User talk:Balsa

Hi.

Balsa 03:28, 3 February 2011 (EST)

ALSA

/etc/asound.names
Gives names and descriptions for ALSA hardware devices. Very useful when using arecord and its '-D, --device=NAME' command line options.

/etc/asound.conf
I do not know what this is currently.

/etc/asound.state
Ditto.

strace - handy utility to see what programs are doing. strace -e file [some program and its args if relevant] - only shows file related stuff.

arecord - ALSA utility to record sound. It's what I'm having trouble with, and is the subject of most of this page.

sox - "Sound eXchange, the Swiss Army knife of audio manipulation" - (man sox; GNU General Public License)

strace -e file arecord -v -d 5 -t au test4.au ----> <snip>
Recording Sparc Audio 'test4.au' : Unsigned 8 bit, Rate 8000 Hz, Mono <snip>
Very interesting, indeed. This gives us the default ALSA device used by arecord, in my case it was /dev/snd/pcmC0D0c. Cross referencing with /etc/asound.names doesn't get us far, because I still don't know the mappings between that file and the stuff in /dev/snd. Some speculation may be in order.

grep -i capture -B 1 /etc/asound.names -->
name 'hw:0,4'
comment 'Physical Device - ALC268 Analog (Capture)'
--
name 'plughw:0,4'
comment 'Physical Device With Conversions - ALC268 Analog (Capture)'

Hmm.

Lets try: strace -e file arecord -v -d 5 -t au -D hw:0,4 test5.au
This opens /dev/snd/pcmC0D4c, and says "Recording Sparc Audio 'test5.au' : Unsigned 8 bit, Rate 8000 Hz, Mono" but exits with "arecord: set_params:954: Sample format non available".

strace -e file arecord -v -d 5 -t au -D hw:0,0 test5.au
Same deal, but opens /dev/snd/pcmC0D0c instead. Strange, since that is the same as the default.

TODO: Look at source code. Edit: nevermind.

Let's try one more thing - that /etc/asound.names gave me an idea...
strace -e file arecord -v -d 5 -t au -D plughw:0,0 test5.au
Hot dog it worked!
open("/dev/snd/pcmC0D0c", O_RDWR|O_NONBLOCK) = 4
open("test5.au", O_WRONLY|O_CREAT|O_LARGEFILE, 0644) = 3

sox test5.au -d --> Lots of clipping, noisy.
sox au: header size 24 is too small

Input File  : 'test5.au'
Channels  : 1
Sample Rate  : 8000
Precision  : 8-bit
Duration  : 00:05.00 = 40000 samples ~ 375 CDDA sectors
Sample Encoding: 8-bit Signed Integer PCM

100% 00:05.00 [00:00.00] of 00:05.00 Out:240k [!=====|=====!]Hd:0.0 Clip:182k
sox effects: rate clipped 120593 samples; decrease volume?
sox sox: alsa: output clipped 61507 samples; decrease volume?
Done.

Aha! The recording is unsigned 8bit, but the playback was signed 8bit. How does one change this?

man sox

sox -u test5.au -d
sox formats_i: `test5.au': overriding encoding type
Sample Encoding: 8-bit Unsigned Integer PCM
OK, it plays as unsigned 8bit. Success.