From alsa-devel-owner@alsa.jcu.cz  Wed Jan  6 13:59:23 1999
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Date: Wed, 6 Jan 1999 13:57:00 +0100 (CET)
From: Jaroslav Kysela <perex@jcu.cz>
To: Andrew Clausen <clausen@alphalink.com.au>
cc: alsa-devel@alsa.jcu.cz,
        "audiotechque@fmc-container.mach.uni-karlsruhe.de" <audiotechque@fmc-container.mach.uni-karlsruhe.de>
Subject: Re: Recording and PSL
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On Wed, 6 Jan 1999, Andrew Clausen wrote:

> > I have started to write an introductory document to audio recording
> > (playing). I will post it here too because there are many question.
> > Are there already documentation available on fine details of audio
> > recording (playing)? I looked at ALSA driver docs but they were quite
> > rough, higher level talk. They didn't tell much about buffers, IRQs,
> > etc. If there are no that level documents, you may want to wait my
> > questions before explaning.

ALSA driver hides things like IRQ etc... We have new document for alsa-lib
functions which explains more things about ALSA buffering, but seems that
person which does corrections (English isn't my native language) is very
busy.. Any volunteer?

> > >ALSA doesn't (afaik) support mmap, but OSS does.
> >
> > To the wishlist. I will eventually look at the code but I can't promise
> > any results. We need it because memory mapping speeds up the recording.
> 
> How significant is the speed-up?

Many people things that mmaped access increase speed, but you save only
ONE copy of data between kernel and user space. This operation is
strongly optimized for most of CPUs today available and I think that this
operation doesn't eat above 1% (real value is probably very much lower) of
CPU time.

Note that mmaped access doesn't give any chance to lowlevel driver do own
data processing for some special hardware - like playback over GUS
synthesizer (GF1 chip) or full-duplex operation for SB16 soundcards.

I think that ALSA offers sufficient support for PCM recording/playback
with enhanced buffer control (you can set both low and high wakeup
watermarks) and overrun/underrun detection for application.

							Jaroslav

-----
Jaroslav Kysela <perex@jcu.cz>
Academic Computer Centre, University of South Bohemia
Branisovska 31, C. Budejovice, CZ-370 05 Czech Republic


