I note that Richard Hess kindly mentioned my name in his comments on
your Decoding Dolby page in relation to using selective (usually
high frequency) EQ
to remove encode/decode Dolby B/C artifacts. I do indeed subscribe to
that view, especially with slow speed tape media like cassettes. I
can bear this out, not in the sense of having made absolute laboratory
measurements of cassette tape signal degradation but having struggled
to decode many Dolby encoded tapes without selectively boosting the
likely weak highs.
Consider a case where the cassette is down by 10db
at 15kHz. It will probably also be down to a lesser extent at say 10
kHz but perhaps is still correct at 5kHz and lower. With standard
will never decode properly. In order to correct the loss at 15kHz w
e can boost the overall gain into the decoder by 10db but in doing
so, we have now created an error of the same magnitude (10db) at 5kHz
and below, and to a lesser extent above it. It makes more sense to
me to only boost the losses where they occur, or are likely to occur,
and leave everything else untouched. A bit like Ray Dolby's own
philosophy of "least treatment."
I note that back in the day, two manufacturers, Yamaha and NAD brought
out cassette decks with a "play trim" control. It was basically a cut
or boost treble control, but operating selectively on higher
frequencies than a normal "treble control", perhaps above 5 kHz,
and - of course - inserted ahead of the Dolby decoder. It was a
rather limited tool as it ganged both left and right channels together
and so couldn't compensate for one channel with more of a loss than
the other, and its corner frequency was fixed. Still, I think that for
what it was, it was a well thought out little feature that the average
home user could make use of on many Dolbyised cassettes with mild
tracking problems, but without getting too technical.
For our purposes of best decode, that tool (hardware or software)
could be built on, made a little more sophisticated. I also agree with
Richard that there is a place for adjusting overall gain pre the
decoder, but usually to a much lesser degree than for the more
troublesome errors in the higher frequencies. Perhaps plus or minus a
few db in normal use.
In 2010 I was appointed audio coordinator of a large cassette
digitisation project. I foresaw that some cassettes would be Dolby
encoded and so pondered the possible issues including "to attempt
Dolby decode at ingest, later, or both?" My recommendation was
that we not decode any cassettes "on the fly".
Instead I allowed for an easier decode digitally if needed, some time
in the future, as your project also aims to allow for. To facilitate
that I maintained one calibrated transfer level from the cassette
machines into the digital recorders. So the "Dolby Level" which
cassette decks use as a set internal reference, I translated to a
level in the digital file. From memory it was something like Dolby
Level equals -15dbFS, or 15 db below digital clipping. This meant
there was but one record level no matter which of the thousands of
tapes was captured. If anyone would like me to explain the rationale
behind that calibration in more detail I'd be happy to.
A lot of this comes down to knowledge of the original Dolby systems
including their strengths and weaknesses, not least when used with
cassettes. I tried recently to access some very helpful in depth
discussion with diagrams of the analog Dolby NR systems which I used
to find on Dolby's own website but I haven't been able to relocate
them. If anyone knows how to access these articles now or has a copy
I'd appreciate hearing from them.
----- Original Message -----
"Association for Recorded Sound Discussion List"
<[log in to unmask]>
<[log in to unmask]>
Tue, 16 Jul 2019 20:56:20 +0000
Re: [ARSCLIST] Preserving both raw and decoded files for tapes
recorded with Noise Reduction?
At Indiana University's MDPI project, we create a preservation master
that is the raw, undecoded output and a preservation
that is decoded. You are not alone! Both are created at the same time
one pass. We believe that both are needed for accurate preservation
content. By keeping the undecoded version, we hold open the
redoing in the future the highly subjective and often inaccurate
which Dolby (or no Dolby) to use and how much gain to apply before
circuit. This meets a basic media preservation principle around the
judgment calls, where they are viewed as potential weak links in the
preservation chain. This leads to our policy to preserve not only the
subjective product of a judgment call, but also a product that is not
result of subjective decisions.
More information on how we handle Dolby-encoded cassettes is on our
Director of Technical Operations, Audio/Video
Media Digitization and Preservation Initiative
From: Association for Recorded Sound Discussion List
<[log in to unmask]> On Behalf Of Richard L. Hess
Sent: Sunday, July 14, 2019 8:53 PM
To: [log in to unmask]
Subject: [External] Re: [ARSCLIST] [EXTERNAL] [ARSCLIST] Preserving
and decoded files for tapes recorded with Noise Reduction?
This message was sent from a non-IU address. Please exercise caution
clicking links or opening attachments from external sources.
