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VSOUND - a virtual audio loopback cable
This program allows you to record the output of any standard OSS
program (one that uses /dev/dsp for sound) without having to modify or
recompile the program. It uses the same idea as the esddsp wrapper
from the Enlightened Sound Daemon (in fact, vsound is based on
esddsp). That is, it preloads a library that intercepts calls to open
/dev/dsp, and instead returns a handle to a normal file. It also
intercepts ioctl's on that file handle and logs them, to help convert
the audio data from its raw form. Vsound then uses sox to convert the
raw data to the desired file format.
The upshoot of this is that instead of playing sound to the sound card
in your computer, the data is recorded to a file. This is similar to
if you connected a loopback cable to the line in and line out jacks on
your sound card, but no DA or AD conversions take place, so quality is
not lost.
One use of vsound is to help convert real audio files to some other
format. Since the real audio format is proprietary, and all we have
is a player, we can use the vsound to create a wave file like so:
vsound -f output.wav realplay input.rm
This will run realplayer under vsound. You will notice that no sound
is produced while the real audio file is being played. When the file
has completed playing, exit realplayer, and the raw audio data will be
converted to a wave file.
With the help of some other encoder, you would then be able to convert
the wave file to MP3 if you wanted. This method is probably the one
that preserves the most data during the conversion.
There are probably many other possible uses for vsound. Just use your
imagination.
A patch by Richard Taylor added an autostop switch to vsound allowing
it to stop recording automatically when /dev/dsp is closed, ideal for
use with RealPlayer for recording BBC radio programmes (a very
popular use for vsound).
Description from Richard;
"I use vsound to record programmes from the BBC to listen to in my
car on the way to work. One of things that I have found annoying is
having to find out the length of the programme before I can set up
my record script to kill realplayer when the programme has finished.
I noticed that realplayer closes the /dev/dsp handle once a
programme has finished. So I have patched vsound to add a new flag
'-a' which will kill the player after a set number of seconds of
inactivity (defined as the time that the /dev/dsp device is closed).
If you have bug reports (or even better, bug fixes!) or ideas for
improvements then please email me.
Nathan Chantrell <nsc@zorg.org>
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