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SubjectRe: Full duplex using 2 sound cards.
On Tue, 28 Jan 1997, Ian Main wrote:

> Hi, as you guessed, I'm trying to get full duplex sound using 2 sound
> cards. One is a mad16, and the other a sb16. Both work fine by
> themselves, but when I try to use them together, the sb16 behaves a little
> strangely. (mad16 works fine.) The sb16 works fine for 8 bit IO, but 16
> bit playback and record is very jumpy. It misses large sections of the
> sound (I assume it's a buffer problem.)
It's very tricky to do recording of CD quality audio to a disk file in
Linux. The process just sometimes get's blocked for disk I/O for too long
time. This seems to happen every time bdflush/sync starts posting dirty
buffers to disk. When using 44.1 kHz/16 bits/stereo format the maximum
possible DMA buffer (64k) becomes full after 0.37 seconds which causes an
overrun. However with a fast (SCSI) disk it's possible to record without
problems. I don't know exactly which causes the difference between SB16
and the MAD16 card but it's possibly related with different DMA overhead
caused by the cards (could this be possible?).

To be able to do simultaneous recording and playback reliably you should
implement extra buffering in the application. This probably means that you
should use multible threads (or processes communicating through a shared
memory segment). One thread reads data from the device as fast as possible
and stores it into a buffer (FIFO). The second thread reads data from the
buffer and posts it to the disk file. For simultaneous playback you
probably need more threads.

Btw, allowing some kind of buffer cache write through would be an usefull
feature. Some applications (like data logging) work better blocks written
by an application are sent to the disk immediately. In this way the disk
I/O caused by the application is distributed over longer period of time
and it doesn't become an bottleneck. This kind of mode could be turned on
using fcntl() or something like it so that it doesn't disturb normal
applications. Is anybody interested in implementing this kind of feature
in Linux?

Best regards,

Hannu Savolainen (, (Open Sound System (OSS)) (OSS Free/TASD/VoxWare)

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