Is hi-fi audio compressed in VHS?
MaGiCa
I don't know if this is the right group for my question. In
it.hobby.hifi I read different opionions about hi-fi audio in VHS VCR
(PAL, as used in Italy). Could somebody tell me if hi-fi audio is
compressed during recording and expanded during playback in order to
obtain 90 dB dynamic? If yes, is the processing similar to DBX or
different?
Thank you.
Gian Carlo
jack schidt
No.
The audio is recorded in an FM manner on the helical scan heads, which
results in very wide bandwidth and low noise. Traditional VHS recorders use
fixed heads, hence slower effective tape speed, higher noise, and lower
bandwidth.
The 90 dB S/N ratio is typical of the modulation/demodulation scheme.
MaGiCa wrote in message <34d054a2...@news.fth.sbs.de>...
Mike NO UCE Schuster
In article <34d054a2...@news.fth.sbs.de>,
MaGiCa <mac...@marina.scn.de> wrote:
>I don't know if this is the right group for my question. In
>it.hobby.hifi I read different opionions about hi-fi audio in VHS VCR
>(PAL, as used in Italy). Could somebody tell me if hi-fi audio is
>compressed during recording and expanded during playback in order to
>obtain 90 dB dynamic? If yes, is the processing similar to DBX or
>different?
Yes it is compressed using a dbx variant and expanded upon playback. This
is one reason why VHS "HiFi" sucks so badly as an interchange medium. It
is so sensitive to tracking error that playback on a different machine (or
even the same machine, after wear and tear alters the alignment) can
result in mistracking and head-switching noise which, when expanded by the
playback circuitry, sounds awful.
--
Mike Schuster | 70346.1745 at CompuServe dot COM
schuster at panix dot com | schuster at pol dot net
HA11747ANT
Gian wrote>
>I don't know if this is the right group for my question. In
>it.hobby.hifi I read different opionions about hi-fi audio in VHS VCR
>(PAL, as used in Italy). Could somebody tell me if hi-fi audio is
>compressed during recording and expanded during playback in order to
>obtain 90 dB dynamic? If yes, is the processing similar to DBX or
>different?
>
>Thank you.
>
>Gian Carlo
Gian, yes the audio is. The audio goes through a 2:1 compression upon recording
and a 1:2 expansion upon playback. In it's raw format, VHS and Beta Hi-Fi only
have about 45 dB of dynamic range, but with the compander, it can reach 90 dB,
similar to a compact disc's dynamic range. It's similar to DBX's 2:1:2
compandering, but I'm not sure of the similarities. Any ideas on that, group,
any other feedback is welcome.
Joe
Jim Lagerkvist
MaGiCa wrote:
>
> I don't know if this is the right group for my question. In
> it.hobby.hifi I read different opionions about hi-fi audio in VHS VCR
> (PAL, as used in Italy). Could somebody tell me if hi-fi audio is
> compressed during recording and expanded during playback in order to
> obtain 90 dB dynamic? If yes, is the processing similar to DBX or
> different?
VHS HiFi audio is definitely companded, as is obvious during loud
passages. I don't know if it's a straight 2:1 or some other curve, but
I've developed a keen ear for these artifacts from my involvement in the
early years (70's) of DBX experiments in the audio recording industry.
Jim Lagerkvist
magica
Thanks for all answers.
However it seems there is no aggreements about this topic, despite
hi-fi VCRs are so widespread.
Here are some aswers I received (also posted to rec.video).
------------
It is not - not anymore. [not DBX - my note]
Only very early Panasonic models (circa -83-85) used patented
dbx-circuitry in their Hi-FI VCRs.
[...]
The official "Depth Multiplex" FM-modulation audio recording format
spec. published by JVC - the founder of VHS format, states clearly
that the compander shall be driven by a peak detector, not rms adopted
by dbx-type companders.
------------
VHS Hi-Fi does use a compression and expansion scheme to get the good
s/n ratio, but I'm not sure if it's the dbx scheme.
-------------
There is no compression. The signal is 'depth multiplexed' on the
tape. That is, the audio is recorded under the video signal. The high
SN ratio is due to an effective writing speed of approximately 200
inches per second compared to conventional tape.
-------------
The audio is recorded in an FM manner on the helical scan heads, which
results in very wide bandwidth and low noise.
[...]
The 90 dB S/N ratio is typical of the modulation/demodulation scheme.
--------------
Yes it is compressed using a dbx variant and expanded upon playback.
This is one reason why VHS "HiFi" sucks so badly as an interchange
medium.
--------------
Gian, yes the audio is. The audio goes through a 2:1 compression upon
recording and a 1:2 expansion upon playback. In it's raw format, VHS
and Beta Hi-Fi only have about 45 dB of dynamic range, but with the
compander, it can reach 90 dB, similar to a compact disc's dynamic
range. It's similar to DBX's 2:1:2 compandering, but I'm not sure of
the similarities.
---------------
VHS HiFi audio is definitely companded, as is obvious during loud
passages. I don't know if it's a straight 2:1 or some other curve,
but I've developed a keen ear for these artifacts from my involvement
in the early years (70's) of DBX experiments in the audio recording
industry.
---------------
Gian Carlo