PAL SYSTEM TELEVISION MEASUREMENT : SIGNAL TO NOISE RATIO

Signal-to-Noise Ratio
DEFINITION
Noise refers to the fluctuations that are present in any electrical system. Noise can be either random or coherent and comes from a variety of natural and man made sources. Although there is always some noise present, an excessive amount is undesirable since it tends to degrade or obscure the signal of interest.
Signal amplitudes do not always remain constant as the video signal is processed and transmitted.
An absolute measurement of noise is there f o re not part i c u l a r l y relevant as a certain amount of noise will have very different effects on signals of different amplitudes.
Since it is the amount of noise relative to the signal amplitude rather than the absolute amount of noise that tends to cause problems, measurements of signal-to-noise ratios, expressed in dB, are made.
PICTURE EFFECTS
Noisy pictures often appear grainy or snowy and sparkles of colour may be noticeable.
Extremely noisy signals may be difficult for equipment to synchronize to and the picture may suffer from blurriness and a general lack of resolution.
TEST SIGNALS
The tangential method can be used on any video signal with a constant luminance level without chrominance. The measurement can be made on a single line in the vertical interval although full field measurements
are more accurate and somewhat easier to make.
Any line with a constant pedestal level can be used to make VM700T Noise Spectrum measurements.
A quiet line in the v e rtical interval is typically used.
The VM700T Chrominance AM/PM noise measurement requires a red field test signal (see Figure 96).

Figure 96. A red field test signal.

MEASUREMENT METHODS
Tangential Method.
Tangential noise measurements can be made with a 1781R. The method is accurate to within 1 or 2 dB,
down to noise levels of about 60 dB. Filters can be inserted in the AUX OUT/AUX IN path to separate noise components of different frequencies.

Figure 97. The 1781R tangential noise measurement mode showing excessive trace separation.

Figure 98. The 1781R tangential noise measurement mode with trace separation properly adjusted. This signal has a signal-to-noise ratio of 30 dB.

Make sure the waveform monitor filter selection is set to FLAT (unless using the auxiliary filter capability) and the DC restorer to OFF or FAST. Select NOISE in the 1781R MEASURE menu. (In the 1481, use the WAVEFORM COMPARISON mode to split the luminance level of interest in half and overlay the two parts).
The measurement is made by adjusting the separation between the two traces until the dark area between them just disappears.
When there is no perceptible dip in brightness between the two traces, the calibrated offset level (in dB) is the amount of noise. In the 1781R, the large knob is used to control the offset and the on screen readout provides the dB reading. In the 1481, the offset function is performed by the two dB NOISE controls in the lower right-hand corn e r. The dB re a d i n g is obtained from the knob settings.
VM700T Automatic Measurement.
Select NOISE SPECTRUM in the VM700T MEASURE menu to make signal-to-noise measurements.
A spectral display and numeric results are provided in this mode (see Figure 99).
Several lowpass, highpass, and weighting filters are available in this mode. Measurement standards typically require three or four measurements made with various combinations of these filters.
The rms signal-to-noise ratio of the entire spectrum is always displayed in the upper righthand corner of the display. A cursor can be used to select a certain frequency for a peak-topeak noise measurement. The
cursors can also be used to define a narrow range of frequencies for S/N measurements.
The CHROMINANCE AMPM selection in the VM700T MEASURE mode, which requires a red field test signal, provides information about the noise that affects the chrominance portion of the signal. Since the chrominance signal is sensitive to both amplitude (AM) and phase (PM) components of noise, two separate
measurements are provided.
A selection of filters is available in this mode.
Noise measurements are also available in the VM700T AUTO mode.

Figure 99. The VM700T Noise Spectrum display.

Figure 100. The VM700T Chrominance AM PM display .

NOTES
24. Quiet Lines.
"Quiet lines” in the vertical interval are sometimes used to evaluate the amount of noise introduced in a
certain part of the transmission path. A line is reinserted (and is therefore relatively noise free) at the transmitting end of the path of interest. This ensures that any noise measured on that line at the receiving end was introduced in that part of the path.


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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