From: dbk@cs.umd.edu (Dan Kozak)
Newsgroups: rec.music.makers
Subject: Re: Beatles & ADT...how?
Date: 22 Jan 93 21:21:55 GMT
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742

warlock@halloween.EBay.Sun.COM writes:
>I've read many articles discussing the Beatles use of ADT (Automatic Double
>Tracking) to 'fatten' up their vocals and I was wondering...  

Just to get the nits out of the way first, that's _Artificial_ Double
Tracking (ADT), an invention of Ken Townshend, one of the Abbey Road
technical engineers.  As Jay Kadis pointed out, they also employed
real double tracking, as well as running vocals (and other things)
through Leslie speakers and sometimes combining more than one of these
together . . .

>1) How exactly did it work?

Let's see if I can confound things with a diagram:

======================================= 4-track master tape
|record/sync head|	|repro head|    
        |		      |---------------------------------> to mixer
	|(playback)
	|
	|-->(another deck, with varispeed and same head spacing)
	         |(recording)
	    ======================================= (variable speed tape)
	    |record/sync head|	|repro head|
				      |-------------------------> to mixer

ADT would be applied during mixing, sometimes during a "reduction mix"
(bouncing down 4 tracks to another machine), sometimes during a final
mix.  The repro head on the four track would feed all 4 tracks to the
mixer for mixdown (whether ADT was being used or not).  To produce the
ADT effect, the track(s) to be effected would simultaneously be taken
off the sync head.  Because of the physical spacing between the heads,
the playback from the sync head would precede the playback from the
repro head by some small amount (probably on the order of 250ms).
The output of the sync head would be run out to another tape deck, set
up as a tape delay with varispeed.  Assuming the same head
spacing/base tape speed of this second deck is the same as the main
4 track, the delayed version of the main 4 track's sync head output
would be "time aligned" with it's repro head output.  By varying the
speed of the second deck, a modulated delay effect is produced, with
one important difference: the delay time can not just go to zero, but
actually become negative, i.e. the "delayed" signal can occur before
the main one.

The production of the varispeed effect was done by taking a normal
audio oscillator set to 50 Hz, running it into a power amp (to juice
it up to 240v--this is Britain ) and then connecting the line cord of
the tape deck across the amp's output.  Since the decks were line
phase synchronous, varying the pitch of the oscillator above or below
50 Hz would cause the deck to speed up or slow down.

>2) Is there a way to get the same effect today with delay/chorus effects?

Not really.  While some of the effects produced with this set up can
be duplicated reasonably well in other ways, the effect that simulates
real double tracking the best utilizes the negative delay time aspect
mentioned above.  Also note that the modulation depth and speed are
controlled manually, i.e. by how far and fast an operator moved the
oscillator's frequency.  This places some obvious physical limits on
what can be done but the very unevenness of this approach is part of
the sound.  And consciously or not, the operator would tend to be
influenced by the music itself (listen to how the "flanger" style ADT
on Eric Clapton's guitar on "While My Guitar Gently Weeps" follows the
peaks and valleys of the part itself).  The best simulation I've
gotten of this kind of effect is to manually modulate the delay time
on a footpedal delay (to try to get close to the sound of the acoustic
guitar on "Cry Baby Cry").  This is hard to do when you're playing.
:-)

BTW, George Martin insists that the term "flanging" has nothing to do
with varispeeding by pressing on the "flange" of the reel, but is from
a string of techno-babble that he gave to the non-technical Lennon to
describe how ADT was done.  As for all the watered down, non-technical
descriptions of how ADT was done with two tape decks running in synch
and pressing on the reel flange . . . they wouldn't be in synch for
very long. :-)

#dan

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