Differential Phase
DEFINITION
Differential phase distortion, sometimes referred to as "diff phase'', is present when chrominance phase is dependent on luminance level. This phase distortion is a result of a system's inability to uniformly process the high-frequency chrominance information at all luminance levels.
The amount of differential phase distortion is expressed in degrees of subcarrier phase.
Since both positive and negative (lead and lag) phase errors may occur in the same signal, it is important to specify whether the peak-to-peak phase error or peak deviation from the blanking level phase is being quoted.
PAL measurement standards most frequently refer to peak deviation differential phase measurements.
Two numbers are typically given to describe the distortion: the peak positive phase deviation and the peak
negative phase deviation from the subcarrier phase at blanking level. Sometimes the larger of these two values is given as a single peak result.
Differential phase distortion should be measured different average picture levels and the worst error quoted.
PICTURE EFFECTS
Since virtually all PAL receivers now employ delay-line decoders, reasonable amounts of differential phase distortion cannot be readily detected in the picture. A delay-line decoder averages each two successive
lines in the field, and the resultant information is displayed.
Chrominance phase shifts are therefore cancelled out and do not result in a hue shift in the picture. (Differential phase is actually converted to differential gain in the resultant, but gain errors are much less objectionable in the picture.)
TEST SIGNALS
Differential phase is measured with a test signal that consists of uniform-phase chrominance superimposed on different luminance levels. A modulated staircase (5 or 10 step) or a modulated ramp (see Figure 62) is typically used. A ramp is normally used when performing measurements on devices and systems that convert
the signal from analogue to digital and back to analogue.
Some generators, such as the Tektronix TG2000, offer a phasealternate modulated ramp test signal. A vector display of this signal is shown in Figure 63.
This signal can help detect distortions that have affected the U and V components differently.
This is most likely to occur if the signal has been demodulated and the U and V components passed through separate processing channels. If this signal is available, it may be desirable to repeat the measurement procedures outlined below for both signal vectors.
MEASUREMENT METHODS
When differential phase is present, the chrominance phase will be different on the different luminance levels of the test signal.
This phase information can be conveniently displayed on a vectorscope after the chrominance has been demodulated.
Although a standard vector display can indicate the presence of large amounts of distortion, a vectorscope equipped with a special differential phase mode or an automatic measurement set such as the VM700T is required for precision measurements.
Vectorscope Display.
In a vectorscope display, the dots corresponding to the various subcarrier packets will spread out along
the circumference of the graticule circle when differential phase is present. When using a ramp signal, the dot will become elongated along the circumference.
To make a measurement, first set the phase of the signal vector to the reference 9 o'clock phase position. Use the vectorscope variable gain control to bring the signal vector out to the graticule circle.
Vectorscope graticules generally have special differential phase and gain marks on the left-hand side to help quantify the distortion.
Peak-to-peak phase deviation can be directly from the graticule. Obtaining peak positive and peak negative
results from the vector display is less straightforward but it is possible when the signal vector lies along the 0 degree or 180 degree axis. In this case, align the bursts with the +135 and -135 degree graticule marks and obtain an approximate peak reading by noting how far positive or negative the dots extend from
the 0/180 degree axis.
Demodulated R-Y Sweep.
Although distortions show up in the vectorscope display, there are some advantages to be gained by
examining the demodulated R-Y (V) signal in a voltage versus time display. (Recall that the weighted R-Y signal drives the vertical axis of a vectorscope, see Figure 65.) First of all, more gain and therefore more measurement resolution is possible in waveform displays. Secondly, the sweep display permits correlation
of the demodulated R-Y signal with the original test signal in the time dimension. This allows determination of exactly how the effects of differential phase vary with luminance level or how they vary over a field.
Precise measurements of differential phase are therefore made by examining a voltage versus time display of the demodulated R-Y information. Distortions manifest themselves as tilt or level changes across the line.
Two different types of demodulated R-Y displays, known as “single trace” and “double trace”, can be used to make this measurement. As described below, different measurement techniques are used with the two displays. In the 1781R, these modes are both accessed by selecting DIFF PHASE in the MEASURE menu. The SINGLE/DOUBLE touchscreen selection determines which of the two displays will appear.
