I'm trying to use a SoundBlaster 16 card with the "sb" driver in a
NetBSD-current snapshot from around March 17, 1995.  It almost works:
when I send a Sun (u-law) audio file to /dev/audio using "cp" or
"cat", the sound that emerges from my speakers is quite recognizable
but it is mixed with a rather loud, crackling interference.

I also tried a driver from early June with an AWE32, with similar
results.

Next, I tried adding the patch from the problem report "kern/1185:
audio driver prematurely considers buffers transferred"; it did not
help.

I also tried increasing the DMA block size to 1024 bytes (by changing
the last line of audio_calc_blocksize() to "return 1024;").  After
this change, the character of the noise changed into a series of
distinct clicks or pops spaced at about 1/8 of a second, suggesting
that the interference is due to some event occurring once per DMA
block.

My final test was to play a 1 kHz sine wave and digitize the resulting
audio output.  At the end of this message is a plot of the waveform
from about 1 ms before to 1 ms after one of the clicks (turn your
head 90 degrees to the right to fully enjoy the ASCII graphics).

Just to be complete, here are the boot-time probe messages:

Jul 10 07:14:54 araneus /netbsd: sb0 at isa0 port 0x220-0x237 irq 10 drq 1: dsp v4.13
Jul 10 07:14:54 araneus /netbsd: audio: unit 0 attached

Does anyone have any insight into the cause of this problem?
-- 
Andreas Gustafsson, gson@araneus.pp.fi

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