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From: newton2@ucbtopaz.CC.Berkeley.ARPA
Newsgroups: net.audio,net.video
Subject: Re: RE: Re: Using Hi-Fi VCRs for Audio Recordings
Message-ID: <588@ucbtopaz.CC.Berkeley.ARPA>
Date: Sun, 28-Oct-84 02:33:34 EST
Article-I.D.: ucbtopaz.588
Posted: Sun Oct 28 02:33:34 1984
Date-Received: Mon, 29-Oct-84 02:39:26 EST
References: <696@ihuxa.UUCP>, <303@tymix.UUCP>, <535@watdcsu.UUCP>, <443@hou2a.UUCP>
Organization: Univ. of Calif., Berkeley CA USA
Lines: 37

These remarks relate to the seeming inconsistency (in Beta/VHS hi-fi
specs) between "45 dB s/n" and "80 dB dynamic range"

	I see two possibilities (omitting degenerate cases like misprint in 
ads, garbled reporting etc.):

	"45 dB s/n" refers to the carrier-to-noise ratio of the FM carrier
which is used for each audio channel. This seems to me the least likely
explanation, both because it's an arcane spec and meaningless as a predictor
of audio performance without knowing the modulation index, and also because
I doubt the carrier-to-noise ratio really *is* as good as 45 dB.

	Therefore, I incline toward this explanation: 45 dB is the *true*
signal-to-noise ratio *with signal present*, i.e. the instantaneous SNR.
Since the noise in any system has some signal-related component (e.g.,
modulation noise in direct analog tape recording), idle-channel noise
is (almost) always less than noise with signal present. The real kicker,
however, is that Beta/VHS are COMPANDED systems, so the noise in the absence
of signal can be 30dB or more below the noise with signal present. Decent
companders make intelligent use of spectral shaping, pre-emphasis, sliding band
filters or various combinations of the well-known techniques expounded by
R. M. Dolby ("An Audio Noise Reduction System", JAES 1967). "Naive" broadband
companders often suffer quite audibly from the different noise levels in
the presence/absence of signal- these "noise modulation" effects have
been discussed extensively here (everywhere).

	The only reason I still have nagging doubt about this being the
true explanation of the VCR spec discrepancy is that I have *never*
seen an ad that correctly and honestly explained this dirty little secret
of broadband companders (except an informational piece put out by Dolby
Labs to explain why dbx's claims were fraudulent (or at least grossly
misleading)).

	Regards,

		Doug Maisel
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