Dolby Enc/Dec: Tangent to List Topic

From: John Lawson <jpl15_at_netcom.com>
Date: Tue Dec 1 18:40:56 1998

On Tue, 1 Dec 1998, Chuck McManis wrote:

> At 03:19 PM 12/1/98 -0800, Cameron wrote:
> >The question is, does source => Dolby encoding => Dolby decoding == source?
> >Dolby encoding, AFAIK, increases the volume of sounds >10KHz, and decoding
> >does the reverse.
>
> Dolby compresses the sound track in the high end causing high frequencies
> to get a lot higher. Generally static is thus moved out of the reproducible
> range of the tape and when the signal is decompressed the "hiss" is gone.
> And yes, it alters the frequency spectrum of the signal source in an effort
> to remove "noise".
>
> --Chuck
>


  Okay... This (I think) is at least *close* to the topic of this
List, since many of our machines are designed with cassette tape I/O
as secondary storage...


HOWEVER: The above posts reflect some general mis-conceptions re:
just what those "D"s do to signals on/in a medium of storage or
transmission.


  Humble Disclaimer:

  I am a complete novice concerning many aspects of the hobby of the
care and feeding of elderly computers... but professional and cinema
audio is how I have made my living for most of my life, and for the
last almost ten years it has been as the Chief Engineer of M.G.M.
Studios.. I *think* I can help y'all with how Dolby A, B, C, SR, and
HQX work... since I have designed and built and use every day
systems with these encoding and decoding schemes, as well as dbx,
Telcom C-64, Ultra*Stereo, NatSemi DNR..... etc.


  First, with most small cassettes, we are talking Dolby 'B' or 'C';
these DO NOT 'alter' the frquency (ie the pitch) of the signals...
they do not shift the spectrum in any way.. [Cameron is closer in his
post], but each uses band-limited compression and decompression to
reduce the overal signal in given frequency bands, store or transport
the signal across a noisy medium, then decompress at the destination.
Any noise *existing* in the original will be reproduced, but noise
from the media will be reduced. These systems can also add distortion
of various kinds when Things go wrong... an understanding of the
nature of the processes can help in restoration of data media made
by older systems and (cheap) cassette decks.

  I have detailed info (many hundreds of pages) on each of the
common Noise Reduction schemes, and I would be glad to correspond
PRIVATELY with anyone interested in the real poop on how NR is
implemented in audio systems. I am also working on a document for
the Archives (maybe a FAQ supplement) on exactly how the common NR
schemes work as they specifically relate to cassette-based computer
data storage.

 At least I hope this will be of use to some of us who have old
tapes to restore and preserve..... and the physics, chemistry, and
electronics of magnetic recording in general is a fascinating and
quite beautiful topic in and of itself... IMHO.


  Cheers

John

   
 PS: Help Prevent Topic-Drift! E-mail me privately unless it's list
oriented... thanx!
Received on Tue Dec 01 1998 - 18:40:56 GMT