[Joe the audio guy]

Another quick question: Is Dolby B/C available as software and, if not,
why?

Joe

[Scott Dorsey]

Joe the audio guy wrote:
Another quick question: Is Dolby B/C available as software and, if not,
why?

This has been discussed repeatedly here, and a google search on the group
is in order.

There is one company selling a "cassette restoration package" that does
Dolby B decoding, but other than that nobody else seems to have implemented
it, and I don't know why.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Don Pearce]

Joe the audio guy wrote:
Another quick question: Is Dolby B/C available as software and, if not,
why?

Joe



It would be totally pointless. Once you have the signal rendered into
numbers, there are no longer any dynamic range limitations that would
merit Dolby (B or C).

d

[Arny Krueger]

"Don Pearce" wrote in message
...
Joe the audio guy wrote:
Another quick question: Is Dolby B/C available as software and, if not,
why?

Joe

It would be totally pointless. Once you have the signal rendered into
numbers, there are no longer any dynamic range limitations that would
merit Dolby (B or C).

What about decoding existing Dolby-encoded recordings?

[Don Pearce]

Arny Krueger wrote:
"Don Pearce" wrote in message
...
Joe the audio guy wrote:
Another quick question: Is Dolby B/C available as software and, if not,
why?

Joe
It would be totally pointless. Once you have the signal rendered into
numbers, there are no longer any dynamic range limitations that would
merit Dolby (B or C).

What about decoding existing Dolby-encoded recordings?



How would you perform the calibration?

d

[Arny Krueger]

"Joe the audio guy" wrote in message
...
Another quick question: Is Dolby B/C available as software and, if not,
why?


http://www.hansvanzutphen.com/tape_restore_live/

"Tape Restore Live! for Winamp

"Tape Restore Live! is a free Winamp plugin that improves the sound of
cassette tapes.

"Software Dolby B remover (*)

"This is an advantage over using a hardware filter because the tape bias
settings can be changed before the filter is used, which is especially
useful if the sound of tapes has become dull because of tape wear and age.
Also, any noise from the sound card is deminished by the Dolby B decoder.
Calibration tones are provided to calibrate the software Dolby B remover
against the encoder in the cassette recorder.

"(*) The filter that is offered by Tape Restore Live! is not a real software
Dolby B decoder, but it is a close enough approximation to get a decent
sound. Because of that it is referred to as a software Dolby B remover
instead of a software Dolby B filter

[Arny Krueger]

"Don Pearce" wrote in message
et...
Arny Krueger wrote:
"Don Pearce" wrote in message
...
Joe the audio guy wrote:
Another quick question: Is Dolby B/C available as software and, if
not, why?

Joe
It would be totally pointless. Once you have the signal rendered into
numbers, there are no longer any dynamic range limitations that would
merit Dolby (B or C).

What about decoding existing Dolby-encoded recordings?

How would you perform the calibration?

By ear.

[Eeyore]

Don Pearce wrote:

Arny Krueger wrote:
"Don Pearce" wrote in message
Joe the audio guy wrote:

Another quick question: Is Dolby B/C available as software and, if not,
why?

It would be totally pointless. Once you have the signal rendered into
numbers, there are no longer any dynamic range limitations that would
merit Dolby (B or C).

What about decoding existing Dolby-encoded recordings?

How would you perform the calibration?

With a calibration tape. I even have one but it's so old that I doubt it's that
accurate. It's got an azimuth alignment on it too. I modded my Akai cassette
deck to allow adjustment of azimuth without tearing it apart.

Graham

[Scott Dorsey]

Don Pearce wrote:
Arny Krueger wrote:
"Don Pearce" wrote in message
...
Joe the audio guy wrote:
Another quick question: Is Dolby B/C available as software and, if not,
why?

Joe
It would be totally pointless. Once you have the signal rendered into
numbers, there are no longer any dynamic range limitations that would
merit Dolby (B or C).

What about decoding existing Dolby-encoded recordings?

How would you perform the calibration?

The same way you ALWAYS have to do it with cassettes that have no tones
on them... by ear.

It is sad and annoying. And it is never quite right.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Don Pearce]

Scott Dorsey wrote:
Don Pearce wrote:
Arny Krueger wrote:
"Don Pearce" wrote in message
...
Joe the audio guy wrote:
Another quick question: Is Dolby B/C available as software and, if not,
why?

Joe
It would be totally pointless. Once you have the signal rendered into
numbers, there are no longer any dynamic range limitations that would
merit Dolby (B or C).
What about decoding existing Dolby-encoded recordings?
How would you perform the calibration?

