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From: karn@petrus.UUCP (Phil R. Karn)
Newsgroups: net.video,net.audio
Subject: Digital audio adapters on VCRs
Message-ID: <697@petrus.UUCP>
Date: Thu, 21-Nov-85 20:16:26 EST
Article-I.D.: petrus.697
Posted: Thu Nov 21 20:16:26 1985
Date-Received: Sat, 23-Nov-85 04:28:44 EST
Distribution: net
Organization: Bell Communications Research, Inc
Lines: 37
Xref: watmath net.video:1680 net.audio:6588

Recently I got the chance to play with a borrowed Sony PCM-F1 digital audio
processor box. This unit consists of two halves. One part digitizes two
audio channels and produces a "video" signal that can be recorded on a VCR.
The other half accepts an encoded video signal and reproduces the original
audio signals.  The sampling rate is 44.056 Khz, 14 bits/sample, and approx
34% redundancy is added in the form of error correcting codes.

When the PCM-F1 was configured in loopback mode (video output feeding video
input) I was completely unable to tell the difference between the PCM
processed signal and the raw input signal. However, there was significant,
audible degradation of the signal if it was recorded and played back on my
VHS VCR (an RCA VKT-385), even at the 2-hour rate.  A look at the video
signal on a scope revealed that while the video signal-to-noise ratio was
quite reasonable, there was considerable intersymbol interference that
caused considerable timing jitter and was almost certainly the cause of the
high bit error rate and audible degradation.  Each bit transition overshot
by almost 50% of the peak-to-peak signal amplitude, and from the asymmetry
of the ringing there was obviously considerable phase distortion.

I'm wondering if this sort of crummy response is inherent in home VCRs, or
if it is an intentional "feature" added to make the picture look
subjectively better. I know that high frequency "peaking" is often added to
video equipment to provide "edge enhancement" and wonder if this is what's
going on in my VCR. If this is so, there unfortunately seems to be no way to
turn it off.

One further observation. If I calibrated the record levels on the PCM-F1 so
that a 0-db signal on my CD player resulted in just saturating the PCM-F1's
A/D converters, then the background noise on even my quietest digitally
recorded classical CDs was still sufficient to randomly toggle the lower 3
or 4 bits of the PCM-F1's A/D output (you can see this by watching the
encoded video signal on a TV).  This tells me that the dynamic range of
current CDs is limited by external factors like microphone and preamp hiss,
room noise and AC hum, and that even 14 bit quantization is sufficient to
record these signals with no loss in dynamic range.

Phil