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Re: Some thoughts on filters

Thanks for the clarifications, Charles. Inevitably when composing a
post on a complicated subject, there is the danger of glossing over the details. I don't disagree with what you wrote but I do ahve a couple of comments.

When you wrote:
> music largely consists of transients and not continuous waveforms,
> especially at high frequencies where these effects occur in digital
> systems. (We are starting now to see the first arguments against
> brickwall filters for music.)

I think we were both incorrect in referring to "transients": the band
limiting of the ADC's antialiasing filter has already done its damage
and smeared the transient information in time.

In this light, perhaps the biggest
step forward in recording hardware has been the recent adoption of
high sample rates with low-pass filters that are better-behaved in
the time domain. When the original data are downsampled to the CD
Standard the filter can be otimized for the musical demands, trading
off some of the out-of-band rejection against reduced dispersion of
transient energy. (See the Keith Howard Stereophile article in January
for some discussion of this.)

I think the point of the debate hinges on your following statement:

> I would like to point out that the signal after being brickwall
> filtered during A/D conversion has been significantly altered from
> the original. Instead of trying to perfectly recreate this
> significantly altered signal by using another brickwall filter at
> playback, perhaps a different filter (or no filter) would allow a
> better recreation of the *original* music signal before the A/D
> conversion.

This is an admirable goal, but there are too many unknowns. If you
as an DAC designer also designed the ADC, you could compensate
exactly for this. (This is, I believe, the crux of HDCD, where both
stages are under the control of one engineer, and embedded metadata tells the DAC how to compensate.) But without that control, you are
in the position of trying to integrate previously differentiated data:
one solution but an infinite series of possible originals.

Allowing image energy into the DAC output to approximate something
like the original waveform might have been is inelegant.

> Food for thought... <

Indeed. Offering up some early morning thoughts in response. -- John

John Atkinson
Editor, Stereophile





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