The same way you ALWAYS have to do it with cassettes that have no tones
on them... by ear.

It is sad and annoying. And it is never quite right.
--scott

So, so true.

d

[Arny Krueger]

"Don Pearce" wrote in message
...
Scott Dorsey wrote:
Don Pearce wrote:
Arny Krueger wrote:
"Don Pearce" wrote in message
...
Joe the audio guy wrote:
Another quick question: Is Dolby B/C available as software and, if
not, why?

Joe
It would be totally pointless. Once you have the signal rendered into
numbers, there are no longer any dynamic range limitations that would
merit Dolby (B or C).
What about decoding existing Dolby-encoded recordings?
How would you perform the calibration?

The same way you ALWAYS have to do it with cassettes that have no tones
on them... by ear.

It is sad and annoying. And it is never quite right.
--scott

So, so true.

In fact not only did the level have to be right, but the FR through the
record/playback cycle really needed to be about perfect. This never happens
on cassettes.

[Mike Rivers]

Arny Krueger wrote:

"(*) The filter that is offered by Tape Restore Live! is not a real software
Dolby B decoder, but it is a close enough approximation to get a decent
sound. Because of that it is referred to as a software Dolby B remover
instead of a software Dolby B filter

Curious that they use the terms "remover" and "filter" rather than
"decoder." Perhaps it's a preset equalizer without the dynamics
expansion. This might be OK with modern listeners who are used to not
hearing full dynamic range audio. So I figured it's just another
processor that you can try, and decide whether it makes a particular
recording sound better or not.


--
If you e-mail me and it bounces, use your secret decoder ring and reach
me he
double-m-eleven-double-zero at yahoo -- I'm really Mike Rivers
)

[Arny Krueger]

"Mike Rivers" wrote in message
news
Arny Krueger wrote:

"(*) The filter that is offered by Tape Restore Live! is not a real
software Dolby B decoder, but it is a close enough approximation to get a
decent sound. Because of that it is referred to as a software Dolby B
remover instead of a software Dolby B filter

Curious that they use the terms "remover" and "filter" rather than
"decoder."

Trying to stay out of trouble with Dolby Labs and our good bud Ray? ;-)

Perhaps it's a preset equalizer without the dynamics expansion.

Sees a tad more complex than that.

I did some tests on it with tones and multitones.

It seems to do some dynamics expansion with a 1 KHz tone, and it also does
low-pass equalization whose corner frequency is very much amplitude
dependent. Both channels get the same basic treatment, even when their
amplitude is 10 dB part.

I don't remember the Dolby 'B" spec precisely, but this does sound like it
is in the ball park, maybe a distant one.

I looked at its performance when turned on via its control panel, and when
turned off.

When turned off, it is -3 dB at about 11 KHz, and it has about 0.5 dB gain.
There is a little added nonlinear distortion but it is all 100 dB down.

When turned on, nonlinear distortion is about 70 dB down @ 1 KHz for a -3
dB FS signal.

[Arny Krueger]

"Arny Krueger" wrote in message
...

"Mike Rivers" wrote in message
news
Arny Krueger wrote:

"(*) The filter that is offered by Tape Restore Live! is not a real
software Dolby B decoder, but it is a close enough approximation to get
a decent sound. Because of that it is referred to as a software Dolby B
remover instead of a software Dolby B filter

Curious that they use the terms "remover" and "filter" rather than
"decoder."

Trying to stay out of trouble with Dolby Labs and our good bud Ray? ;-)

Perhaps it's a preset equalizer without the dynamics expansion.

Sees a tad more complex than that.

I did some tests on it with tones and multitones.

It seems to do some dynamics expansion with a 1 KHz tone, and it also
does low-pass equalization whose corner frequency is very much amplitude
dependent. Both channels get the same basic treatment, even when their
amplitude is 10 dB part.

I don't remember the Dolby 'B" spec precisely, but this does sound like it
is in the ball park, maybe a distant one.

I looked at its performance when turned on via its control panel, and when
turned off.

When turned off, it is -3 dB at about 11 KHz, and it has about 0.5 dB
gain. There is a little added nonlinear distortion but it is all 100 dB
down.

When turned on, nonlinear distortion is about 70 dB down @ 1 KHz for a -3
dB FS signal.

Correction - some of the funnies mentioned above were due to having Winamp's
own equalizer turned on. Turning that off, cleaned things up appreciably.

