Luminance nonlinearity can be evaluated using a staircase signal having five or ten steps, with or without high-frequency modulation. However, the modulated staircase may give a different measure of luminance nonlinearity -- the high-frequency signals may affect the lower- frequency processing. The TSG 170A contains both a five-step luminance staircase and a five-step modulated staircase (contained in the NTC 7 Composite signal). The five-step luminance staircase and the NTC 7 Composite signal are shown in Figures 5-15 and 5-16, respectively.

 

Figure 5-15. The five-step luminance staircase signal is used to measure luminance nonlinearity.

 

Figure 5-16. The five-step staircase of the NTC 7 Composite test signal can also be used to measure luminance nonlinearity. To make the measurement, though, the waveform monitor's filter must be set to low-pass to remove the chrominance information.

To make the luminance nonlinearity measurement on the waveform monitor using a luminance staircase, ensure the waveform monitor's filter selection is set to flat. When making the measurement on a modulated staircase, you must first engage the low-pass filter (LPASS). This eliminates the high-frequency modulation, as shown in Figure 5-17 (but remember, it does not eliminate it from the equipment under test). Then calculate the nonlinearity error as the difference in height between the largest and smallest steps. The error is then expressed as a percentage of the largest step.

 

Figure 5-17. With the waveform monitor in low-pass filter mode, the undistorted modulated staircase appears as five luminance levels with equal steps.

In Figure 5-18, for example, the largest step covers about 23 IRE, and the smallest step covers about 16 IRE. From these two values, the luminance nonlinearity error is computed to be 30%. This is a fairly large error that was chosen simply for the purpose of illustration.

 

Figure 5-18. This staircase shows increased gain (taller steps) around the middle of the luminance range and decreased gain (shorter steps) at the highest luminance level. The maximum linearity error on this signal is about 30%.

More precise measurements can be made by using the waveform monitor's X5 magnifier to expand the steps. With X5 magnification, a 20 IRE step vertically covers 100 IRE on the graticule scale. By positioning the waveform vertically, you can view each step individually over the full graticule for high-resolution measurements.

Some waveform monitors -- such as the Tektronix 1780R -- include a differentiated step filter. This filter can be used for luminance nonlinearity measurements on an unmodulated staircase signal.

The differentiated step filter converts each step to a spike of a height proportional to the step height (Figure 5-19). Any variations in step height become immediately apparent as changes in spike heights. By using the waveform monitor's variable gain control, the largest spike can be set to 100 IRE and the percent nonlinearity error can be read directly from the graticule.

 

Figure 5-19. This differentiated staircase, which is displayed on the 1780R Video Measurement Set, shows a luminance nonlinearity of about 10%.

If the amount of luminance nonlinearity measured exceeds the manufacturer's specifications, the equipment requires service or repair.

Differential Gain

Differential gain testing allows you to determine when chrominance gain is being affected by luminance level. Differential gain, sometimes referred to as diff gain or dG, occurs when a video system doesn't process the chrominance signal consistently at all luminance levels. This can cause unwanted chrominance amplitude increases and decreases, all in the same signal. In other words, chrominance can be too high at one luminance level and too low at another.

Differential gain causes color saturation to have an unwarranted dependence on luminance level. This is seen in the television picture as saturation being affected by variations in brightness. When a colored object moves from sunlight to shade, for example, the color intensity will increase or decrease abnormally.

A five-step modulated staircase, found on the NTC 7 Composite signal, can be used to measure differential gain with either a waveform monitor or a vectorscope.

A modulated ramp, available in the TSG 170A, can also be used, as well as a 10-step modulated staircase.

To make measurements on a waveform monitor, set the instrument for one- or two-line sweep with the chrominance filter selected. This filter passes only the 3.58 MHz chrominance signal and eliminates the luminance steps. Also, for convenience, adjust the gain control so the maximum peak-to-peak amplitude of the signal is 100 IRE. Disregarding the color burst signal, the top and bottom of the chrominance signal should be flat, as shown in Figure 5-20. If differential gain is present, packet amplitudes will not be uniform, as shown in Figure 5-21.

 

Figure 5-20. The waveform monitor's chrominance filter eliminates varying luminance levels from the modulated staircase or ramp signal. The flat top and bottom of this chrominance signal indicate an absence of differential gain.

