RE: arsclist reel to reel player/recorder

["Copeland, Peter" <Peter.Copeland@xxxxx>]

Dear All,  Once again I am having to apologise for being out-of-synchronism
in this debate, because I am on another continent. And I would like to thank
Richard Hess for his comments.
    When we are at the leading edge of technology, my little grey cell
thinks we ought to test sound storage media (whether digital or analogue)
using *both* our ears *and* suitable test-equipment. This was universal in
analogue sound-recording days, but unfortunately digital gurus tend to sit
on one side of the fence or the other. So I adopted this principle when we
began digitising sounds in 1982 with the Betamax F1 system. I did all the
usual analogue tests (which the equipment passed), and using the best
loudspeakers in listening-rooms which were familiar to the victims, we
compared signals which had been through a PCM-F1 coder-and-decoder with
those which had not. All the victims could near no difference, even when the
operator was free to compare "before" with "after" on a changeover switch.
And that was in 1982! (For the pedants, I will agree we didn't use "double
blind" test procedures).
    Our current policy is that operators should continue this process, and I
do random checks upon the results. Frankly, I am baffled that people claim
to be able to hear differences between different CD players playing copies
of the same discs, let alone different types of analogue-to-digital
converters. However, I am prepared to admit that my hearing may not be what
it was; so I am trying (with the aforementioned limited resources) to
understand why.
    Classical information theory describes something called "the impulse
response test". In this, a very short transient spike is presented to the
equipment under test, and the resulting output is recorded. The British
magazine "Hi-Fi News" was recently been comparing top-end CD players using
an impulse response. Perfect impulses are easy to generate in the digital
domain. You have a large number of consecutive zeros, then change just one
sample to +32767 (for sixteen-bit tests), and you can buy CDs with this
test. The resulting analogue signals coming out of Hi-Fi News' CD players
were all totally different from each other! The reviewer didn't go into the
differences very deeply, but being familiar with the information-theory
issues, I could see that some were made to give the widest uncorrupted
frequency ranges, others made to give the best transient responses, etc.
    And this is only high-end digital *reproduction*! We are currently
attempting to tackle analogue-to-digital *converters* in a similar way, and
develop a standard methodology so we may document the performance of such
converters for all time. As far as I know, the AES has made no
recommendations for such a test, but we are not in the main British Library
building, so I cannot confirm this; can anyone correct me?
    My aim is that a load of work done on (say) a particular Macintosh
computer can have the performance of its converter stored alongside its
resulting CD-Rs. It is still early days to present my conclusions, but I
hope to talk to some of you at the ARSC/IASA Conference in September.
*Correcting* distortions shown up by such a test will be the next stage; I
am sure John Watkinson could describe the limits of what would ever be
possible.
    I know I count as a snotty old fart too; I can only plead that I am
doing my best!
Peter Copeland
<peter.copeland@xxxxx>

-----Original Message-----
From: Doug Pomeroy [mailto:pomeroyaudio@xxxxxxx]
Sent: 14 June 2001 02:55
To: ARSCLIST@xxxxxxxxxxxxxxxx
Subject: Re: arsclist reel to reel player/recorder


Don,

Thanks for clarification. I definitely disagree.

You can't copy *anything* forever without SOME loss.
But high-quality digital equipment DOES allow virtually endless
copying WITHOUT loss, digital bit-for-bit. This has been demonstrated.
To suggest that this is "not good enough" strikes me as extreme
stogginess, even without mentioning the weaknesses of the alternatives!

I agree with your admonition about "processing in the digital domain",
but this is *precisely* why archival/preservation work must NEVER
include any digital signal processing. I believe this is fully understood
by all those doing archival preservation work these days.

The matter of "lossy compression" is totally irrelevant, as you seem
to acknowledge.

You say

> "given an old tape or disc, playing it back so
> that nothing is lost is hard,

but I would assure you that there are qualified engineers who CAN
do this extremely well. In spite of what you may have read in the
popular press, which always makes it out as some kind of black art,

> and converting from analogue to digital
> without losing more is also hard.

Wrong. There is nothing "hard" about it. It just requires good equipment,
musical sensibility and a knowledge of the medium(s) involved. And
experience doesn't hurt, of course. I'd venture to say it's only "hard" if
you are dealing with people who don't understand the music and/or the
medium.

I don't mean to sound like a snotty old fart, but there has been SO MUCH
nonesense spread around about these matters, I eventually start to loose
my temper a little bit. Forgive me.

Cheers,

Doug
-------
>From: Don Cox <doncox@xxxxxxxxxxxxxx>
>To: ARSCLIST@xxxxxxxxxxxxxxxx
>Subject: Re: arsclist reel to reel player/recorder
>Date: Wed, Jun 13, 2001, 7:36 PM
>

> On 13-Jun-01, Doug Pomeroy wrote:
>
>>> The problem is getting from one to the other without losing
>>> information.
>>
>> What does this mean? Why should there be a loss? I havn't read
>> Watkinson's book, so maybe the answer is there. My understanding is
>> that the advantage of digital is precisely that program can be
>> migrated *without* loss of info, error correction being extremely
>> robust. Contrast that with analog tape, where frequency-response and
>> phase variations are *invariably* imposed with every copy, not to
>> mention the matter of added tape hiss and modulation noise.
>
> Once you have a digital signal, you can copy it repeatedly with no loss.
> If you start processing it in the digital domain, there may be problems
> with aliasing.
>
> Analogue signals, as everyone knows, degrade with each stage of copying.
> So do any digital signals on which lossy compression has been used (MP3,
> Minidisc, etc), but nobody would use those for archiving.
>
> What I am saying is that, given an old tape or disc, playing it back so
> that nothing is lost is hard, and converting from analogue to digital
> without losing more is also hard.  This is what other people have said
> in their mails.
>
> Regards
> --
> Don Cox
> doncox@xxxxxxxxxxxxxx


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