From: JROSSI@EAGLE.AAMRL.WPAFB.AF.MIL
Newsgroups: rec.music.synth
Subject: Re: EM reviews the Alesis ADAT... and loves it
Date: 30 Sep 92 15:21:00 GMT


On a multi-track digital recorder the tracks are independent of eachother
only in the context that there is no crosstalk between them.  That is to say,
if you record something on track-1, a ghost image of it won't appear on
track-2.  It seems clear, by a lot of people's attitudes concerning the
smaller size of the 8mm format, that there is considerable misconception as
to how digital multitracks actually record information.  While it is apparently
open to debate whether there is a substantial dropout rate difference between
S-VHS and Hi-8mm (espec on evaporated metal tape), the fact remains that the
two formats are approximately equal in regard to signal bandwidth.  What this
means is that both formats have approximately the same capability of storing
PCM encoded digital data.  The most important thing to consider in the digital
world is that there are no physically discreet tracks on the tape.  Basically,
(this concerns helical scan technologies, I have no idea how stationary head
digital decks work), what you have is a very long stream of pulse code
modulated digital data which is serial in nature.  The necessary information
concerning the 'tracks' is imbeded in the stream.  I am not sure of the exact
format, but it probably is a sequential scan system where a PCM sample from
track 1 enters the stream, followed by the same time sample form track 2, etc
until you get to 8 (which seems to be the number of tracks supported by the
helical scan formats).  Obviously, since these things are capable of working
at 48KHz, there is a lot of PCM data going to the tape.  Now, assuming there
is some kind of provision for error correction, and other overhead to take care
of indexing, etc., there is quite a data stream going to the tape.  So, as
long as the digital stream remains intact (or correctable), there is no
chance of crosstalk, or inter-track modulation.  Also, since amplitude is
encoded as a digital numeric value, there is no possibility of tape saturation
effects.  So, in regard to track to track anomalies, there should be no
theoretical difference between S-VHS and Hi-8mm.  I think that the whole
argument will eventually come down to the relative contribution of dropouts
(which is a critical problem for digital serial media), and the actual
physical wear a tape can take.  As I said above, the dropout question remains
to be resolved (in terms of error correction, etc), as for the physical wear
on the tape, ther is no question thart a VHS mechanism mauls the tape just
by the way it loads it onto the heads.  The 8mm mechanism is a lot more
tape-friendly.  The shuttling mechanism in the two formats is equally
different and 8mm, again, is a lot easier on the tape.  Finally, VHS tape
is a lot more open to damage when it isn't in the machine.  Go out and shale
a VHS tape from side to side and watch the tape bang around on the loosely
fitted hubs.  8mm, which is more like a miniature Beta tape than anything
else, works in a much more precise mechanism.  Anyway, when Tascam finally
gets the 8mm recorder out and people have some time to compare the two formats
over more than a couple of days, we will find out which format is
superior.  Personally, I am waiting for the Tascam deck because I hate the
VHS format.  I mean, Sony was the real inventor of VHS (I think, this might
just be some rumor I picked up a long time ago) and sold the rites to JVC
so they could develop it.  As the story goes, Sony sold VHS to JVC because they
thought it would never be able to sucdessfully compete with Beta.  So, call
me near sighted and prejudiced but I hate VHS (if for nothing other than
the shell and mechanism are so ugly.  If nothing else 8mm (i.e., son of Beta)
at least has elegance going for it.

John
Newsgroups: rec.music.synth