All by itself, "Tape Restore Live" was far more transparent when turned off.
The FR was about perfect, for example.

[William Sommerwerck]

How would you perform the calibration?

By ear.

If this were a professional tape, it would probably have calibration tones.

[Scott Dorsey]

William Sommerwerck wrote:
How would you perform the calibration?

By ear.

If this were a professional tape, it would probably have calibration tones.

True, but if it were a professional tape, it would probably not have
used Dolby B or C.

Some of the earlier cassette decks that came with Dolby actually had provision
for putting a 1KC reference tone down and then adjusting the reference level
on playback. The Advent was one of the more common units that did.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Eeyore]

Scott Dorsey wrote:

Don Pearce wrote:

How would you perform the calibration?

The same way you ALWAYS have to do it with cassettes that have no tones
on them... by ear.

It is sad and annoying. And it is never quite right.

You DON'T have a cassette calibration tape ?

Graham

[Chris Hornbeck]

On Mon, 27 Oct 2008 17:06:49 +0000, Don Pearce
wrote:

Arny Krueger wrote:
What about decoding existing Dolby-encoded recordings?

How would you perform the calibration?

I'll have to disagree with several august posters about this.
Dolby B and C levels are referenced to magnetic levels, so any
properly recorded tape will be properly de-encoded if the
playback machine has been calibrated with a reference-level
tape, 400Hz at Dolby reference level of 200nW/m.

Assuming that everything else is up to snuff, natch. A pretty
big assumption these decades later... but fundamentally right.
Dolby B and C do not require tones on every tape for de-encode
tracking.

Much thanks, as always,
Chris Hornbeck

[Richard Crowley]

"Don Pearce" wrote ...
Arny Krueger wrote:
"Don Pearce" wrote ...
Joe the audio guy wrote:
Another quick question: Is Dolby B/C available as software and, if
not, why?

Joe
It would be totally pointless. Once you have the signal rendered into
numbers, there are no longer any dynamic range limitations that would
merit Dolby (B or C).

What about decoding existing Dolby-encoded recordings?

Do you really not see the benefit of a software-based solution.

How would you perform the calibration?

Same way you would with hardware-based Dolby decoding.
(whether accurate or otherwise).

[Richard Crowley]

"Scott Dorsey" wrote...
Joe the audio guy wrote:
Another quick question: Is Dolby B/C available as software and, if not,
why?

This has been discussed repeatedly here, and a google search on the group
is in order.

There is one company selling a "cassette restoration package" that does
Dolby B decoding, but other than that nobody else seems to have
implemented
it, and I don't know why.

Are you saying that there are no patent barriers?

[Don Pearce]

Richard Crowley wrote:
"Don Pearce" wrote ...
Arny Krueger wrote:
"Don Pearce" wrote ...
Joe the audio guy wrote:
Another quick question: Is Dolby B/C available as software and, if
not, why?

Joe
It would be totally pointless. Once you have the signal rendered into
numbers, there are no longer any dynamic range limitations that would
merit Dolby (B or C).
What about decoding existing Dolby-encoded recordings?

Do you really not see the benefit of a software-based solution.

How would you perform the calibration?

Same way you would with hardware-based Dolby decoding.
(whether accurate or otherwise).


Everybody is clearly reading this question differently to the way I am.
If the question is "Would it be a good idea to have an application that
could rescue a tape recorded in B/C when a proper playback system is
unavailable?" Then yes, I can see that it might have some limited
application for a while.

But that is not how I read it. My understanding of the question is that
it is asking if it would be a good idea to routinely offer Dolby B and C
as companding methods in a DAW, on the basis that they might offer some
sonic advantage. The answer to that is a definite No.

d

[William Sommerwerck]

"Richard Crowley" wrote in message
...
"Scott Dorsey" wrote...
Joe the audio guy wrote:

There is one company selling a "cassette restoration package"
that does Dolby B decoding, but other than that nobody else
seems to have implemented it, and I don't know why.

Are you saying that there are no patent barriers?

How could there be? The Dolby A, B, and C patents expired long ago. The
Dolby trademark is another matter.

[William Sommerwerck]

Everybody is clearly reading this question differently to the way I am.
If the question is "Would it be a good idea to have an application that
could rescue a tape recorded in B/C when a proper playback system
is unavailable?" Then yes, I can see that it might have some limited
application for a while.

But that is not how I read it. My understanding of the question is that
it is asking if it would be a good idea to routinely offer Dolby B and C
as companding methods in a DAW, on the basis that they might offer
some sonic advantage. The answer to that is a definite No.