Single Trace Method.
In the single trace mode, distortions are quantified by comparing the R-Y waveform to a vertical graticule
scale. To make a measurement, first use the vectorscope display to set the signal vector to the reference
9 o'clock phase position.
Use the vectorscope variable gain control to bring the signal vector out to the edge of the vectorscope graticule circle.
Make sure the 1781R waveform monitor gain is in the calibrated (1 volt full scale) setting. The R-Y display appears on the waveform (right-hand) screen in the 1781R. Each major division (100 mV) on the vertical graticule scale corresponds to one degree when the R-Y waveform is being displayed. The amount of peak-to-peak differential phase can be determined by measuring the largest vertical deviation between any two parts of the signal. To obtain peak results, measure how far positive and negative the signal extends from the level that cor responds to blanking level subcarrier.
The double trace method provides a more accurate way of measuring the tilt in a one-line sweep of the R-Y information. Instead of comparing the waveform to a graticule, the vectorscope calibrated phase shifter is used to quantify the amount of distortion. The double trace display, which also appears on the waveform
screen in the 1781R, is produced by displaying the single trace R-Y information non-inverted for half the lines and inverted for the other half. Since phase changes affect the amplitude of the R-Y signal, the inverted and non-inverted traces can be moved vertically with respect to each other by shifting phase.
Measurements can therefore be made by introducing calibrated amounts of phase shift with the vectorscope phase control. The basic technique involves nulling the blanking level part of the signal by bringing the inverted and non-inverted traces together at that point. The amount of phase shift that is then required to
overlay the two traces at the point of maximum level shift is the amount of differential phase.
Select DOUBLE in the 1781R DIFF PHASE mode to make this measurement. First look at the vectorscope screen and use the phase shifter to set the signal vector to the reference 9 o'clock phase position. Neither vectorscope nor waveform monitor gain is critical in this mode (see Note 18), but setting the vector to the graticule circle is a good starting point. Now refer to the waveform monitor (right-hand)
screen and use the phase shifter to overlay the blanking level portions of the two waveforms.
Press REF SET to set the phase readout to 0.00 degree (see Figure 67).
The next step is to use the phase shifter to overlay the point in the R-Y waveforms that deviates most from blanking level. The phase readout now indicates the amount of differential phase distortion (see Figure 68). In this example the phase error is all in one direction so peak and peakto-peak results will be the same.
If the signal has both positive and negative phase errors (the R-Y signal extends both positive and negative from blanking), repeat the process for the largest positive and largest negative signal excursions.
The double trace technique is similar when using a 521A Vectorscope. Start by setting the “calibrated phase” dial to zero.
Use the A phase or B phase control to null the blanking level and then use the calibrated phase shifter to null the largest excursion. The number above the calibrated phase dial will now give the amount of differential Phase distortion.
Figure 68. The double trace DIFF PHASE display with the measurement results indicated on the readout.
VM700T Automatic Measurement.
To make an automatic measurement of differential phase with the VM700T, select DGDP in the MEASURE mode. Both differential phase and differential gain are shown on the same display (the lower graph is differential phase). Measurement results are also available in the AUTO mode.
18. 1781R Waveform and Vector Gains.
In the single trace mode, the vector gain must be set so the signal vector extends to the graticule circle. The waveform gain must be in the calibrated (1 volt full scale) position. The graticule is calibrated to one degree per division only under these conditions.
With the double mode display, however, more gain may be introduced for greater resolution.
Additional vectorscope gain and/or waveform vertical gain can be selected without affecting the results.
19. Noise Reduction Filter.
A digital recursive filter is available in the 1781R to facilitate differential phase and gain measurements in the presence of noise. Select the NOISE REDUCTION ON touchscreen selection in the DIFF PHASE or DIFF GAIN menu to enable this filter. The filter removes about 15 dB of noise from the signal without any loss of bandwidth or horizontal resolution.
This mode is particularly useful for VTR and transmitter measurements.
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Table of Contents
DEFINITION
Differential phase distortion, sometimes referred to as "diff phase'', is present when chrominance phase is dependent on luminance level. This phase distortion is a result of a system's inability to uniformly process the high-frequency chrominance information at all luminance levels.
