Digital Drive: Re: A DAC's "Brickwall Filter" is *Not* the Analog Post Filter... by Werner Upsamplers, DACs, jitter, shakes and analogue withdrawals, this is it.

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Posted by Werner (A ) on August 27, 2003 at 03:58:47

In Reply to: A DAC's "Brickwall Filter" is *Not* the Analog Post Filter... posted by Todd Krieger on August 27, 2003 at 01:04:25:

"since in the case the 320/147 method is not utilized, the signal at the new rate will have to be calculated off an *interpolated* (filtered) signal"

It remains asynchronous, interpolated, and 'lossy' even in the 320/147 case. Think it through.

"The Fourier *transform* of the sinc function in the time domain is the "brickwall" response in the frequency domain."

Still missing out on the fact that the transform of the step in the time domain (as output by a practical ZOH DAC) is the Sinc function in the frequency domain. Which is a very interesting thing in that it tells us, and should tell yourself, that you have remarkable large holes in your knowledge of signal theory. Holes large enough to sit back and reconsider a thing or two.

"The upsampler first filters the signal like an oversampling DAC, and *then* does the conversion."

Care to explain this? How can one filter without first (actually at the same time) extending the frequency space that is available for the very filtering, i.e without actually up sampling the data stream.

"But which DAC would you rather listen to- one that rings from transients, or one that does not?"

And pray, where do you get a transient from in a correctly-digitized source signal?

"Try to find an audio upsampler chip or DAC data sheet that mentions "dither" in it..."

Actually a practical audio up/oversampler without using dither would be a rather incompetent design.

Link below is a Google search on "oversampling filter datasheet dither". "Upsampler" is not quite an accepted term outside of consumer audio marketing.

• http://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=oversampling+filter+datasheet+dither   (Open in New Window)

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