PAL SYSTEM TELEVISION MEASUREMENT : DEPTH OF MODULATION

Depth of Modulation

DEFINITION
Depth of modulation (percentage of modulation) measurements indicate whether or not video signal levels are properly represented in the RF signal.
The PAL System I modulation scheme (see Note 29) yields an RF signal that reaches its maximum peak-to-peak amplitude at sync tip (100%). In a properly adjusted signal, blanking level corresponds to 76% and white to 20%. The zero carrier reference level corresponds to 0% (see Figure 106).

Figure 106. Depth of modulation levels for System I.

Figure 107. The zero carrier reference pulse as it appears in a baseband signal (System I).

PICTURE EFFECTS
Overmodulation often shows up as nonlinear distortions such as differential phase and gain and picture effects correspond to those caused by the various distortions. ICPM or white clipping may also result.
Undermodulation often results in degraded signal-to-noise performance.
TEST SIGNAL
A signal with black and white levels is required for depth of modulation measurements. This signal is used in conjunction with the zero carrier reference pulse, which the demodulator typically places on one line in the vertical interval. In the composite signal the zero carrier pulse appears as a 0.95 volt (above blanking) bar approximately 30 microseconds in duration (see Figure 107).

Figure 108. A signal that extends to 700 mV, such as this staircase signal, is used in conjunction with the zero carrier pulse to verify modulation levels.

MEASUREMENT METHODS
Modulation depth is measured at the output of a precision demodulator by verifying that the ratios between the parts of the signal are correct. Overall amplitude is not critical, but it should be adjusted in the system
to be approximately 1.25 volts from sync tip to zero carrier at 100% transmitter power. This will minimize the effects of nonlinearities in the measurement system.
Waveform Monitor. 
Most waveform monitors provide a depth of modulation scale on the graticule.
Use the variable gain to position the zero carrier reference pulse at 1.25 volts and sync tip at 0 volts. Verify that blanking level and white level occur at the prescribed points (0.3 and 1.0 volts respectively).
The voltage cursors can also be used for this measurement.
NOTES
28. Envelope Detection Mode.
Depth of modulation measurements should be made with the demodulator in the envelope detection mode to minimize effects of ICPM. (Quadrature distortion will not affect modulation depth.)
29. Depth of Modulation Numbers.
The depth of modulation numbers used in this section are for System I PAL. For PAL Systems B, G, D and K, the CCIR specifies blanking level at 75% ±2.5% of peak carrier, and peak white at 10% to 12.5%. To make measurements that correspond to these specifications, use an overall video amplitude of approximately
1.12 volts. Verify that the white level is at about 11% of the overall amplitude, and that blanking is at about 73%. Since different countries may use different RF levels, be sure to note the recommendations of your
broadcast authority.


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Table of Contents

Preface	3
Good Measurement Practices	4
EQUIPMENT REQUIREMENTS	4
CALIBRATION	6
INSTRUMENT CONFIGURATION	6
DEMODULATED RF SIGNALS	8
TERMINATION	8
DEFINITION OF THE PAL TELEVISION STANDARD	8
PERFORMANCE GOALS	8
Waveform Distortions and Measurement Methods	9
I VIDEO AMPLITUDE AND TIME MEASUREMENTS	9
Amplitude Measurements	10
Time Measurements	12
SCH Phase	15
II LINEAR DISTORTIONS	18
Chrominance-to-Luminance Gain and Delay	19
Short Time Distortion	24
Line Time Distortion	26
Field Time Distortion	28
Long Time Distortion	30
Frequency Response	31
Group Delay	36
K Factor Ratings	38
III NONLINEAR DISTORTIONS	41
Differential Phase	42
Differential Gain	46
Luminance Nonlinearity	50
Chrominance Nonlinear P h a s e	5 2
Chrominance Nonlinear Gain	5 3
Chrominance-to-Luminance Intermodulation	54
Transient Sync Gain Distortion	55
Steady-State (Static) Sync Gain Distortion	56
IV NOISE MEASUREMENTS	57
Signal-to-Noise Ratio	58
V TRANSMITTER MEASUREMENTS	6 0
ICPM	61
Depth of Modulation	63
GLOSSARY OF TELEVISION TERMS	64
APPENDICES
APPENDIX A - PAL COLOUR BARS	67
APPENDIX B - SINE-SQUARED PULSES