Re: [linux-audio-dev] Realtime

New Message Reply About this list Date view Thread view Subject view Author view Other groups

Subject: Re: [linux-audio-dev] Realtime
From: eli+@gs211.sp.cs.cmu.edu
Date: Thu Jun 29 2000 - 01:34:40 EEST


Tom Pincince wrote:
> As surround sound makes it's
> way into society, a significant opportunity will present itself
> regarding the psychoacoustic perception of location and movement of
> individual sounds within the sound field. The ability to stream audio
> as single samples and control the rate of playback with sub-sample
> precision would open up some impressive opportunities.

I'm intrigued but confused. You're suggesting we could achieve
sub-sample timing by making use of sub-sample scheduling latency,
right? And presumably achieving it in a manner less expensive than
the one (long FIR filters) we use in the absence of sub-sample
scheduling. How?

I think you want to control the phase of the D/A clock, so as to place
samples wherever you choose. If that's not it, disregard the rest.

If you can build a D/A that supports this phase control (that
anti-alias filtering has got to be a big old pain...), you could make
it take an instantaneous "right now" phase modulation register,
requiring usec realtime control as you describe. But you could
instead build it to take a phase-mod signal, clocked at N times the
audio rate, which you lay out alongside the audio for it to read. So
to get sub-sample phase control you don't need sub-sample scheduling,
any more than sample-precise audio needs sample-precise scheduling.

Also, I'm only seeing how this works when a sound is cued against a
background of silence. If two sounds need to be mixed, you need to
resample one to the other's sample grid (or you need two of these
D/As), as far as I can see. Or is there some way to finesse this
without trashing the stereo image? Depends on spatial perception,
so I don't know.

As long as we're talking blue-sky, I'm tempted to just up the audio
clock rate until sub-sample precision is unnecessary and time-shifting
becomes trivial. Use that monstrous Sony one-bit-at-megahertz-clock
format, even.

-- 
     Eli Brandt  |  eli+@cs.cmu.edu  |  http://www.cs.cmu.edu/~eli/


New Message Reply About this list Date view Thread view Subject view Author view Other groups

This archive was generated by hypermail 2b28 : Wed Jun 28 2000 - 23:00:25 EEST