 

Figure 5-21. This waveform monitor display of a chrominance-filtered modulated staircase shows top and bottom distortions representing a differential gain of about 20%.

To determine the amount of differential gain, find the difference in peak-to-peak values for the largest and smallest packets of the distorted signal. The difference is then expressed as a percentage of the larger number. For example, the differential gain in Figure 5-21 is about 20%. This is a fairly large distortion, used for purposes of illustration.

You can also use a vectorscope to check differential gain. On the vectorscope, the modulated staircase will appear as shown in Figure 5-22.

 

Figure 5-22. This vectorscope display of a modulated staircase test signal shows little or no differential gain. The vectorscope's variable gain has been increased for this measurement.

To evaluate differential gain, adjust the vectorscope's variable gain so that the dot representing the staircase chrominance touches the graticule circle (Figure 5-22). (If undistorted, the staircase chrominance will overlay the burst vector -- they are each 40 IRE p-p at reference phase at the generator.) Any horizontal elongation of the dot is due to differential gain.

Most vectorscopes have special graticule marks, such as shown in Figure 5-23, to help measure the amount of differential gain. These graticule markings allow you to measure fairly large amounts of distortion. But for more precise measurements, especially on smaller amounts of distortion, you'll need a more sophisticated measurement tool with a special DIFF GAIN mode. The Tektronix 1780R Video Measurement Set, for example, provides a DIFF GAIN mode.

 

Figure 5-23. Elongation of the chrominance vector dot indicates about 15% differential gain.

Differential gain, like other nonlinear distortions, cannot be corrected by a simple front panel adjustment. Internal adjustments or repairs are necessary to correct excessive amounts of diff gain.

Tektronix has produced a videotape that provides further details on differential gain and how to measure it. You can obtain this videotape by ordering Tektronix Television Division Measurement Series #1, "Differential Gain" (068-0330-04, NTSC VHS).

Differential Phase

Measuring differential phase determines whether chrominance phase is affected by luminance level. Differential phase, also referred to as diff phase or dP, occurs when the video system fails to process chrominance consistently at all luminance levels. This is similar to differential gain, except luminance level changes affect chrominance phase rather than gain.

Differential phase causes the chrominance signal phase to either lead or lag the desired phase. Such phase errors cause colors to change hue when picture brightness changes. For example, when an object moves from sunlight to shadow, it appears to change color. The incorrect reproduction of color is most likely to occur in the high-luminance portions of the picture.

Differential phase can be tested using the same five- or ten-step modulated staircase signal used for differential gain testing. For digital systems, however, the modulated ramp signal is generally preferred.

Measurements are made using a vectorscope. As with differential gain, the vectorscope's variable gain is adjusted so the end of the modulated staircase vector touches the graticule circle. Any widening of the dot along the graticule circumference represents differential phase. This can be measured using the special graticule marks provided on most vectorscopes (Figure 5-24).

 

Figure 5-24. Widening of the chrominance vector dot along the graticule circumference indicates 5 degrees of differential phase.

Higher resolution measurements, however, need to be done with a more sophisticated vectorscope having a DIFF PHASE mode. A DIFF PHASE mode is provided, for example, on the Tektronix 1780R Video Measurement Set. The VM700T Video Measurement Set also provides a DG DP mode for automatic measurement of differential gain and phase. This is particularly convenient since differential gain and phase often occur simultaneously, as shown in Figure 5-25.

 

Figure 5-25. This display shows simultaneous differential gain and phase of about 10% and 9 degrees.

Correcting differential phase, like differential gain, requires servicing by a qualified technician.

Tektronix has produced a videotape that provides further details on differential phase and how it is measured. To obtain this videotape, order Tektronix Television Division Measurement Series #1, "Differential Phase" (068-0331-04, NTSC VHS).

Conclusion

To maintain optimal signal quality, regular system testing is a must. The tests described in this booklet, while not exhaustive, do detect most of the distortions commonly responsible for
picture quality problems. More importantly, these tests can warn you of impending problems that may not yet be visible as actual picture impairments.

It's wise to establish standard test procedures for each system and document the results from each test. Such test histories allow you to spot system deterioration trends even before the video signal violates acceptable limits. The reward for this effort will be video productions of consistently high technical quality while avoiding costly reshoots and reediting.