, 30 mV has been selected as the recommended amplitude of the superimposing dither signal for measuring any DUTs irrespective of the number of their bits. 4 Measuring methods 4.1 Signal-to-noise ratio (the measurement of continuous random noi

33、se including dispersed quantizing noise) 4.1.1 Definition The signal-to-noise ratio (SIN) is defined as the ratio of the 100 IRE units p-p amplitude of a luminance signal to the r.m.s. value of the random noise including the quantizing noise, denoted in decibels. COPYRIGHT International Telecommunic

34、ations Union/ITU RadiocommunicationsLicensed by Information Handling Services230 Rec. ITU-R BT.1204 4.1.2 Test signals - A horizontal shallow-ramp signal of about 20 to 25 IRE units amplitude with a pedestal level of 40 IFS units. If this shallow-ramp test signal is not available, a 50 IRE units fla

35、t-fieid signal with the dither signal could be used. - These test signals are shown in Fig. 3. FIGURE 3 Test signais for SIN measurrments 30 rnV dither signai d-7 3 20-25 40iREunits RE units a) Shaiiow-ramp signai b) Flat-field with dither signai w, 4.1.3 Measuring instruments - A test signal genera

36、tor which can generate a shallow-ramp signal or a flat-field test signal. - A dither signal generator and dither signal superimposer when the flat-field test signal is applied. - A video noise meter or video analyser, with a tilt-null function and band-limiting low-pass and high-pass filters. - An e

37、xternal 200 kHz high-pass filter, if necessary. 4.1.4 Band-limiting The measuring instrument should use band-limiting filters. The lower band limit is normally 10 kHz to eliminate hum noise from the power supply. The 200 kHz high-pass filter (HPF) described in Annex II to Part C of IT-T Recom- menda

38、tion J.61 may be used to remove the slant component of the shallow-ramp signal, if necessary. The upper band limit is the highest frequency of the DUT. For 525-line systems, the upper band limit is 4.2 MHz, and for 625-line systems 5.0 MHz. When a 200 kHz high-pass filter is used, the measuring band

39、 must be indicated in the data sheet. 4.1.5 Measuring method 4.1.5.1 Measurement using the shaiiow-ramp test signal A measurement block diagram is shown in Fig. 4. The external 200 kHz high-pass filter can be eliminated if the tilt-null function of the instrument is enough to eliminate the ramp comp

40、onent of the test signal. If an external high-pass filter is used, an external synchronization signal for the measuring instrument may be required to generate a measuring window signal. The measuring window for the shallow-ramp test signal should be set to cover the ramp signal part only and not to

41、cover the flat part of the test signal. ITU-R RECMNSrBT. 1204 95 m Y855212 0526104 TI4 = COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesRec. ITU-R BT.1204 ACURE 4 Block diagram for SIN measurement using a shallow-ramp signal M - High-

42、pass filter 200 kHz Test signal generator DUT 23 1 Video noise meter + with low-pass filter with tilt-nuli function 4 4.1.5.2 A measurement block diagram is shown in Fig. 5. The dither frequency should be kept within the null-point range of the low-pass filter of the instrument. Measurement using th

43、e 50 JRE units flat-field signai with mixed dither Dither signal El- superimposer Test signal generator* FIGURE 5 Block diagram for SIN measurement using a flat-field signal with the dither signal Video noise meter with low-pass filter _c DUT - or video analyser * Fiat-field signal m 4.1.6 Notes 4.1

44、.6.1 has a special frequency spectrum depending on the shallow-ramp amplitude. 4.1.6.2 bit higher resolution is desirable. 4.1.6.3 An external 200 kHz high-pass filter may be required if the tilt-null function of the video noise meter is not sufficient to eliminate the ramp-signal component or to av

45、oid the saturation of the head-amplifier in the measuring instrument. It is not recommended to use the weighting network because the quantizing noise using the ramp test signal The bit resolution of the digitally generated shallow-ramp signal should be better than that of the DUT. Two Use of the 200

46、 Wz high pass filter has little effect on the quantizing noise component, because the basic frequency component is above 700 Wz even for the 20 IRE units shallow-ramp signal used for measuring an 8-bit DUT. The high-pass filter, however, decreases the random flat noise component. In the case of 525

47、systems, use of the 200 kHz high-pass filter increases the measured SIN data by 0.4 dB. ITU-R RECMN*BT= 32oq 95 m 4855232 0526305 950 COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services232 Rec. ITU-R BT.1204 4.1.6.4 video equipment, becau

48、se the 100 IRE units ramp signal covers the full dynamic range of DUTs. Assuming a 100 IRE units ramp signal of 40 p duration as a test signal, the quantizing noise spectrum of 8-bit DUTs, which has 140 quantizing steps, appears at about 3.75 MHz, while that of 9-bit DUTs appears at about 7.5 MHz wh

49、ich is beyond the measuring frequency band. The spectrum of 8-bit quantizing noise obtained with a 100 IRE units ramp signal (625/50 line-rate) without added dither and using 13.3 MHz sampling frequency (Le. three times subcarrier frequency) is illustrated in Fig. 6. In some cases, a ramp test signai with an amplitude of 100 TRE units is used in SIN measurement of digital FIGURE 6 Spectrum of %bit quantid, 100 IRE units, line-rate ramp with no added dither - 10 - 20 - 30 -40 v -50 $ -60 43 f! .- d - 70 - 80 - 90 - loo O 1 2 3 4 5 6 Frequency (MHz) Noise spectrum : Amplitude O d