No offense, but I think you misread it.

By the way, many years ago I tried superimposing dbx II on top of Dolby B,
the idea being that the Dolby would reduce the noise sufficiently to prevent
audible breathing from the dbx. The results were horrendous.

[Don Pearce]

William Sommerwerck wrote:
Everybody is clearly reading this question differently to the way I am.
If the question is "Would it be a good idea to have an application that
could rescue a tape recorded in B/C when a proper playback system
is unavailable?" Then yes, I can see that it might have some limited
application for a while.

But that is not how I read it. My understanding of the question is that
it is asking if it would be a good idea to routinely offer Dolby B and C
as companding methods in a DAW, on the basis that they might offer
some sonic advantage. The answer to that is a definite No.

No offense, but I think you misread it.


Not offended - but here is the original question "Another quick
question: Is Dolby B/C available as software and, if not, why?". I
think my interpretation is probably rather more reasonable (in that it
doesn't add a whole load of unstated stuff) than the 'rescuing old
tapes' one.

By the way, many years ago I tried superimposing dbx II on top of Dolby B,
the idea being that the Dolby would reduce the noise sufficiently to prevent
audible breathing from the dbx. The results were horrendous.



I can imagine. I still have an analogue dbx companding box I made many,
many years ago with some purpose-designed vca chips - can't remember the
numbers now. It worked pretty well with my cassette deck, but not if
Dolby was enabled too.

d

[Arny Krueger]

"Eeyore" wrote in message
...


Scott Dorsey wrote:

Don Pearce wrote:

How would you perform the calibration?

The same way you ALWAYS have to do it with cassettes that have no tones
on them... by ear.

It is sad and annoying. And it is never quite right.

You DON'T have a cassette calibration tape ?

I have one. Never use it.

However, I may put it into service if I ever try to exploit this software
Dolby B decoder that we've found.

[Arny Krueger]

"Chris Hornbeck" wrote in message
...
On Mon, 27 Oct 2008 17:06:49 +0000, Don Pearce
wrote:

Arny Krueger wrote:
What about decoding existing Dolby-encoded recordings?

How would you perform the calibration?

I'll have to disagree with several august posters about this.
Dolby B and C levels are referenced to magnetic levels, so any
properly recorded tape will be properly de-encoded if the
playback machine has been calibrated with a reference-level
tape, 400Hz at Dolby reference level of 200nW/m.

You've got to calibrate the entire transcription process including both the
playback machine and the recording process on the computer. I presume that's
what you mean by calibrating the playback machine. In this case the
"playback machine" is two interconnected boxes and 2 pieces of software.

Assuming that everything else is up to snuff, natch. A pretty
big assumption these decades later... but fundamentally right.
Dolby B and C do not require tones on every tape for de-encode
tracking.

After testing the software Dolby B decoder, and also examining the Dolby B
specs on their web site, here's what I would do:

(1) Play the calibration tape and digitize it, making sure that I also knew
the settings of the various level controls that would be involved in the
recording. In my case there would be two - one the output level on the
cassette machine, and the other the record input gain on the digital audio
interface.

(2) It appears that the software Dolby B decoder is set up so that Dolby
level = digital full scale. So, I would find out how much amplification I
had to apply in the digital domain to get the tone I recorded off the
calibration tape to be FS. I would then apply that much gain to any tapes I
transcribe, before passing them through the software Dolby digital decoder.

This whole process strikes me as being the Y2K analog for how one used an
outboard Dolby Digital decoder in the day of. Been there, done that. ;-)

[Arny Krueger]

"Don Pearce" wrote in message
...
Everybody is clearly reading this question differently to the way I am. If
the question is "Would it be a good idea to have an application that could
rescue a tape recorded in B/C when a proper playback system is
unavailable?" Then yes, I can see that it might have some limited
application for a while.

That's how I see the question.

But that is not how I read it. My understanding of the question is that it
is asking if it would be a good idea to routinely offer Dolby B and C as
companding methods in a DAW, on the basis that they might offer some sonic
advantage. The answer to that is a definite No.

I agree with that, too. Dolby A, B, C, and S as well as the various forms of
DBX were work-arounds for a problem that has been solved for all practical
purposes by means of modern digital recording.

One interesting spec comes from one of the Dolby boxes that supports A, B,
C, and S. They seem to be claiming 100's of dBs of dynamic range. If I was
in some kind of esoteric situation where a mere 120 dB wouldn't do the job,
I might think about dredging up one of these boxes and see what it could do.
;-)

[Scott Dorsey]

Eeyore wrote:
Scott Dorsey wrote:

Don Pearce wrote:

How would you perform the calibration?

