5121cdd@houxm.UUCP (C.DORY) (10/10/85)
Aha! Someone has made the plunge into digital recording -- I hope with
noble intentions (i.e., live recording).
As I am not sure which PCM processor you are speaking of, I can't make
specific comments; however, in general they are very similar.
First, a short tutorial for those unfamiliar with PCM processors: There
are two "consumer" digital formats accepted by the EIAJ (the Japanese
"international" body). These are 14-bit linear PCM and 16-bit linear PCM.
At present there are two sampling rates commonly used by manufacturers:
44.056 KHz and 44.1 KHz. The Sony PCM F1, for instance, uses the slower
of the two while the new PCM 501 uses the faster rate. (By the way,
a tape made at one rate can be played back at the other. There is an
audible change in pitch and timbre, however.) The digital processors
currently available for the consumer "semi-pro" market utilize VCRs as
storage devices. In other words, the digital processor takes as input
mic-level or line-level analog audio, converts this to an interleaved digital
bit stream formatting this bit stream as a video signal and interfacing
directly to a VCR for storage.
There are numerous problems with using video as a storage medium (as well
as a few advantages). The problems begin with dropouts. Dropouts on the
video tape can kill you in playback. They can occur from a tiny spec of
junk between the video head and tape (i.e., poor bonding etc.) or from
wear on the tape, etc. The slower the recording speed the more susceptible
you are to dropouts.
A few general-practice hints:
- Always use Hi-Grade tape by a well-known manufacturer
or, even better, a Pro or industrial-grade tape. These
tapes are tested for low dropout counts and the bonding
is beefed up for repeated plays.
- Before recording, completely fast-forward and rewind the
tape. This tends to knock any loose junk from the tape,
exercise the tape, and remove any electrostatic charge that
has built up in storage. (This goes for playback as well.)
- Record at the fastest speed your VCR will operate. For Beta,
this is the BI or BII speeds; for VHS use SP. Tape is cheap,
one-time live recordings are not.
- Use a back-up machine (i.e., use two VCRs paralleled from the
digital processor output). Be careful that the video line is
properly terminated (most video sources like to drive 75-Ohm
loads, not 37.5 Ohm loads).
For those interested, there is an article in the latest Recording Engineer /
Producer Magazine on how the digital information is stored on video tape.
This article also explains how to "watch" this information on a video monitor
to spot trouble. This is required reading for anyone doing any serious
recording.
Craig Dory
AT&T Bell Laboratories