From: engelund@vab02.larc.nasa.gov (Walter C. Engelund)
Newsgroups: rec.music.phish
Subject: Dolby and record levels
Date: 10 Apr 92 18:12:46 GMT
Article-I.D.: uvaarpa.1992Apr10.181246.28542
Reply-To: engelund@vab02.larc.nasa.gov
Organization: The Internet


Pandion (Lee Silverman) writes:

->On a related note: If you're copying using dolby, particularly dolby C,
->you should cut your record levels by 1 or 2 db.  Why?  Well, the way
->dolby works is that it boosts the signal that goes onto the tape
->in certain ranges.  In the proccess of boosting, if the levels are
->already high, you may pass the saturation point of the tape, and
->you'll get distortion.

Actually, I don't think this is true. Recording levels should be set
independent of whether or not you are using Dolby. Lee and I had a
private e-mail discussion regarding this, and he's since revised the
above statement. The fact is that Dolby encoding and decoding (both B
and C types) is a dynamic process and does not have any effect on record
levels above the Dolby level (usually somewhere between 0 and +3db).

About a year ago there was some discussion on the tape-heads list regarding
the use of Dolby NR. Ihor Slabicky (iws@lionel.ssd.ray.com) went right to the
source and called Dolby Labs to clear up some of the most common mis-
conceptions. Unfortunately, Dolby (and noise reduction in general) is probably
the most misunderstood concept amongst tape traders. I spoke with Ihor this
morning and asked him if he would mind if I reposted some of the text from
his conversation with Bob from Dolby Labs. He said go right ahead, so I'll
append it below.

I have some of the follow up conversation that the following article
generated, and Ihor was good enough to send me some additional info. If
anyone would like to see more, e-mail me. I think most of this probably
belongs on the tape-heads list (which I highly recommend subscribing to
if your serious about taping trading in general). 

I hope you all find it as interesting as I did. 

Happy taping!

							- Walt

______________________________________________________________________________

From: Ihor Slabicky (iws@lionel.ssd.ray.com)
 
We could actually ask the source - Dolby Labs ...

|	Dolby Laboratories Inc.
|	100 Potrero Avenue
|	S.F., CA 94103
|
|	(415) 558-0200

I called Dolby Labs, and spoke with 'Bob' there.
Here's roughly what we discussed...

Dolby is dynamic - depending on the level of signal
being recorded, there is a different amount of 'Dolby'
applied.  Roughly, at high levels, there is no Dolby
effect, in the middle there is some, and at very low
levels, there is a lot - up to 10 dB worth.

The Dolby symbols on your deck's meters (usually the
'double-d') is where the Dolby level is for your deck
(i.e. where there is little or no Dolby effect).
It is ~ 200nW/m.  If there is no 'double-d', then the 
Dolby level is 0 dB on the meters.

(If you check your meters for the placement of the
Dolby symbol, you'll see it somewhere very near to
the max recommended recording levels for your deck.
What this means to me is that if you record your
tapes at the max recording levels which are recommended
for your deck, the recording level will match the
Dolby level, and there will be little or no Dolby
effect for those passages which are at that level.
Those passages which are below the Dolby level will
get more of the Dolby effect applied to them.  I guess
that if you record a tape with no Dolby, but the levels
are set to the Dolby level, as shown on your deck
meters, then this tape should sound just fine.  This
is just my wild speculation, folks.  This also
supports my theory that you should record tapes at the
highest recording level recommended by your deck's
manual for each tape type, setting the recording levels
so that the PEAKS just hit the max recording level.
On my deck, this means the average recording level
is about -3 to 0 dB, which gives some stretching
room out to the ~ +5 to +10 dB max for Type II tapes.
This way you avoid distortion, which is what starts
happening the more you go over that recommended level.
End of my wild speculation, based on known facts.)

Dolby C has a greater dynamic range than Dolby B.

