Posted by Smarty on 12/05/06 15:39

The technically correct way to set the deviation is with a spectrum analyzer
centered on the audio subcarrier of the modulator's output. The audio
subcarrier (for NTSC) is perched 4.5 MHz above the video carrier, and is
nominally modulated with plus and minus 25 KHz deviation for 100%
modulation. This will produce spectral components out to about 150 to 200
KHz (Bessel functions at work!). The FCC spec allows 200 KHz max I believe,
but most cable operators use a bit less to avoid adjacent channel splash.

Absent a spectrum analyzer, you could slowly increase the modulation index /
audio gain control until you begin to see / hear interference on the next
higher channel. You will also find that a point will be reached where
perceived audio output level does not increase proportionate to your gain
adjustment, and audio distortion begins to rise. This is the point where
your receiver's FM discriminator / demodulator is swinging fully through
its' maximum bandwidth. Assuming your receiver has been designed correctly
(and most TVs work pretty well in this regard), you can assume that you have
set the modulator's audio setting to about the correct value.

The latter method is not likely to be really accurate, and the resulting
setting would fail an FCC inspection if you were doing it on a commercial
broadcast transmitter, but it is probably sufficient for a modulator setting
where public broadcasting is not involved.

Smarty


"Richard Crowley" <rcrowley@xpr7t.net> wrote in message
news:12nb35eclj5pa7b@corp.supernews.com...
> "Rick Merrill" wrote ...
>>I have a color bars and audio tone program
>> that cycles through several frequencies - how do I use that
>> to set the audio deviation (etc.) on the modulator that
>> goes to the cable TV headend?
>
> Are you saying that you get different deviation at different
> frequencies? That would be an indication of a frequency-
> response that was not flat (i.e. broken). Assuming that your test source
> is "flat".
>
> As for the actual deviation level, it would depend on the
> level of your test source, and the practice of that particular
> cable operator, I would think. Their head-end technician
> should know what the standard is, whether documented or
> in his head.

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