[Flac] Standard encoding rates?

[Curt Sampson]

On Wed, 6 Apr 2005, Brian Willoughby wrote:

> 7 kHz would require a tighter brick-wall filter than even CD, are you even
> sure it's possible to go from flat to silent in 100 Hz (between 3400 Hz and
> 3500 Hz)?

It's not how many Hz, but how many octaves. As well, "flat" to "silent"
is relative, you can probably afford to start rolling off a bit earlier
(say, put the filter at 3400 Hz, since filters are spec'd at their -3db
points if I recall correctly) and it just has to get "silent enough"--if
you're using 8-bit audio the spectrum at 3500 Hz needs be reduced only
half as much as it would for 16-bit audio.

Still, that is a filter about twice as steep as one used for a CD. On
the other hand, oversampling and digital filters can help greatly with
this, and with the slower sample rates and having fewer bits to process
you can oversample further for the same price.

> Almost all telephone connections are digital. Certainly long distance,
> and probably local as well.

Oh, yes, definitely. You would have to have a very, very old switch
to have analogue past your local loop. Last one I saw served only a
few thousand people spread across a few villages on the west side of
Vancouver Island, and even that was retired in the mid-90s.

> In other words, the
> standard sampling rate for telephone voice is certainly 8 kHz.

That's certainly what it's sampled at when converted (at your ISDN TA,
at a multiplexer, or at the switch end of your local loop); 64 Kbps
is the standard for a digital channel in telephone equipment. (The
reason you sometimes get only 56 Kbps for data is due to encoding and
synchronisation issues; with some encoding systems sending long streams
of certain values, say a constant stream of zeros, will cause the other
end to lose sync.)

So have to resample to change that sampling rate. Data compression is
probably your better bet.

cjs
-- 
Curt Sampson  <cjs at cynic.net>   +81 90 7737 2974   http://www.NetBSD.org
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