MEASUREMENTS

Group Delay
Figure 51. The Multipulse test signal.
Figure 52. Multipulse 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.
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• DEFINITION
Group delay distortion is present when signal components of
different frequencies experience different delays as they pass
through a system. Distortions are 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, which is described on page 37, 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 VM700A.
• MEASUREMENT METHODS
Group delay is measured by analysing the baseline distortion
of the modulated sine-squared pulses in the Multipulse signal.
As discussed on page 26, delay errors between the lowfrequency
and high-frequency components of the pulse appear
as sinusoidal distortion of the baseline. See Figure 52. The
measurement methods for group delay are very similar to those
used for chrominance-to-luminance delay, but in this case it is
necessary to examine the delay at a number of different
frequencies within the video passband.
Waveform Monitor & Nomograph
The baseline distortion of each pulse must be individually
measured and applied to a nomograph when group delay
measurements are made with Multipulse. Normalise each
pulse height to 100%, and measure the positive & 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 on page 27 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, and the
others are 10T pulses. The nomograph works for any modulated
20T pulse, regardless of the modulation frequency. For 10T
pulses, however, you will need to divide the delay number
from the nomograph by two. In practice, it is often easy to see
which of the pulses exhibits the most delay. If you are just
interested in the largest delay error, measure only that pulse.
1781R Semi-Auto ma tic Procedure
Group delay can be measured with the CHROMA/LUMA
selection in the 1Z81R MEASURE menu. Go through the
measurement procedure for the pulse which exhibits the most
delay, or for each pulse if you wish.