[linux-audio-user] 192kHz

[Gene Heskett]

On Tuesday 07 February 2006 04:26, David Cournapeau wrote:
[...]
>
>Also, both frequency range and bit width have influence on maximum
>SNR: bitwidth change is obvious; in the case of frequency range, you
>can think as a way to "spread" the "floor noise" on higher bandwith,
>effectively decreasing the level on the hearing range. In ideal
>conditions which never happen in the reality, one bit more is 6 dB, and
>a doubling of frequency bandwith is 3 dB.
>
I've taken the liberty of a spelling correct above s/on/one to make it 
correct.

>Nyquist is an *exact* theorem. Mathematically, any function without
>any frequency above fr/2 can be *exactly* characterized by its sampled
>coefficient. Now, it assumes a perfect filter (infinite slope, no
>modification below the frequency cut) which does not exist.
>
>David

My thoughts on this run toward a 1 bit sample at 256khz (or more since 
we could do it at many megahertz today) driving both the output data 
stream, and a 24 to 32 bit counter whose input is the same clock, and 
the direction bit is this 1 bit a/d's output.  The catch here is that 
the 1 bitter is comparing the output from a d/a driven by this counter 
with the next sample to make its data bit that then slews the counter 
one way or the other by this one bit.

Such a device would be slew rate limited at the higher frequencies, but 
should not generate any aliasing products whose magnitudes would be 
greater than the bit ratio allows, and its input anti-aliasing filter 
could be very simple.  Its output would already be pretty well 
compressed, particularly as the slew rate approaches the limit, and 
further digital processing of the 32 bit range down to 24 bit might 
offer a considerable amount of additional compression (you cand read 
that as noise reduction) to be done.  It would of course assume a 
counter driving the d/a that did not overflow in either direction, and 
some sort of circuitry to attempt to ac couple the signal by doing a 
very slow slew toward the 50% point exerted over a t=rc of about .2 
seconds, which should be ignored for the most part by the reproduction 
system.

Such a devices nyquist limitations and the subsequent aliasing would be 
a long way below the human ears range of being able to detect them 
under any conditions I can imagine, even by those who can hear an 
acoustic doppler burglar alarm.

That said, its also an old idea/technique, waiting only for sufficiently 
well behaved electronics which we've had for what, 2 decades or more 
now?

Or has this quietly occured in the utilitarian devices we buy today 
without being an important enough detail to make it to the propaganda 
on the box?  Are these then considered to be implementation details 
that are considered proprietary/trade secrets & best not mentioned, 
yadda yadda?

I don't know, so you all tell me please.

-- 
Cheers, Gene
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