The amount of differential phase distortion is expressed in degrees of subcarrier phase.
Since both positive and negative (lead and lag) phase errors may occur in the same signal, it is important to specify whether the peak-to-peak phase error or peak deviation from the blanking level phase is being quoted.
PAL measurement standards most frequently refer to peak deviation differential phase measurements.
Two numbers are typically given to describe the distortion: the peak positive phase deviation and the peak
negative phase deviation from the subcarrier phase at blanking level. Sometimes the larger of these two values is given as a single peak result.
Differential phase distortion should be measured different average picture levels and the worst error quoted.
PICTURE EFFECTS
Since virtually all PAL receivers now employ delay-line decoders, reasonable amounts of differential phase distortion cannot be readily detected in the picture. A delay-line decoder averages each two successive
lines in the field, and the resultant information is displayed.
Chrominance phase shifts are therefore cancelled out and do not result in a hue shift in the picture. (Differential phase is actually converted to differential gain in the resultant, but gain errors are much less objectionable in the picture.)
TEST SIGNALS
Differential phase is measured with a test signal that consists of uniform-phase chrominance superimposed on different luminance levels. A modulated staircase (5 or 10 step) or a modulated ramp (see Figure 62) is typically used. A ramp is normally used when performing measurements on devices and systems that convert
the signal from analogue to digital and back to analogue.
Some generators, such as the Tektronix TG2000, offer a phasealternate modulated ramp test signal. A vector display of this signal is shown in Figure 63.
This signal can help detect distortions that have affected the U and V components differently.
This is most likely to occur if the signal has been demodulated and the U and V components passed through separate processing channels. If this signal is available, it may be desirable to repeat the measurement procedures outlined below for both signal vectors.
Figure 62. A modulated ramp test signal.
Figure 63. Vector display of the TG2000 phase-alternate modulated ramp.
MEASUREMENT METHODS
When differential phase is present, the chrominance phase will be different on the different luminance levels of the test signal.
This phase information can be conveniently displayed on a vectorscope after the chrominance has been demodulated.
Although a standard vector display can indicate the presence of large amounts of distortion, a vectorscope equipped with a special differential phase mode or an automatic measurement set such as the VM700T is required for precision measurements.
Vectorscope Display.
In a vectorscope display, the dots corresponding to the various subcarrier packets will spread out along
the circumference of the graticule circle when differential phase is present. When using a ramp signal, the dot will become elongated along the circumference.
To make a measurement, first set the phase of the signal vector to the reference 9 o'clock phase position. Use the vectorscope variable gain control to bring the signal vector out to the graticule circle.
Vectorscope graticules generally have special differential phase and gain marks on the left-hand side to help quantify the distortion.
Peak-to-peak phase deviation can be directly from the graticule. Obtaining peak positive and peak negative
results from the vector display is less straightforward but it is possible when the signal vector lies along the 0 degree or 180 degree axis. In this case, align the bursts with the +135 and -135 degree graticule marks and obtain an approximate peak reading by noting how far positive or negative the dots extend from
the 0/180 degree axis.
Figure 64. A vectorscope display showing a peak-to-peak differential phase distortion of about 7 degrees. Differential gain distortion is also present.
Demodulated R-Y Sweep.
Although distortions show up in the vectorscope display, there are some advantages to be gained by
examining the demodulated R-Y (V) signal in a voltage versus time display. (Recall that the weighted R-Y signal drives the vertical axis of a vectorscope, see Figure 65.) First of all, more gain and therefore more measurement resolution is possible in waveform displays. Secondly, the sweep display permits correlation
of the demodulated R-Y signal with the original test signal in the time dimension. This allows determination of exactly how the effects of differential phase vary with luminance level or how they vary over a field.
Precise measurements of differential phase are therefore made by examining a voltage versus time display of the demodulated R-Y information. Distortions manifest themselves as tilt or level changes across the line.
Figure 65. Differential phase distortion affects the R-Y (V) signal.
Two different types of demodulated R-Y displays, known as “single trace” and “double trace”, can be used to make this measurement. As described below, different measurement techniques are used with the two displays. In the 1781R, these modes are both accessed by selecting DIFF PHASE in the MEASURE menu. The SINGLE/DOUBLE touchscreen selection determines which of the two displays will appear.
