[BC] Is overprocessing illegal?

[Jim Tonne]

Gary:

Note that they don't say you need any filtering at all.
The requirement is that you not interfere with the pilot
tone region.  How you manage that is up to you.

If you do even light clipping of the audio, a lowpass
filter is by far the best way to go (as opposed to a
notch filter).  This is because harmonics of the main
channel stuff will fall into the stereo subchannel.  They
will then demodulate in a strange fashion (due to aliasing)
and not sound good at all.

Do this test to see what I am talking about.  Connect an
audio oscillator to your stereo generator input.  Sweep
the audio tone upward, starting at say 1000 Hz, up to
20 kHz or beyond.  Up to 40 kHz if the oscillator will go
that far.  The "beat notes" you hear are a result of not having
a lowpass filter in the audio channel.  Especially near 38 kHz
you will hear the beat notes big time.  A 15 kHz lowpass
filter would prohibit such nonsense.

I would never consider a notch filter in a stereo generator
that I would design.  I always used lowpass filters.  Even
a most basic, simplistic, design using only two inductors
can have a notch at 19 kHz to clearly protect the pilot tone
region, along with some suppression of trash in the stereo
subchannel region.

I think the Canadians did, however, require a notch filter
or perhaps even a lowpass but I am going to leave the
sure answer to that to someone else.

- JimT



----- Original Message ----- 
From: "Kevin Tekel" <amstereoexp at yahoo.com>
To: <broadcast at radiolists.net>
Sent: Sunday, December 24, 2006 10:06 PM
Subject: [BC] Is overprocessing illegal?


>I found this in the FCC rules....
>
>
> Sec. 73.1570  Modulation levels: AM, FM, TV and Class A TV aural.
>
> (c) If a limiting or compression amplifier is employed to maintain
> modulation levels, precaution must be taken so as not to substantially
> alter the dynamic characteristics of programs.
>
>
> What I was trying to find is some definition of the allowed audio
> bandwidth for monaural FM stations... the rules constantly refer to "50 to
> 15,000 Hz" but the only definite regulation regarding FM audio bandwidth
> seems to be that "an FM broadcast station shall not use 19 kHz +/-20 Hz,
> except as the stereophonic pilot frequency".  So is it correct that a mono
> FM station only needs a 19 kHz notch filter in its audio to avoid
> triggering the MPX pilot decoder in receivers, and not the traditional 15
> kHz brickwall filter?
>
>
>
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