Updated 1/12/02
Hints, Aural Carrier Level
This is another case in which the test should be conducted on all channels at the tap and at the output of a converter. It is also required in the headend. As with the visual signal level test, we run the test on all channels at the tap and, if a baseband converter is being used, only on the test channels at the output of the converter — with the appropriate ("the level doesn't change") comment in the report. For non-baseband converters, we run the test on all channels at the tap and at the converter.
Its not unusual for the frequency response of non-baseband converters to roll off the aural carrier a dB or two. This can cause the aural carrier level for some channels (those that were marginal at the headend or tap) to fail to meet the requirement.
This test can also cause problems for scrambled channels that use sync suppression. To adjust the aural carrier
level, many systems turn the scrambler off, then adjust the aural level for, maybe -16 dBc. When the scrambler
is turned back on, data pulses that amplitude modulate the aural carrier may cause the (now peak) of the aural
carrier to be greater than -10 dBc. Reducing the aural carrier level may cause problems for some set-top converters.
One thought on this is that, as long as the amplitude of the aural carrier isn't high enough to cause problems
on other channels, its probably not a significant issue.
If the aural level was OK in the headend but doesn't meet the requirements at the tap in the field, a plot of the visual carrier levels (typically done for the 24 hr test results) may show sharp peaks and valleys in the system response that is causing the aural carrier level problem. For example, the plot below shows a "suck-out" at approximately 360 MHz. The aural carrier levels of the channels near the "suck-out" are likely impacted.