[Flac] Standard encoding rates?

martin@kfib.org
Tue, 05 Apr 2005 08:45:28 -0000

Ralph Giles <giles@xiph.org> writes:

> On Tue, Apr 05, 2005 at 02:26:03AM -0500, Hal Vaughan wrote:
>
> > Is there a list somewhere of "standard" encoding rates?  I
know, for example,
> > CDs are encoded at 44100, as is a lot of digital sound, but I've
seen
> > programs that specify different levels of quality (like radio, phone,
tape,
> > CD) and I'd like to know if there are some encoding rates that are
accepted
> > as standardized for recording at different levels of quality.
>
> Well, where analog formats are concerned these are estimates, and in
> there is no *standard*. "CD quality" is 44100 Hz stereo with 16
bits per
> channel. FM radio is limited to 17 kHz iirc, so in theory you could
> sample at 32 kHz, but in practice people usually use 44.1 or 48 kHz and
> just lowpass filter. AM radio is lower quality (mono) but I don't know
> what the digital equivalent would be. Telephone is nominally 8 kHz mono
> (i.e. really bad) though I think the use of digital voice codecs in the
> last 20 years may have improved on this a bit.
>
> Maybe someone else can comment on tape fidelity. I think the issue there
> has more to do with recording artefacts than bandwidth.

Typical sample rates for music is 32, 44.1, 48, 96 and 192 kHz. FM
radio is limited to 15, not 17, kHz so 32 kHz is an adequate sample
rate for such broadcasts. But, as already stated, it's common to
sample in 44.1 or 48 kHz, although in that case you should make sure
you notch out 19 kHz. Hard. Otherwise you'll sample the stereo sync
tone as well. It's already filtered with -60 dB (approx) in the
reciever, but even so it might add information that makes it harder to
compress the material. Also, FM-recievings usually contains a lot of
high frequency artefacts that will make it hard for flac to work
efficiently. If you can't filter these things out, I'd recommend 32
kHz.

Tape can reach anything from 14 to 24 kHz (lousy recordings might have
even lower quality than 14 kHz, of course) but as an general rule I'd
say that 32 kHz sample rate is sufficient if we're talking normal
tapes. Chrome and metal tapes might be a reason to use higher sample
rates (depending on the equipment), or, of course, sheer lazyness.
Except from noise there shouldn't be any need for filtering anything
out.

Reel-to-reel-tapes on higher speeds easily mark 20 kHz so there a high
sample rate is a good thing. AM-broadcast is limited to 4.5 kHz. That
means that 8 kHz sample frequency might filter out some of the
bandwidth, but I doubt you'll notice much difference considering the
crappy signal to begin with. If you do, use 11.025 or 16 kHz sample
rate.

LP:s etc are usually limited to 16 kHz in the mastering, so there's no
real point sampling those at higher rate as 32 kHz as well. Also,
they're usually gradually going from stereo to mono between 150 to 50
Hz so if you can "monoify" the signal from 50 Hz and down you'll
also
filter the rumble (it's in 180? phase between the channels) which also
will improve the possibility to compress the material, aside from the
possibility to play it back LOUD without wasting amplifier power or
risk the speakers.

In general; use your ears. If you can't hear the difference, does it
matter...?
--
Martin Persson           martin@kfib.org
http://ss.kfib.org/

  Friendly fire isn't.