From: gutierre@noc.arc.nasa.gov (Robert Michael Gutierrez)
Subject: Re: 8MM Digital Audio
Originator: tjk@csd4.csd.uwm.edu
Organization: NASA Science Internet Network Operations
Date: Mon, 14 Sep 1992 12:59:04 GMT
  (5.65c/IDA-1.4.4 for <rec-audio-high-end@ames.arc.nasa.gov>); Sat, 12 Sep 1992 20: 06:26 -0700
  (5.65c/IDA-1.4.4 for rec-audio-high-end@ames.arc.nasa.gov); Sat, 12 Sep 1992 20: 06:25 -0700

aj008@cleveland.Freenet.Edu (Aaron M. Barnes) writes back in Mon, 31 Aug 1992:

|> I would like to here some of your experiances with using the 8mm
|>  videotape digital format.
|> First,what is the word size for this format?I know the sampling 
|> rate is 32kHz,but is the word rate 14 bit or 16 bit?
|> Also,is the dynamic range as great as the CD?How about S/N?

8mm PCM mode is 32khz, 10 bit non-linear (compressed from 14 bit if
I remember correctly).  This is even worse from the standard DAT LP
mode (32khz, 12 bit non-linear).  Rated f-r is supposedly 20hz-14khz,
but I've heard tests of 30hz+ to 12khz+ (the rating of MTS TV stereo!).

|> The most attractive thing to me is the 24 hours audio only mode 
|> on the Sony machines.

Yes, you have a 24 hour mode, but it is in 4 hour increments.  That is,
it divides up the tape into 6 "channels" that you select from the front
panel.  The tape then runs for the maximum length (4 hours in the LP
mode), then you have to rewind it, select the next channel, then play
again.  Yes, 4 x 6 = 24, but not continiously.

|> Do the sony machines have digital inputs and outputs like DAT's 
|> do?

Nope.  No SPDIF at all.

|> Is the overall quality of reproduction using the 8MM digital for
|> mat higher then the VHS Hi-Fi?

This is a loaded question.  Yes, the frequency response is much lower
and the S/N is a bit less, but this is *flat* response because in VHS
HiFi, the audio signal is companded.  Attached is a description of using
a Sony PCM unit to record digital audio on VHS (or Beta, if you prefer)
if the moderator would permit me to use.

|> All this came up since I have an oppertunity to buy one used for
|> $475.

I've seen ones for less.  Like $300/less.  $475 is on the high side.
This is for the EVO-800, I assume? (non-Hi8 model).

|> Might anyone know where to get a new Sony 8mm with the 24 hour au
|> dio mode at a more attractive price?

The old Good Guys Clearance Center in San Jose (California) had them
for $450 shelf price, and less if you bargained.

|> Just curious,since I am using VHS Hi-Fi as my main recording for
|> mat,and my machine lacks input level controls,so I am an expert 
|> on pumping,of course.

Good.  Take a look at the following I wrote for rec.video and rec.audio.pro.
A strong argument for using some old technology that's been around for 10
years now.


   rob.


-------------------

>From: gutierre@noc2.arc.nasa.gov (Robert Michael Gutierrez)
>Subject: Re: VHS/SVHR Hi-Fi VCR recommendations needed!  HELP!
>Sender: usenet@news.arc.nasa.gov
>Organization: NASA Science Internet Network Operations
>Date: Fri, 11 Sep 1992 06:53:16 GMT

davisonj@en.ecn.purdue.edu (John M Davison) writes:

|>         Could someone direct me to a more appropriate VCR for someone in my
|> price range?  (Mitsubishi puts out a model around the price of the SLV-696HF,
|> but with input level controls.  Is it any good?)  Also, I am curious about the
|> following issues:

I can try since I do time shifting via VCR, but I use a different system...

|> 1. Would it be worthwhile to use a broken cassette deck as an input volume
|>    control for something like the SLV-696HF, or should I demand a VCR with
|>    input level controls?

This VCR has ALC, so adjusting levels before the VCR won't make any difference.
The ALC is a bit dampened in attack/delay (as opposed to the standard linear
track which always reminds me of a CBS Labs Volumax at maximum settings), but
the ALC will eventually creep up and compensate for your lower levels.

|> 2. Is better audio fidelity among the advantages of SVHS Hi-Fi (i.e. does it
|>    sound any better than VHS Hi-Fi)?

Nope.  No difference.  It's because of the setup of the companding system.

|> 3. All visual considerations aside, is it possible to do any sort of sonic
|>    editing wih VHS or SVHS?  Would a flying erase head do me any good?

Only with an editing system.  With say 2 Sony SLV-R5's and a good controller
(like a Future Video Pro Mk-II) you'd be within +- 2-3 video frames (~1/10 to
1/15 second accuracy).  This is expensive enough (~$2200.  With 2 Panasonic 
AG-7760's (with the time code option) and a Panasonic controller, you'd still
be looking at exactly 1/30th second accuracy.  This is a stratopherically
expensive system ($10k+).  Anyway, that comparison is overkill at this point...

|> 4. What is a good tape for audio mastering purposes?

