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Tag - general (GNU-) Linux stuff

Fil des billets - Fil des commentaires

samedi 7 avril 2012

Oh PulseAudio, you are an interesting little beast

So, one can record anything going through PulseAudio (and therefore really, any sound being played back on Ubuntu or Debian) very simply.

Of course, this comes as no surprise.
After all, everything is a copy on a computer, no matter how many layers of impracticality you add for a user (cumbersome flash players à la myspace, silly DRM, etc.) to prevent them from accessing that copy.

No, what's great here is how simple it is to dump the sound being played back, once you look into it.

Without any further ado, here is how to dump anything being played back as mp3, Ogg Vorbis or the almighty FLAC:

mp3: parec -d alsa_output.pci-0000_00_1b.0.analog-stereo.monitor | lame -r -V0 - test.mp3

Ogg Vorbis: parec -d alsa_output.pci-0000_00_1b.0.analog-stereo.monitor | oggenc --raw -q 8 -o test.ogg -

FLAC: parec -d alsa_output.pci-0000_00_1b.0.analog-stereo.monitor | flac --force-raw-format --endian=little --channels=2 --sample-rate=44100 --sign=signed --bps=16 -o test.flac -

parec is the command to record from a PulseAudio device, and -d the flag to specify which device to record from (this must be an input device, and will vary from system to system) We then pipe the data (using the pipe symbol | ) to an encoder, either lame for mp3, or oggenc for ogg vorbis.

For lame, we specify the data is coming in raw format (-r) and we ask the sound to be encoded in V0 quality (~256 KBps). We tell it to use the stdin input (represented by single dash -) and to output to a file called test.mp3.

For oggenc, we specify the data is coming in raw format with the —you guessed it— --raw flag and ask for a quality of 8 (out of 10) with the -q 8 flag. We also request an output file named test.ogg with -o test.ogg, and tell it to use stdin as the input file (to use the data piped through with the |) with the final -.

For flac, things are a little more complicated, as flac demands more information when you encode from stdin. You have to specify the endianness of the data coming in with --endian=little (I tried both big or little, you want little), the number of channels (1 for mono, 2 for stereo, 6 for 5.1 etc.) with --channels=2, the sample rate (44 100 KHz is what CDs use, DVDs are usually 48 000) with --sample-rate=44100, --sign to “Set the sign of samples (the default is signed)” (yes, I'm quoting from the manual, no idea what this does) and finally, the number of bits per second, which is 16 on CDs (hence --bps=16). We then ask for the data to be written to an external file called test.flac with -o test.flac and tell flac to get its input data from stdin (and therefore from parec through the pipe) with the final dash.

That's it.

Neat, eh?

dimanche 3 juillet 2011

Comment faire une goldcard pour votre téléphone sous GNU/Linux

Un petit billet technique, aussi parce que je sais qu'il risque de m'être utile à l'avenir.

Une goldcard permet de forcer un téléphone à accepter une « mise à jour » qui est plus ancienne que la version du système déjà présente sur le téléphone. Un retour en arrière donc. Ça permet de descendre de version ou downgrader. Ce qui est parfois bien pratique quand on a besoin de rooter son téléphone.

C'est pas bien rassurant sur la sécurité de son téléphone non plus, mais bon. Si on veut de la sécurité, on chiffre[1], non ?

Bref, vous verrez souvent écrit ici et là que c'est chiant à faire sous Linux.
Que nenni, c'est probablement plus rapide, si on sait où on va.


[1] Au sens utiliser des moyens cryptographiques.

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