From: gmw1@cunixa.cc.columbia.edu (Gabe M Wiener)
Subject: Re: Dolby B/C dubbing on Dual Decks
Reply-To: gmw1@cunixa.cc.columbia.edu (Gabe M Wiener)
Organization: Columbia University
Date: Sun, 8 Nov 1992 03:31:52 GMT

In article <1992Nov5.224618.24416@ll.mit.edu> rhoades@ll.mit.edu (Captain Chaos) writes:
>I'm finding this discussion interesting but confusing.  What's not clear to
>me is how you are _supposed_ to make copies if you have two decks, each of
>which can be set to Dolby B, C, or none and you want to copy a B-encoded
>tape.  By _supposed_ to I mean how the system was intended to be used to
>avoid double-encoding (if that's possible), mismatched encoding (with B &
>C), or improper decoding (C from B, none from B, etc).

Okay folks, let's clear up this Dolby business once and for all.

Dolby is what we in the pro audio world call a _double ended_ process,
meaning that a tape is either recorded with it or it isn't, and the proper
Dolby process must be used at the playback end.  This is true for all the
consumer Dolby formates, B, C, and S, and for the professional Dolby systems
too, namely A and SR.

When a tape is recorded with Dolby, the Dolby-encoded signal is an intrinsic
part of the tape, and you can never change that.  All dual-deck machines are
designed so that when you put the unit into copy mode, the Dolby circuitry is
switched completely out of the line.  Why, you may ask?  Because, you are
copying a tape that is *already* Dolby encoded.  You needn't do anything 
except copy the tape.  If you do a 1:1 copy, the duplicate will have the
same encoding as an original.  Thus, it is impossible to a) accidentally
double-encode a tape, or b) use a dual-deck to "put" Dolby onto an un-encoded
tape.

Speaking of which, if you want to put Dolby on an unencoded tape, let me 
tell you this.  FORGET IT.  You're trying to encode something that *already*
has tape noise from the first generation...don't waste your time.  You should
only put Dolby on a new line recording.  Why?  Because Dolby is meant to
preserve whatever's coming into the deck...if what's coming in is a hissy
recording, you can't reduce that noise.

As for making copies, here's the way to do it:

	- In a dual-deck machine, just drop it in and hit copy.
	  Dolby or not, it's gonna copy and that's that.
	- In two single deck machines, you have two options.  
	  You can turn BOTH Dolby circuits off and copy the
	  tape, thus copying any encoding present on the tape,
	  *or* you can decode on the source and re-encode on
	  the destination machine.

The decode/reencode system is only useful if you feel compelled to put,
say, C on a B-encoded tape.  You have to decode the B and then put C 
on.  However, I'd advise against this, as you're going to be doing so 
much companding and EQ on the recording that by the time you're done,
it's going to sound, ah, less than desirable.

You also can just get a DAT, DCC, MD, CDR, DAW, 1/4" w/Dolby SR, etc. and
be done with all this. :-)


--
Gabe Wiener - Columbia Univ.     "This 'telephone' has too many shortcomings 
gmw1@cunixa.cc.columbia.edu       to be seriously considered as a means of 
N2GPZ in ham radio circles        communication. The device is inherently of
72355,1226 on CI$                 no value to us." -Western Union memo, 1877