With HiFi, any tape will do that has a good bonding system (ie: amount of
bonding vs. amount of oxide).  For sure, the industrial tapes (Ampex 199,
Sony BA-120 or 3M/Scotch Color Plus) would be the start.  After that, I
would not go below HiFi consumer grades (Maxell HGX-HiFi, TDK HiFi, etc).
What you are looking for in a tape is the fewest dropouts possible on
the tape.  A "dropout" is where the oxide either wears off the tape backing
because of normal playing over the video heads, or just literally flakes
off because of a weak bonding system.  Some of the so-called high output
tapes get higher video frequency response by packing in more oxide at the
risk of the binding system not being able to retain against the backing
over time.  Literally, it can vary also on the "batch" of tape made (batches
made on large rolls not unlike newsprint, and the tape cut into 1/2 inch
strips as it is rolled off and packaged).

|> 5. Am I correct in thinking that audio input level controls are largely the
|>    domain of SVHS machines?  If so, why?  (Do rich people really prefer more
|>    knobs?)

No, the loss of manual level controls come from "idiotizing" the machines
themselves.  All of the HiFi machines at the start of their manufacture were
equipped with *only* level controls, no ALC at all!  With the advent of
people who complained where they were supposed to keep the levels at, the
manufacturers did the only thing to stop the complaints...they got rid of
the controls except for the highest end machines.  And even with those,
they have ALC switches, just in case (like my Panasonic PV-4990 S-VHS).

There's always Beta!  They all have level controls...

|>         Any and all comments would be appreciated, especially if they were
|> mailed to me in addition to being posted.  Thank you very much!

Well, the current system I use is the old PCM system, which is ironically
used now to sumbit CD masters to major manufacturers.  The PCM system is
where white and black values on the video portion are used to represent
digital bits, and those values are converted to digital audio, and then
processed as such.  Depending on the PCM unit you use, it uses a standard
sampling rate and frequency (44.1 khz, 16 bit ... standard CD sampling).
Newer PCM units have SPDIF (digital) inputs and outputs which uses exactly
the same digital stream like any normal CD player with a digital output.
Older PCM units do not have digital in/outputs, and sometimes use a different
sampling rate (44.056 khz, 14 or 16 bit).  I have one of those non-standard
units (a Sony PCM-F1), but the "tapes" made on it can be played back on
any of the newer players (though digital copying from 44.056 khz to 44.1 khz
may be impossible without an expensive sampling rate converter).

Using PCM units affords the same uses as VHS tape, like 9 hour record/play
(with a T-180) and using an existing available machine (my VCR), and it
avoids the problems with VHS HiFi tracks.  What problems...???

Well, VHS HiFi is a companded system.  It uses 2:1 companding because
the dynamic range of VHS HiFi without any noise reduction is 50-55 db!
They had the frequency range, but because of the apparent high speed
of the tape and to hide the head switching noise of the HiFi heads, they
had to clean it up somehow.  Companding was the only way to do it.  Problem
is, it's a lousy system because the tracking of the videotape is more
critical (due to the "depth multiplex" system of recording the audio
to begin with, ie: recording the audio at a different head azmuth so
as to not interfere [yeah, right!] with the video portion of the
tape), and you have that awful "pumping" of typically cheap and cheezey
companding systems.  And that head switching noise always in the background!
(sounds like slow buzzing).  Oh yeah, copies of HiFi tapes just compound
the problem with expanding/recompanding and more head switching noise...

(P.S...I don't put up with my HiFi system at all.  The HiFi tracks are
used for [Left channel] SMPTE time code and [Right channel] WWV time clock,
the video portion of course containing the PCM video).

Just Say NO to HiFi!

Anyway, hope this helps...


   rob.


p.s..some more short notes, just in case you're interested:

Experimenting with my S-VHS unit and my ED Beta unit (yes, I have LOTS of
toys!), there was some interesting things I found.

Seems I had some problems recording in S-VHS for the PCM unit.  The sound
had over-emphasized highs (making the sound very, very "bright"), but without
any error correction indication, I could not figure out whay this was
happening.  I had concluded at the time that it was just white peaks extending
beyond the original spec for the F1, and the video input was internally
pre-emphasized for the low bandwidth of regular VHS (or the originally
intended Beta in the case of the F1).  This theory was promptly shot
down when I tried a test recording using my ED Beta machine in ED Beta
mode (ED Beta tape and drop out compensator disabled via the "PCM" switch
on the ED Beta).  Theories now run rampant from problems with the color
signal not disabling on the S-VHS (being the conversion from 3.58 mhz to
688 khz not stable at all!) to Panasonic playing tricks in doing their
own bandwidth pre-emphasis to make the picture look better, to god-knows-
what-else...

Also, Beta is very well known for droputs and skew (tape strech) on their 
tapes due to the "U" loading system (full tape wrap loading), and so
that makes me squeamish in using anything PCM related on any Beta tape,
and always using 14 bit mode for the extra CRC error correction.  Bleah!
Comments?


-end-


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