Single Trace Method.
In the single trace mode, distortions are quantified by comparing the R-Y waveform to a vertical graticule
scale. To make a measurement, first use the vectorscope display to set the signal vector to the reference
9 o'clock phase position.
Use the vectorscope variable gain control to bring the signal vector out to the edge of the vectorscope graticule circle.
Make sure the 1781R waveform monitor gain is in the calibrated (1 volt full scale) setting. The R-Y display appears on the waveform (right-hand) screen in the 1781R. Each major division (100 mV) on the vertical graticule scale corresponds to one degree when the R-Y waveform is being displayed. The amount of peak-to-peak differential phase can be determined by measuring the largest vertical deviation between any two parts of the signal. To obtain peak results, measure how far positive and negative the signal extends from the level that cor responds to blanking level subcarrier.
Figure 66. A single trace display indicating about 7 degrees of differential phase distortion.
Double Trace Method.The double trace method provides a more accurate way of measuring the tilt in a one-line sweep of the R-Y information. Instead of comparing the waveform to a graticule, the vectorscope calibrated phase shifter is used to quantify the amount of distortion. The double trace display, which also appears on the waveform
screen in the 1781R, is produced by displaying the single trace R-Y information non-inverted for half the lines and inverted for the other half. Since phase changes affect the amplitude of the R-Y signal, the inverted and non-inverted traces can be moved vertically with respect to each other by shifting phase.
Measurements can therefore be made by introducing calibrated amounts of phase shift with the vectorscope phase control. The basic technique involves nulling the blanking level part of the signal by bringing the inverted and non-inverted traces together at that point. The amount of phase shift that is then required to
overlay the two traces at the point of maximum level shift is the amount of differential phase.
Select DOUBLE in the 1781R DIFF PHASE mode to make this measurement. First look at the vectorscope screen and use the phase shifter to set the signal vector to the reference 9 o'clock phase position. Neither vectorscope nor waveform monitor gain is critical in this mode (see Note 18), but setting the vector to the graticule circle is a good starting point. Now refer to the waveform monitor (right-hand)
screen and use the phase shifter to overlay the blanking level portions of the two waveforms.
Press REF SET to set the phase readout to 0.00 degree (see Figure 67).
The next step is to use the phase shifter to overlay the point in the R-Y waveforms that deviates most from blanking level. The phase readout now indicates the amount of differential phase distortion (see Figure 68). In this example the phase error is all in one direction so peak and peakto-peak results will be the same.
If the signal has both positive and negative phase errors (the R-Y signal extends both positive and negative from blanking), repeat the process for the largest positive and largest negative signal excursions.
The double trace technique is similar when using a 521A Vectorscope. Start by setting the “calibrated phase” dial to zero.
Use the A phase or B phase control to null the blanking level and then use the calibrated phase shifter to null the largest excursion. The number above the calibrated phase dial will now give the amount of differential Phase distortion.
Figure 67. The 1781R double trace DIFF PHASE display with the phase readout zeroed.
VM700T Automatic Measurement.
To make an automatic measurement of differential phase with the VM700T, select DGDP in the MEASURE mode. Both differential phase and differential gain are shown on the same display (the lower graph is differential phase). Measurement results are also available in the AUTO mode.
Figure 69. The VM700T DG DP display.
NOTES18. 1781R Waveform and Vector Gains.
In the single trace mode, the vector gain must be set so the signal vector extends to the graticule circle. The waveform gain must be in the calibrated (1 volt full scale) position. The graticule is calibrated to one degree per division only under these conditions.
With the double mode display, however, more gain may be introduced for greater resolution.
Additional vectorscope gain and/or waveform vertical gain can be selected without affecting the results.
19. Noise Reduction Filter.
A digital recursive filter is available in the 1781R to facilitate differential phase and gain measurements in the presence of noise. Select the NOISE REDUCTION ON touchscreen selection in the DIFF PHASE or DIFF GAIN menu to enable this filter. The filter removes about 15 dB of noise from the signal without any loss of bandwidth or horizontal resolution.
This mode is particularly useful for VTR and transmitter measurements.
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APPENDICES
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