From: Frank Caherty <caherty@wizard.nrl.navy.mil>
Subject: Revisiting the Dolby Nightmare (lightly technical)
Reply-To: caherty@wizard.nrl.navy.mil
Organization: The Internet
Date: Fri, 6 Nov 1992 23:56:21 GMT

jamie@access.digex.com (James Treworgy) writes:

|Another observation.  One of the biggest misconceptions about it
|arises from its name, Dolby _noise reduction_.  In fact dolby does not
|_reduce_ noice once it is already there.  It encodes the sounds on the
|tape in such a way that there will be less _noise addition_ from the
|tape surface.  The standard garbage in, garbage out applies to dolby
|as with most things.  If you copy a hissy tape and use dolby on the
|copy, the copy will be hissy as well.  The point is, there is no
|advantage that I can think of to making a copy of a tape which was _not_
|made with dolby and using dolby on the copy. 

There's a bit of misinformation here.  When Jamie says that "dolby does not
_reduce_ noice once it is already there.  It encodes the sounds on the
tape in such a way that there will be less _noise addition_ from the
tape surface"  he is correct that the process does not change or alter the
physical causes of tape noise, it simply (?!) records higher frequency audio
signals at a higher signal amplitude level and then on playback cuts that
band of frequencies by "the same ammount" as they were boosted on record.
This improves the _signal_to_noise_ratio_ by reducing the amplitude of the
hiss (noise) relative to the amplitude of the signal.  If the hiss level
on a blank tape produces a signal of 5x10-3 volts on playback, and the 
signal we are recording is 1x10-2 volts we have a signal to noise ratio
of 2 (6 dB).  If we boost the signal level we record to say 1x10-1 (a gain of
ten) volts, record it on the tape, and then on playback reduce all of the 
output from the tape head (signal + hiss) by the same factor of ten, our
desired signal amplitude is still 1x10-2 volts but the noise is now 5x10-4
volts giving us a signal to noise ratio of (1x10-2)/(5x10-4) = 20.  The
actual performance of dolby B gives about 10 db of Signal to Noise Ratio
improvement.

Jamie goes on to say: "If you copy a hissy tape and use dolby on the
copy, the copy will be hissy as well.  The point is, there is no
advantage that I can think of to making a copy of a tape which was _not_
made with dolby and using dolby on the copy. "  

Here is where the problem creeps in.  If you have a tape that has a certain 
level of noise on it, and you copy that tape _without_ dolby, you will get
a copy that has the original noise _plus_the_noise_on_the_new_tape_.  This
will total about twice the noise on the original tape.  If you had made that 
copy using dolby on record, you'd have a copy that has the original noise
plus only a small fraction (ratio is actually about -10 dB) of the noise of 
the new tape when played back.  So there is a _very_good_ reason to use 
dolby B on record when making a copy of a tape made without dolby B.  This
only applies to decks where you have control of the state (on or off) of
the dolby circuitry when duping tapes (I don't think the dual well dubbing
decks give you this control).

For the experimentalists out there, try this:  make a copy of some music
on a tape without using dolby (you'll only need a couple of minutes of 
music)  copy this tape segment to _another_ tape without using dolby.
Copy this segment (call it B) back to tape A (after the original music)
and call it A-prime.  Copy A-prime to tape B (after the previously recorded
segment B) _without_dolby_ and call it B-prime.  repeat this process a few
cycles until you have say 5 segments on tape A, each 1 gen removed from
the preceeding segment.  Listen to this tape without dolby.  You should 
hear each succeeding segment get noisier and noisier.   Repeat the process
_using_dolby_ this time and listen to the result.   Specifically compare
segment 5-without-dolby to segment 5-with-dolby.  This should convince
all but the most ardent anti-dolby people.  

The most common causes of dissatisfaction with dolby have, in my experience
been mismatches between channel levels recorded on tape, and improper bias
current adjustment for a given tape type.  If there is enough interest I
could write up a couple procedures for bias current adjustment and channel
balance, and post them.

Till next time,
Frank.