Hi, Corey and Gary,
Thanks for your kind remarks about the decoder. My colleague and
Dyson has done a wonderful job with the code. His acid tests have
Dolby recordings of 70s pop music--some of them sound so bad until he
them...but they are tougher than the stuff I've recorded and obtained
What has happened is the intermod that is normally generated by fast
changes on decoding is vastly reduced.
As to my question, am I the only proponent of recording the raw,
output? It's saved my bacon more than once, and I've been insisting
on it for
at least a decade. I was hoping that some standards/best practice
recommended it. I did not think I was alone.
John Chester, thanks for the info on 384 kHz sampling frequency and
Remember my effort here?
The only major recorders that are problematic (i.e. bias frequencies
Ampex ATR-100 (432 kHz)
Sony APR-5000 and probably multitracks (400 kHz) Studer A80VU (240
late models are 150 or 153.6 kHz,
the A77 is 120 kHz)
Otari MTR-10/12 and MTR-90 (246-250 kHz)
On 2019-07-14 7:16 p.m., Gary A. Galo wrote:
> Hi Richard,
> I echo Corey Bailey's email in congratulating you on the
> decoder. I'm sure there will be a considerable market for it.
> The issue of preserving the "original" data - whether analog of
digital - is
> a sticky and controversial one. When I gave my ARSC presentation on
> transferring PCM-F1 format digital recordings for the NY ARSC
> 2018, I was taken to task by one attendee for not preserving the
> bits. I go from the S/PDIF output of my PCM-601ESD digital
> directly into a Tascam DA-3000 digital recorder. The Tascam has a
> switchable sample rate converter based on the Cirrus Logic CS8422
> (which doubles as the S/PDIF input receiver). I set the Tascam to
> 88.2 kHz, so the CS8422 is converting 44.056 to 88.2. An
> feature" of the DA-3000 recorder is that the CS8422 SRC chip also
> uSec de-emphasis, which take care of another issue with F1
> Tascam fails to mention this anywhere in their manual or product
> is beyond me, because the de-emphasis feature is clearly stated on
> page of the CS-8422 data sheet, and it's an extremely useful
> With this method, only the inter-channel time delay and DC offset
> to be addressed once the 88.2 kHz data is on your computer.
> My method does not save the original 44.056 kHz bits. Guilty as
> But, the CS8422 does a beautiful job with the SRC and the
> has ultra-low jitter clock recovery to boot, so I sleep well at
> you feel the need to preserve the original bits, you could run a
> transfer directly into your computer, if your computer will lock
> kHz. Or, you could use a digital distribution device to split the
> data stream, sending it to both the computer, and the DA-3000
> simultaneously. But, I just don't see the need.
> So there is no misunderstanding, I can well understand the desire
> preserve the non-decoded Dolby-A analog signal in case better
> conversion becomes available down the road. It makes sense to do
> perhaps I'm being inconsistent. These are thorny issues, and
> have their own viewpoints.
> Gary Galo
> Audio Engineer Emeritus
> The Crane School of Music
> SUNY at Potsdam, NY 13676
> "Great art presupposes the alert mind of the educated listener."
> Arnold Schoenberg
> "A true artist doesn't want to be admired, he wants to be
> Igor Markevitch
> "If you design an audio system based on the premise that nothing is
> audible, on that system nothing will be audible."
> G. Galo
> -----Original Message-----
> From: Association for Recorded Sound Discussion List
> [mailto:[log in to unmask]] On Behalf Of Richard L. Hess
> Sent: Sunday, July 14, 2019 5:42 PM
> To: [log in to unmask]
> Subject: [EXTERNAL] [ARSCLIST] Preserving both raw and decoded
> tapes recorded with Noise Reduction?
> Hi, I think many of us agree that it's necessary to preserve both
> raw transfer and the decoded version of a file which has been
> with Dolby or DBX type noise reduction.
> When I first thought about it, I never imagined I'd be part of a
> that would produce a better decoder for Dolby A encoded tapes than
> Dolby, but it's happening and humbling... So, it is a good idea to
> save as much raw data as possible because who knows what else will
> Plangent is wonderful, but a bit problematic as it is still
> inconvenient to properly archive the bias, but that's another
> and I think in the long run it would be good if we could do that.
> MY QUESTION is: Are there any standards or recommendations that say
> "keep the raw undecoded copy as well as keeping the decoded copy?
> It's for a paper that Federica and I are writing.
Richard L. Hess email: [log in to unmask]
Aurora, Ontario, Canada 647 479 2800
Quality tape transfers -- even from hard-to-play tapes.
Email sent using Optus Webmail