The same way you ALWAYS have to do it with cassettes that have no tones
on them... by ear.

It is sad and annoying. And it is never quite right.

You DON'T have a cassette calibration tape ?

It's futile, because the tapes being transcribed were never recorded with
correct calibration. What's worse, plenty of them vary in physical alignment
from one end of the tape to the other, since most cassette transports are
pretty sloppy.

If folks actually calibrated machines and laid tones down, 90% of the
transcription problems would be gone.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Scott Dorsey]

Chris Hornbeck wrote:
On Mon, 27 Oct 2008 17:06:49 +0000, Don Pearce
wrote:

Arny Krueger wrote:
What about decoding existing Dolby-encoded recordings?

How would you perform the calibration?

I'll have to disagree with several august posters about this.
Dolby B and C levels are referenced to magnetic levels, so any
properly recorded tape will be properly de-encoded if the
playback machine has been calibrated with a reference-level
tape, 400Hz at Dolby reference level of 200nW/m.

This is true. However I have never actually seen a machine that was
properly calibrated at the time it was used.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Richard Crowley]

"Don Pearce" wrote ...
Everybody is clearly reading this question differently to the way I am. If
the question is "Would it be a good idea to have an application that could
rescue a tape recorded in B/C when a proper playback system is
unavailable?" Then yes, I can see that it might have some limited
application for a while.

But that is not how I read it. My understanding of the question is that it
is asking if it would be a good idea to routinely offer Dolby B and C as
companding methods in a DAW, on the basis that they might offer some sonic
advantage. The answer to that is a definite No.

Yes, of course. It would be absurd to try to use any kind of traditional
Dolby noise reduction when recording digitally. Which is why we all
jumped to the obvious assumption that the OP was talking about the
decoding of previously recorded tapes. Admittedly, that assumption
should have been explicitly stated.

[Scott Dorsey]

Richard Crowley wrote:
"Scott Dorsey" wrote...
Joe the audio guy wrote:
Another quick question: Is Dolby B/C available as software and, if not,
why?

This has been discussed repeatedly here, and a google search on the group
is in order.

There is one company selling a "cassette restoration package" that does
Dolby B decoding, but other than that nobody else seems to have
implemented
it, and I don't know why.

Are you saying that there are no patent barriers?

There are none on Dolby B. I believe the whole purpose of Dolby C was to
get something to markey when the Dolby B patent expired.

Dolby A also is no longer in patent, but SR sure is.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Scott Dorsey]

Richard Crowley wrote:

Yes, of course. It would be absurd to try to use any kind of traditional
Dolby noise reduction when recording digitally. Which is why we all
jumped to the obvious assumption that the OP was talking about the
decoding of previously recorded tapes. Admittedly, that assumption
should have been explicitly stated.

Folks actually DID try using NR through digital systems back in the
early eighties. I knew a couple folks who used DBX systems running into
PCM F-1 machines and it did seem to tame some of the low-level buzziness
problems of the F-1.

Thank God that's all over.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Don Pearce]

Richard Crowley wrote:
"Don Pearce" wrote ...
Everybody is clearly reading this question differently to the way I am. If
the question is "Would it be a good idea to have an application that could
rescue a tape recorded in B/C when a proper playback system is
unavailable?" Then yes, I can see that it might have some limited
application for a while.

But that is not how I read it. My understanding of the question is that it
is asking if it would be a good idea to routinely offer Dolby B and C as
companding methods in a DAW, on the basis that they might offer some sonic
advantage. The answer to that is a definite No.

Yes, of course. It would be absurd to try to use any kind of traditional
Dolby noise reduction when recording digitally. Which is why we all
jumped to the obvious assumption that the OP was talking about the
decoding of previously recorded tapes. Admittedly, that assumption
should have been explicitly stated.



I always think it is better to assume that the poster meant what he said
and answer that, rather than instantly assuming he must have meant
something different. He can always clarify later.

d

[Mike Rivers]

Don Pearce wrote:

I always think it is better to assume that the poster meant what he said
and answer that, rather than instantly assuming he must have meant
something different. He can always clarify later.

Far too many posters here don't know the proper, or at least widely
accepted, terminology and can often be misleading if you take their
posts too literally. How many people write "jack" when they mean "plug,"
or "mix" when they mean "edit" or vice versa?

