PAL SYSTEM TELEVISION MEASUREMENT : GROUP DELAY

Group Delay

DEFINITION
Group delay distortion is present when some frequency components of a signal are delayed more than others. Distortion is expressed in units of time. The difference in delay between a reference low frequency and the highest frequency tested is typically quoted as the group delay error.
PICTURE EFFECTS
Group delay problems can cause a lack of vertical line sharpness due to luminance pulse ringing, overshoot, or undershoot.
TEST SIGNAL
The multipulse test signal, described in the Frequency Response section, is used to measure group delay. It is also possible to obtain a group delay measurement from the (sin x)/x signal, but only with an automatic
measurement set such as the VM700T.

Figure 50. The multipulse test signal.

MEASUREMENT METHODS
Group delay is measured by analyzing the baseline distortion of the modulated sine-squared pulses in the multipulse signal. As discussed earlier, delay errors between the low frequency and high frequency components of the pulse appear as sinusoidal distortion of the baseline (see Figure 51). The measurement
methods for group delay are very similar to those used for chrominance-to-luminance delay differing only in the number of frequencies at which delay is measured.

Figure 51. The multipulse signal exhibiting group delay distortion. Group delay differences between the high frequency and low frequency components of the pulse appear as sinusoidal distortion of the baseline.

Waveform Monitor and Nomograph.
When making group delay measurements with the multipulse signal, the baseline distortion of each pulse must be individually measured and applied to a nomograph. Normalize each pulse height to 100% and measure
the positive and negative peaks of the baseline distortion.
Voltage cursors in the RELATIVE mode can also be used for these measurements. Apply the numbers to the nomograph (in the Chrominance-to-Luminance Gain and Delay section of this booklet) to obtain the delay number. The largest delay measured in this way is typically quoted as the group delay error.
The first pulse in a multipulse signal is generally a 20T pulse and the others 10T pulses. The nomograph works for any modulated 20T pulse regardless of the modulation frequency. For a 10T pulse, however, the delay number from the nomograph must be divided by two. In practice, it is often easy to see which of the
pulses exhibits the most delay necessitating only one measurement when maximum delay is the value of interest.
1781R Semi-Automatic Procedure.
Group delay can be measured with the CHROMA/LUMA selection in the 1781R MEASURE menu. Repeat the measurement procedure for each frequency of interest.


Automatic Measurement — (Sin x)/x.
The VM700T uses the (sin x)/x signal to make group delay measurements.
This method offers the advantage of providing delay information for a large number of frequencies rather than just at the six discrete frequencies of the multipulse signal. Select GROUP DELAY (SIN X)/X in the
VM700T MEASURE mode (see Figure 52).

Figure 52. The VM700T Group Delay & Gain measurement made with the (sin x)/x signal.

NOTES
15. Group Delay Definition.
In mathematical terms, group delay is defined as the derivative of phase with respect to frequency
dØ/dω. In a distortion free system, the phase versus frequency response is a linear slope and the derivative is therefore a constant (see Figure 53).
If the phase versus frequency response is not linear, then the derivative is not a constant and group delay distortion is present.
The largest difference in dØ/dω that occurs over the frequency interval of interest is the amount of group delay (see Figure 54).
16. Envelope Delay.
The term “envelope delay” is often used interchangeably with group delay in television applications.
Strictly speaking, envelope delay is measured by passing an amplitude modulated signal through the system and observing the modulation envelope.
Group delay, on the other hand, is measured directly by observ i n g phase shift in the signal itself.
Since the two methods yield very nearly the same results in practice, it is safe to assume the two terms are synonymous.

Figure 53. Response of a distortion free system.

Figure 54. Response of a system with amplitude and phase distortion.

Lihat juga

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