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[tomdolan <tomdolan@xxxxxxxxxxxx>]

"Copeland, Peter" wrote:

> Dear All,
>     Well, I'm sorry this topic has now reached the stratosphere, but George
> has raised some interesting points. The rest of you may go back to sleep,
> since (as I said last time) we are now at the leading edge of technology.
>
> Point 1: Yes, I agree it's an artificial test, in the sense that one sample
> could not occur in a proper Nyquist-filtered analogue-to-digital converter.
> But I was talking about digital-to-analogue converters at the time, and a
> default standard test-procedure like I described seems to be evolving to
> document the performance of D-to-As. But I fully accept that I ought to have
> made it clear that I *include* an anti-aliasing filter in *both* A-to-Ds and
> D-to-As; they are in practice (and often physically) inseparable, and their
> overall performance should be documented (if only to show cases where
> anti-aliasing has been omitted, as in some digital signal processes!)
> Point 2: Sorry George, you are just plain wrong here. The widest uncorrupted
> frequency range (let's pick a nominal frequency of 22.0499999kHz to show
> what I mean) will inevitably mean a very considerable smearing along the
> time axis. No filter, analogue or digital, can cut sounds between
> 22.0499999kHz and 22.05kHz without generating what looks like "ringing" on
> (say) square waves. If "ringing" isn't synonymous with destruction of
> transient response, I don't know what is! It is possible our moderator may
> have something to say here. Analogue video signals must be "brick-walled"
> without video artefaces like ringing, and I have often wondered how it is
> done! But, for documenting the performance of A-to-D converters (or
> D-to-As), *including* their anti-aliasing filters, I cannot think of a
> better test than an impulse test.
> Point 3: A well-made point, but I am simply trying to document the
> performance of imperfect hardware. 24-bit test-gear will not help with
> 16-bit (or 8-bit) recordings, which we are now beginning to acquire!
> Peter Copeland
> <peter.copeland@xxxxx>
>
> -----Original Message-----
> From: George Brock-Nannestad [mailto:pattac@xxxxxxxx]
> Sent: 15 June 2001 13:09
> To: ARSCLIST@xxxxxxxxxxxxxxxx
> Subject: RE: arsclist reel to reel player/recorder
>
> From:                   "Copeland, Peter" <Peter.Copeland@xxxxx>
> To:                     "'ARSCLIST@xxxxxxxxxxxxxxxx'"
> <ARSCLIST@xxxxxxxxxxxxxxxx>
> Subject:                RE: arsclist reel to reel player/recorder
> Date sent:              Thu, 14 Jun 2001 11:47:01 +0100
> Send reply to:          ARSCLIST@xxxxxxxxxxxxxxxx
>
> Dear Peter, for me the third lesson has not ended yet. On 14 June
> 2001 at 11:47:01 you wrote (among other wise things):
>
> >     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.
>
> ----- Now, what is the relevance of this digital signal? It is a signal
> which would never be generated by an A/D converter in the 44.1, 16
> bit realm, because, due to the criterion promulgated by Nyquist,
> there are at least *two* samples to represent the highest frequency
> that one desires to keep track of. For this reason, an anti-aliasing
> filter is fitted before the A/D conversion, so that we never have any
> signal component above 20 kHz. The kind of signal you mention
> can only be generated by extremely stupid digital editing, and the
> smoothing filter fitted after the output of the D/A converter ought to
> reduce it to - nothing. However, trigger times (jitter) and other
> electronic problems may feed this brief high-level signal on the
> output of the D/A converter into various undesired places, and
> certainly some capacitors will be DC "pumped".
>
>  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.
>
> ----- Again, I am not entirely sure what you mean: the widest
> uncorrupted frequency range *will* give the best transient response -
>  unless there are time delay problems.
>
> >     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 comment here is that if you desire the precision that 24 bit
> really represents, you would want to look at some Agilent
> measuring equipment. They have digital voltmeters possessing that
> precision. What I would like the audio community to look at is price
> and the sample rate that is supported at that precision. Also the
> performance over time is interesting!
>
> Kind regards to all,
>
> George
> Preservation Tactics
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