MEASUREMENTS

Chrominance-to-Luminance Gain & Delay Inequalities
• DEFINITION
Chrominance-to-luminance gain inequality (relative chrominance
level) is the difference between the gain of the chrominance
components and the gain of the luminance components as
they pass through a system. The difference is expressed in
percent or dB, and the number is negative for low chrominance
and positive for high chrominance.
Chrominance-to-luminance delay inequality (relative chrominance
time) is the difference between the time it takes for the chrominance
portion of the signal to pass through a system and the time it
takes for the luminance portion to pass through. The amount of
distortion is expressed in units of time, typically nanoseconds.
The number is positive for delayed chrominance and negative
for advanced chrominance.
Figure 21. A combination signal which includes a 201
modulated pulse (CCIR Line 17).
• PICTURE EFFECTS
Gain errors most commonly appear as attenuation or peaking
of the chrominance information, which shows up in the picture
as incorrect colour saturation.
Delay distortion will cause colour smearing or bleeding, particularly
at the edges of objects in the picture. It may also cause poor
reproduction of sharp luminance transitions.
H TEST SIGNALS
Chrominance-to-luminance gain and delay inequalities are
measured with a 10T or 20T modulated sine-squared pulse,
sometimes called a composite pulse. Many combination ITS
signals include such a pulse.
Figure 22. The chrominance and luminance components
of a modulated sine-squared pulse.
The frequency spectrum of a composite pulse includes energy
at low frequencies and energy centred around the subcarrier
frequency. Selection of an appropriate pulse width is a tradeoff
between occupying the PAL chrominance bandwidth as fully as
possible, and obtaining a pulse with sufficient sensitivity to
delay errors. (The 10T pulse is more sensitive to delay errors
than the 20T pulse, but does not occupy as much of the
chrominance bandwidth.) CCIR specifications generally
recommend the use of 20T pulses, while 10T pulses are
commonly used in the U.K.
A modulated bar is also sometimes used to measure
chrominance-to-luminance gain inequalities.
• MEASUREMENT METHODS
Conventional chrominance-to-luminance gain and delay measurements
are based on analysis of the baseline of a modulated
sine-squared pulse. (See Appendix B for a definition of the
time interval T.) This pulse is made up of a sine-squared
luminance pulse and a chrominance packet with a sinesquared
envelope, as shown in Figure 22.
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