User Manual - TRS-RenTelco

Option SD/HD Detailed Measurement Information
Multiburst
The Multiburst measurement is used to verify frequencies and corresponding
amplitudes of sinusoidal bursts. It can be used to verify the proper reconstruction
of a band-limited video signal across the frequency spectrum. For example, a
frequency roll-off (decrease in amplitude with increasing frequency) is seen with
video from an analog-to-digital converter that lacks a reconstruction filter with
sin(x)/x correction. In this example, quality is reduced by things like loss of
detail in the video or color shifts in details.
Deviations in frequency from the reference test signal frequency (for example, a
30 MHz burst is rendered as a 20 MHz burst) may indicate things such as a
frequency shift due to format change, frequency aliasing due to digital and/or
analog re-sampling, and/or frequency aliasing due to the lack of a proper
reconstruction filter.
The flag amplitude measurement can be used to verify the channel gain across a
wider frequency range than each burst. Burst amplitudes are measured relative to
the flag amplitude.
Signal Source
You can use the matrix signal, or you can use any multiburst signal with a
reference flag preceding the bursts. The more cycles present in each burst, the
more robust and generally more accurate the measurement will be. A burst with
less than 2 cycles is not recommended.
The frequencies are always displayed from lowest to highest regardless of the
order of burst frequencies in the multiburst signal. To avoid confusion regarding
the correspondence between the results burst numbering and the signal burst
ordering, a signal with increasing burst frequencies (from left to right) is
recommended.
Interpreting Results
For multiburst signals free of non-linear distortion and with increasing burst
frequencies, the ‘Frq(MHz)’ column displays the fundamental frequency of burst
specified in each row. The “Amp(dB)” column has the magnitude of the
corresponding burst.
However, if non-linear distortion is present, interpreting results may be less
straight forward.
In many consumer set-top boxes, lack of a reconstruction filter (with out-of-band
rejection plus sin(x)/x correction) leads to roll-off in the fundamental frequency
component of the bursts even though the envelope of the bursts may be flat on
average. In this case, the amplitude results for the highest frequency may be low
by a few dB even though the waveform shows the same burst with peak-to-peak
amplitude closer to 0 dB. This indicates that the burst is no longer a pure
windowed sinusoid, but has other spectral content (at least one alias frequency in
VM5000 Automatic Video Measurement Set User Manual 3-33