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RE: Some DVD-A not delivering Hi Def Content

Posted by Werner (A) on March 28, 2010 at 01:31:15

In Reply to: RE: Some DVD-A not delivering Hi Def Content posted by antoneb on March 27, 2010 at 12:38:28:

"a characteristic sound of a lot of digital recordings using FIR filters is a subtle chirp or pre ring?"

The pre-ringing induced by the ADC's anti-aliasing filter (and not the DAC's anti-imaging filter, which can be proven not to ring at all when stimulated with a correctly band-limited signal!) is situated above 20kHz. It is on its own not audible by the majority of people. There are voices that it could be detectable by some: when the pre-ringing extends in time beyond the temporal width of the highest cochlear filter (~400 us), then it might trigger undue desensitization of the inner ear, which would sound as a form of compression. This of course only with pathological test signals, as in normal music there is so much going on simultaneously that an isolated pre-ring has no chance to surface.

"I paper I read on a FIR filtering a square wave at 2khz resulted in a very noticeable chirp, vs a negative feedback filter."

Yes. But that is entirely predictable and hardly a secret.

"Have the filter designers found a way to throw away the negative portion of the impulse response and keep phase linear?"

That would be mathematically impossible.

"My conversation about zero phase filters happened about 4 years ago sky walkers in house acoustician. Who was responsible for all if the modernization of the screening rooms post THX dude. There seemed to be a lot of dis satisfaction with FIR filters at least in his circle. "

This must be seen in its context, which most probably was NOT anti-aliasing and anti-imaging at the edge of the audible band.

FIR pre-ringing is detrimental if it is applied in the audible band. This happens for instance in the bandpass filters used in perceptual coding (i.e. MP3, AC3, DTS, ...) although eventually all induced pre-ringing cancels as the several bands are summed again upon decoding.

Another important case is that of digital loudspeaker crossovers: if FIRs are used then the low-pass and high-pass sections both pre-ring. This, again, is canceled when the signals are acoustically summed. Now this summing is non-perfect as the two speakers are not coincident, and the acoustic paths of the sounds are not identical. In this scenario pre-ringing can become clearly audible.


regards,

Darth

And then you wake up and realise that your classmates of old ... are running most of the TV shows.

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