HP 8591C User Manual page 278

Selecting the MORE INFO softkey displays the low frequency spectrum
around the visual carrier. The marker can be used to identify the frequency
and level of interference relative to the carrier. Obtaining meaningful results
even in the presence of video modulation is a great benefit. No one needs to
be present at the headend to temporarily insert a carrier wave signal, and
subscribers are not deprived of service. On a properly operating system,
results with video modulation present will usually be under the 3 percent
limit. Low frequency disturbances on channels above the limit are often a
combination of video Eeld time distortion on the received signal and cable
system hum. The dominant contributor may be identified by removing the
signal input to the headend, inserting a clean carrier wave signal on that
input, and retesting.
More than 3 percent Eeld time distortion on off-air signals has been observed,
but it also has been seen to be caused by multi-path interference between the
transmitter and the headend antenna. Field time distortion on the signal can
also be qualitatively separated from cable hum by observing whether the
distortions vary with time or picture level since cable hum won't change with
picture level.
Test Descriptions
Hum Test Description
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