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RE: arsclist Dolby and unintended consequences
Dear All,
When I was working professionally for a certain British broadcasting
corporation which shall be nameless, the problems of Dolby C on analogue
video had just begun to surface. I believe the Dolby C system has a fatal
defect. Its compression system uses *peak detection* (rather than RMS or
some form of slugged action), and because Dolby C basically comprises two
Dolby Bs in series (with the latter one handling frequencies an octave lower
at a lower threshold and with double the timeconstants), *any* amplitude or
phase distortion can result in audible artefacts. In the dubbing theatre, we
tried arbitrarily routing encoded sounds through rep-coils or parametric
equalisers until the non-linearities disappeared, and then decoding them
using a standalone Dolby B/C processor made by JVC. But there was never time
for this on news items, and (to my ears) the artefacts happened on at least
one news-story in every news bulletin.
For your information Richard, there were at least three Telefunken
systems: "C4" was the professional version dividing the frequency range into
four and compressing at 1.5 : 1. There was another one called the Model 112S
for analogue satellite communication which compressed at 2.5 : 1, and a
third down-market one called "Telefunken Hi-Com", licensed to other
manufacturers and sold to the home studio market. I should also mention the
"CX" system invented by U.S. Columbia Records as being *not* audible in the
encoded state; but this never appeared on this side of the pond - it was
killed by being marketed by technical ignorami. And several cassette
manufacturers effectively combined something like Dolby B and dBx, to get
around both patents.
Peter Copeland
-----Original Message-----
From: Richard L. Hess [mailto:arclists@xxxxxxxxxxxxxxx]
Sent: 25 June 2002 23:48
To: Association of Moving Image Archivists; ARSCLIST@xxxxxxxxxxxxxxxx
Subject: arsclist Dolby and unintended consequences
Hi,
A couple of weeks ago, Jim Wheeler pointed me to an article by Dave Moulton
in TV Technology
http://www.tvtechnology.com/features/Inside-Audio/f-dm-badaudio.shtml that
addressed some "bad audio" problems related to Dolby noise reduction.
The article specifically addressed Dolby C encoding on Beta video decks.
According to the article, the early Beta decks had a switch to turn the
Dolby on and off (much like most cassette recorders). Further, the article
states that with the advent of BetaSP, that switch was eliminated and
permanently wired "on."
I'm sending this email out just in case someone doesn't realize that the
Dolby and dbx noise reduction processes are double-ended and a
fully-functional unit is required to properly reproduce the encoded tape.
There are no plug-ins to DAW software (to the best of my knowledge) that
will decode any of these noise reduction systems. You're stuck either with
the hardware built into the player or with the outboard decoders.
These processes will sometimes magnify errors if the tapes or playback
machines are not in great condition. In addition, it is possible to have
defective decoders (don't ask me how I know). In all cases, trust your
ears. If you think you're hearing a noise reduction problem, try and get to
the bottom of it sooner rather than later.
Dolby C is built into Sony's Beta decks as well as Panasonic's MII decks.
Dolby A was optional on 1-inch Type C videotape. I suspect that a few quad
tapes might have been recorded with Dolby A, and that Dolby A might have
been used on other formats as well in limited quantities.
As to audio-only recordings, perhaps this would be a good time to just
mention the different flavors of noise reduction companding that have
existed. I may even have missed one or two:
DOLBY
A-the first studio standard (interrupted tone)
SR-the second studio standard (noise reference)
B-the first consumer standard (widely used on cassettes)
also used on FM broadcasts
C-the second consumer standard
also used on some semi-pro multi-tracks and video formats
S-the third consumer standard (used on some cassettes
and some semi-pro multi-tracks
DBX
I - the pro standard
II - the consumer standard also used on discs, FM broadcasts,
and in the stereo TV standard for the difference and pro channels.
BURWEN
Although noted for his single-ended noise reduction products
they also produced for a time a 3:1 linear compander.
TELEFUNKEN
C4D ? this was yet another, incompatible, compander system with
some penetration into Europe.
FYI, I have multiple copies of processors for Dolby A, SR, and B, and dbx
I, and II. I don't have any for Dolby C (other than on cassette) or Dolby S
nor the Burwen nor Telefunken systems.
Remember, the metadata on the tape box about the noise processing MIGHT BE
WRONG. (I've had that happen more than once).
Good luck.
Any questions, please email or post to the list.
Cheers,
Richard
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