Dolby C is much more sensitive to different makes of
tapedecks than Dolby B.  That's why Dolby C on one deck
can sound different on another brand of deck.  Theoretically,
all tapedecks have the same Dolby systems (B or C), because
they are all under license from Dolby Labs.  However, if
one deck manufacturer uses components that are not in spec,
or has a wider tolerances on components, than the
components used by another deck manufacturer, the Dolby
will behave differently between decks.  It's the effect
of hardware...

To make the best copies of tapes, dubbing should be
done from a high quality source to a low quality copy...
Going from a low quality source to a higher quality
machine does nothing - 'you can't get blood from a 
turnip'...  When copying cassette tapes, you lose
a little bit (or gain a bit of noise/hiss) with
each generation you make.  You can't get back something
which is not on the tape.

In general, one should playback the tape as it is encoded,
(i.e., if the tape is encoded in Dolby B, then play it
back in Dolby B, with the tape type set correctly) and
record the tape on another deck any way you want (your
choice of Type I, II, or IV tape, no noise reduction,
Dolby B, Dolby C, dbx, etc).

In general, the best (easiest, less prone to cause problems,
add hiss/noise) way to make copies is to have a tape made with
Dolby B, playback in Dolby B, and record it in Dolby B.

Some finer points...

A)	If a Dolby B encoded tape is played and the copy
is recorded with Dolby C, this will NOT give you a better
copy.  You don't gain anything from the extra dynamic range
of Dolby C, because of the lesser dynamic range of Dolby B.
In effect, your Dolby C tape can record all these great
dynamic ranges, but because the Dolby B tape has a lower
dynamic range, there just isn't anything to record...

B)	A Dolby C encoded tape dubbed to a Dolby B copy
will give you a better sounding copy (than that mentioned
in paragraph A) above), because the Dolby C tape
has a greater dynamic range, and you are capturing
more of it on the Dolby B tape.

C)	A Dolby B encoded tape dubbed to a Dolby B copy
will have a very slight loss of dynamics - this is inevitable,
due to the copying process.

D)	A Dolby B tape dubbed to a Dolby C copy (paragraph A)
above) theoretically will sound only slightly better
than a Dolby B tape that was dubbed to a Dolby B copy
(paragraph C) above).  The difference would be very slight,
but you probably would NOT notice any difference because
of the inherent losses that do occur when making copies.

E)	A Dolby C tape dubbed to a Dolby C copy will
experience the same type of slight loss as described in
paragraph C) above, due to the inherent losses that occur
when copying tapes.  However, remember that Dolby C is
more sensitive to differences in tapedecks, so you
may not benefit from using Dolby C that much...

(In all of these cases, the differences are VERY small.)

F)	You can make copies of Dolby encoded tapes without
any Dolby.  You would do this by setting the Dolby to off
on the playback machine, setting the playback output levels
to match the Dolby level (by using a Dolby Level Tape),
setting the recording level on the second deck to the Dolby
level (again using a Dolby Level Tape) and recording the copy. 
You do need that Dolby level tape...  Btw, this is what is
meant by matching the levels of the two tapes.

(This to me sounds like a lot of work, besides having to get
a Dolby level tape, and matching the levels of the two decks.)

That was pretty much the discussion.  The conclusions,
imho, are as follows:

Play the tape as it was encoded.  Record the copy the
way you would like it to be recorded.

You'd get a better copy by using a Dolby C 'master'
and making a Dolby B copy.

You won't go wrong by having the master in Dolby B and
making the copies in Dolby B.

Theoretically, a Dolby B tape copied to Dolby C should be
better, than if it was copied to Dolby C, but not by any
amount noticeable...

In general, the best (easiest, less prone to cause problems,
add hiss/noise) way to make copies is to have a tape made with
Dolby B, playback in Dolby B, and record it in Dolby B.

The recording levels are very important to reducing the
inherent losses due to making copies.  You should record
tapes at the highest recording level recommended by YOUR
deck's manual for each tape type, setting the recording
levels so that the PEAKS just hit the max recording level.
On my deck, this means the average recording level
is about -3 to 0 dB, which gives some stretching
room out to the ~ +5 to +10 dB max for Type II tapes.
This way you avoid distortion, which is what starts
happening the more you go over that recommended level.

Direct all flames to me - Ihor