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From: rjr@mgweed.UUCP
Newsgroups: net.audio
Subject: FM stations and limiting
Message-ID: <6306@mgweed.UUCP>
Date: Tue, 17-Jan-84 11:19:07 EST
Article-I.D.: mgweed.6306
Posted: Tue Jan 17 11:19:07 1984
Date-Received: Wed, 18-Jan-84 07:22:06 EST
Organization: AT&T Technologies - Montgomery Illinois
Lines: 39


I agree with Dave Sewhuk's comments on FM stations.
As far as I know, peak limiters are still required in all radio stations,
AM or FM (and TV audio too). Theses devices prevent audio peaks from
exceeding the 100% level (in FM this equates to 75kHz deviation).
The peak limiter is an AGC amplifier with fast attack and release times
as opposed to compression amps that have a slow decay time (1 second or so).

It's been a while since I worked in broadcast but it seems to me there was
a requirement (at least for AM stations) that the average percentage of
modulation had to be above 75 or 80 percent. That is impossible to do
without some kind of processing.

I don't know what WFMT in Chicago does except perhaps to keep their level
down low enough so the peaks seldom reach the limiting point. At times
their level is awfully low, but they certainly have dynamic range.

Someone on the net said the received quality would depend how far one
was from a station. Distance makes no difference if the signal is
full quieting and there is minimal multipath. I am 40 miles from the
Chicago stations and I use a 12 element yagi with foam coax to my tuner
to get the best signal strength.

Dave, I think FM stations can change over from the standard 75us pre-
emphasis when sending Dolby type source audio. 

The pre-emphasis at the transmitter "gooses up" the highs.
The receiver has the opposite curve (75us de-emphasis) so the overall
response is flat. This improves the signal to noise ratio. The same type
of thing is even done with FM 2-way radios (20db from 300 to 3000 Hz).
Tapes and discs use different types of equalizations (NAB and RIAA to 
mention a couple). I don't consider these "treatments" of the response
curves as being "processing" as such.
They are used to correct an undesirable condition that would cause
overload in the bass region that would occur if the recording were
flat.

Bob Roehrig