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From: karn@allegra.UUCP
Newsgroups: net.audio
Subject: Re: DIGITAL TUNING
Message-ID: <2233@allegra.UUCP>
Date: Mon, 23-Jan-84 19:37:52 EST
Article-I.D.: allegra.2233
Posted: Mon Jan 23 19:37:52 1984
Date-Received: Fri, 27-Jan-84 06:33:31 EST
References: <917@druxy.UUCP>
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 37

A few comments.

FCC rules (at least as of the time I was active in FM broadcasting, which
was 8 years ago) require an FM carrier frequency accuracy of +/- 2khz.  (By
comparison, TV is +/- 1 khz, even for UHF, and AM radio is +/- 20 hertz.)
American FM channel assignments are ALL on odd 200 khz spacings.

Maximum deviation is +/- 75 khz. This is fairly broad compared to most
tuning errors, except that tight IF filters may cause distortion with
even small amounts of mistuning, given the way that stations like to
drive right up to the legal deviation limits.

Digital tuning allows the use of crystal-referenced synthesizers, which
are much more stable than conventional "VFOs" (continuous free-running
oscillators).  The state of the art now in synthesizers is to hit
accuracies of a few hundred hertz right from power on.  In fact, it ought
to be cheaper to build synthesized tuners than "analog" types because
the latter require more mechanical complexity and precision temperature
compensated components.  Even with good design, continuous tuning receivers
such as those common on the HF bands are less stable even though they operate
at lower frequencies.

The only things which would keep me from buying a digital tuner are:

1. The possibility that the FCC would start assigning "split" frequency
assignments on even 200 khz steps.

2. Direct connection to cable TV systems which may use oddball,
imprecise channel assignments.

3. Listening to FM "pirates" just outside the normal channels.

4. Listening from low earth orbit where doppler shift is a problem :-)

Other than that, I wouldn't worry about the choice too much.

Phil