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Hi Mike,

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
playback, it
 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.

Tim Gillett

Perth,
Western Australia

 

----- Original Message -----
From:
 "Association for Recorded Sound Discussion List"
<[log in to unmask]>

To:
<[log in to unmask]>
Cc:

Sent:
Tue, 16 Jul 2019 20:56:20 +0000
Subject:
Re: [ARSCLIST] Preserving both raw and decoded files for tapes
recorded with Noise Reduction?

 Hi Richard,

 At Indiana University's MDPI project, we create a preservation master
file 
 that is the raw, undecoded output and a preservation
master-intermediate file 
 that is decoded. You are not alone! Both are created at the same time
during 
 one pass. We believe that both are needed for accurate preservation
of the 
 content. By keeping the undecoded version, we hold open the
possibility of 
 redoing in the future the highly subjective and often inaccurate
choice of 
 which Dolby (or no Dolby) to use and how much gain to apply before
the Dolby 
 circuit. This meets a basic media preservation principle around the
nature of 
 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
the 
 result of subjective decisions.

 More information on how we handle Dolby-encoded cassettes is on our
blog:

 https://blogs.iu.edu/mdpi/

 Mike

 ---------------
 Mike Casey
 Director of Technical Operations, Audio/Video
 Media Digitization and Preservation Initiative
 Indiana University

 https://blogs.iu.edu/mdpi/
 https://mdpi.iu.edu/

 -----Original Message-----
 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
both raw 
 and decoded files for tapes recorded with Noise Reduction?

 This message was sent from a non-IU address. Please exercise caution
when 
 clicking links or opening attachments from external sources.
 -------

 Hi, Corey and Gary,

 Thanks for your kind remarks about the decoder. My colleague and
friend, John 
 Dyson has done a wonderful job with the code. His acid tests have
been leaked 
 Dolby recordings of 70s pop music--some of them sound so bad until he
decodes 
 them...but they are tougher than the stuff I've recorded and obtained
from 
 other sources.

 What has happened is the intermod that is normally generated by fast
gain 
 changes on decoding is vastly reduced.

 As to my question, am I the only proponent of recording the raw,
undecoded 
 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
body 
 recommended it. I did not think I was alone.

 John Chester, thanks for the info on 384 kHz sampling frequency and
bias.

 Remember my effort here?
 http://richardhess.com/notes/2008/02/02/tape-recorder-bias-frequencies/

 The only major recorders that are problematic (i.e. bias frequencies
above 180 
 kHz are:

 Ampex ATR-100 (432 kHz)
 Sony APR-5000 and probably multitracks (400 kHz) Studer A80VU (240
kHz, most 
 late models are 150 or 153.6 kHz,
 the A77 is 120 kHz)
 Otari MTR-10/12 and MTR-90 (246-250 kHz)

 Cheers,

 Richard

 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
software-based NR 
 > 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
chapter April 
 > 2018, I was taken to task by one attendee for not preserving the
original 
 > bits. I go from the S/PDIF output of my PCM-601ESD digital
processor 
 > directly into a Tascam DA-3000 digital recorder. The Tascam has a
built-in, 
 > switchable sample rate converter based on the Cirrus Logic CS8422
SRC chip 
 > (which doubles as the S/PDIF input receiver). I set the Tascam to
record at 
 > 88.2 kHz, so the CS8422 is converting 44.056 to 88.2. An
"undocumented 
 > feature" of the DA-3000 recorder is that the CS8422 SRC chip also
does 50/15 
 > uSec de-emphasis, which take care of another issue with F1
recordings. Why 
 > Tascam fails to mention this anywhere in their manual or product
literature 
 > is beyond me, because the de-emphasis feature is clearly stated on
the front 
 > page of the CS-8422 data sheet, and it's an extremely useful
feature.
 >
 > With this method, only the inter-channel time delay and DC offset
still need 
 > 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
charged. 
 > But, the CS8422 does a beautiful job with the SRC and the
de-emphasis, and 
 > has ultra-low jitter clock recovery to boot, so I sleep well at
night. If 
 > you feel the need to preserve the original bits, you could run a
second, raw 
 > transfer directly into your computer, if your computer will lock
onto 44.056 
 > kHz. Or, you could use a digital distribution device to split the
44.056 kHz 
 > data stream, sending it to both the computer, and the DA-3000
recorder 
 > simultaneously. But, I just don't see the need.
 >
 > So there is no misunderstanding, I can well understand the desire
to 
 > preserve the non-decoded Dolby-A analog signal in case better
software 
 > conversion becomes available down the road. It makes sense to do
this. So, 
 > perhaps I'm being inconsistent. These are thorny issues, and
everyone will 
 > have their own viewpoints.
 >
 > Best,
 > Gary
 >
 > ____________________________
 >
 > 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
believed."
 > 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
files for 
 > tapes recorded with Noise Reduction?
 >
 > Hi, I think many of us agree that it's necessary to preserve both
the
 > raw transfer and the decoded version of a file which has been
recorded
 > with Dolby or DBX type noise reduction.
 >
 > When I first thought about it, I never imagined I'd be part of a
team
 > 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
come 
 > along.
 >
 > Plangent is wonderful, but a bit problematic as it is still
 > inconvenient to properly archive the bias, but that's another
story,
 > 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.
 >
 > Thanks!
 >
 > Cheers,
 >
 > Richard
 >

 -- 
 Richard L. Hess email: [log in to unmask]
 Aurora, Ontario, Canada 647 479 2800
 http://www.richardhess.com/tape/contact.htm
 Quality tape transfers -- even from hard-to-play tapes.

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