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.

Simple tests from command line

netstat - man netstat

look for services that you don't know, read their manual pages. See if the service would be better listening to fewer interfaces [like localhost]. Use the ps command to figure out what's running. Try the netstat flags "--inet -anp" and read what they mean in the netstat manual.

look at /etc/fstab

man fstab man mount

Read a security article [fixme] about this file and the options. Skim the LVM and partitions pages here on slackwiki. Read about the noexec option (don't use this without understanding the consequences)

Try "hostname --fqdn" and see what it does. Check the exim page or qmail page to see about local mail delivery. Some security tools (and other stuff too) will email you when important events occur.

Read about the sudo and su commands.

--Balsa 16:15, 20 February 2011 (EST)