--
If you e-mail me and it bounces, use your secret decoder ring and reach
me he
double-m-eleven-double-zero at yahoo -- I'm really Mike Rivers
)

[Don Pearce]

Mike Rivers wrote:
Don Pearce wrote:

I always think it is better to assume that the poster meant what he
said and answer that, rather than instantly assuming he must have
meant something different. He can always clarify later.

Far too many posters here don't know the proper, or at least widely
accepted, terminology and can often be misleading if you take their
posts too literally. How many people write "jack" when they mean "plug,"
or "mix" when they mean "edit" or vice versa?


Of course, but this wasn't simply a matter of terminology but one of
basic concept; that of using Dolby B or C in the digital domain.

d

[Paul Stamler]

"Arny Krueger" wrote in message
...

(2) It appears that the software Dolby B decoder is set up so that Dolby
level = digital full scale. So, I would find out how much amplification I
had to apply in the digital domain to get the tone I recorded off the
calibration tape to be FS. I would then apply that much gain to any tapes
I
transcribe, before passing them through the software Dolby digital
decoder.

Oh dear. That's bad news, since very often tapes have signal recorded at
higher than Dolby level. What was in on cassettes, 185nW/m? Or 250?

Peace,
Paul

[Arny Krueger]

"Paul Stamler" wrote in message
...
"Arny Krueger" wrote in message
...

(2) It appears that the software Dolby B decoder is set up so that Dolby
level = digital full scale. So, I would find out how much amplification I
had to apply in the digital domain to get the tone I recorded off the
calibration tape to be FS. I would then apply that much gain to any tapes
I
transcribe, before passing them through the software Dolby digital
decoder.

Oh dear. That's bad news, since very often tapes have signal recorded at
higher than Dolby level. What was in on cassettes, 185nW/m? Or 250?

185nW/m comes to mind. Yes, it is my recollection that there were cassettes
that were recorded with peaks above Dolby level. I'm not sure that this is
as Dolby intended, but it did happen.

http://www.essex.ac.uk/dces/research...ubdocs/G43.pdf

"In practice the system employs a reference tone for calibration, where this
tone
is generated at the encoder to allow the decoder to be calibrated in level
prior to use. The
tone is set at the standard Dolby level, that in tape based applications is
related also to a
specific magnetic flux density on the tape (e.g. 185 nW/m Ampex NAB level
and 320 nW/m
DIN level)."

Dolby level corresponded to +3 Vu on a standard-conforming cassette deck.
My recollection is that going over 0 vu on a casstte machine was bad form.

[Scott Dorsey]

Paul Stamler wrote:
"Arny Krueger" wrote in message
...

(2) It appears that the software Dolby B decoder is set up so that Dolby
level = digital full scale. So, I would find out how much amplification I
had to apply in the digital domain to get the tone I recorded off the
calibration tape to be FS. I would then apply that much gain to any tapes
I
transcribe, before passing them through the software Dolby digital
decoder.

Oh dear. That's bad news, since very often tapes have signal recorded at
higher than Dolby level. What was in on cassettes, 185nW/m? Or 250?

It's not only bad news, it's very foolish, because even if Dolby B is
properly set up, you can expect VU meters to jump as much as 3 dB over
the Dolby reference level... which means peak levels are a lot higher
than that.

The reference level needs to be adjustable, and if it's not adjustable it
should be somewhere around -18 dBFS so that we have room to adjust the input.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Mike Rivers]

Arny Krueger wrote:

185nW/m comes to mind. Yes, it is my recollection that there were cassettes
that were recorded with peaks above Dolby level. I'm not sure that this is
as Dolby intended, but it did happen.

Oh, for sure. The Dolby level is, like everything else with analog tape,
a nominal level. While an analog tape deck doesn't have as much headroom
as we usually allow with a digital recorder, peaks of at least 10 dB
above the nominal level are expected. 185 nW/m is 0.5 to 1 dB lower than
cassette decks were normally aligned, so Dolby level should probably
correspond to somewhere in the ballpark of -15 dBFS.


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[William Sommerwerck]

185nW/m comes to mind. Yes, it is my recollection that there were
cassettes that were recorded with peaks above Dolby level. I'm not
sure that this is as Dolby intended, but it did happen.

I think 185nW/m is the right reference level.

Dolby's intent had nothing to do with the peak level, which was controlled
by the quality of the tape, heads, electronics, etc.

Interestingly, good cassette machines could tolerate higher peak levels with
dbx II than with Dolby. I never figured that out.