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[tomdolan <tomdolan@xxxxxxxxxxxx>]

George Brock-Nannestad wrote:

> 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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