Amarok Blog

I might be sick, or just crazy, or both of them, but I still think I’m quite more productive when I have fever, or the days around that time. Yesterday I had fever, and I was knock out till late afternoon, but then I started feeling better, and I started producing.

First of all, rbot’s init script in my overlay has been updated: Subversion trunk will now create a rbot.pid file inside the bot’s directory without need for start-stop-daemon to create it itself, which means I can finally let rbot fork on its own rather than forcing it to background (which is not really a good thing to do, if it can be avoided). Thanks to tango and jbn for looking after my raw patch.

Then I decided to finish the work with apcupsd; again in my overlay you can find a new ebuild for 3.14.0 based on the one found on bugzilla, but with a new apccontrol file and a totally renewed init script. This script can be multiplied, which means you can have a /etc/init.d/apcupsd.foo link, which would then start apcupsd looking for /etc/apcupsd/foo.conf configuration file. Thanks to apcupsd authors for implementing this feature in 3.14!

I’ve also attached the two UPSes to Farragut instead of Enterprise, as the latter is not a server and might as well be offline when Farragut is still up (for instance this is the case most of the times I’m outside for the whole day, or if there is noone at home); apcupsd on FreeBSD works nicely, and doesn’t require any fiddling with configuration, neither kernel side (the ugen support is built by default) nor with permissions (as the default is to run as root, this might change in the future, but as it is it’s fine to me; I’ll be working on a better handling of permissions on device nodes for Gentoo/FreeBSD, but it’s not in my priority list at the moment). Also here, they work perfectly fine.

I was also able to fix a bug in xine-lib, with mp4/mov files playback that used version 1 rather than version 0 of the media header atom, such as files generated by FFmpeg. The bug was reported on sourceforge already but I wasn’t sure what it actually meant and where to find a sample file; when I generated the same condition by chance here, I decided to take a deeper look; unfortunately MultimediaWiki doesn’t provide much information about that, but I asked Mike to give me an account there so I can try to write something useful, maybe next time someone else needs a mdhd atom description they won’t have to look at the sources of FFmpeg to see how it’s read and generated.

Then tonight I wanted to resume my work on implementing audio conversions inside the audio output loop instead of doing it for every decoder; it’s an hard work as it probably will require rewriting a good deal of code, but it should be rewarding once it’s done. Right now there are a bunch more of flag values for capabilities, so for instance I can say if a drivers supports integer or float 32-bit samples, 64-bit samples, and if it can accept streams in a different endianness. This is important because there’s little point in doing the job of the output plugin, that might handle that transparently, for instance a big-endian stream might be decoded on a little-endian machine, then sent through PulseAudio to a big-endian machine where it will be reproduced: in this setup, xine’s endian reversal of the stream (from big to little endian) would have been superfluous, as PulseAudio would have accepted the big-endian samples, then sent them to the other machine that needed not to reverse them to reproduce them.

Anyway, right now the code is quite fragile, there’s no conversion being done, there are mostly only things that are totally broken out, there are asserts @1 == 0@ used to mark the code that needs to be rewritten. But something works: I was able to remove a lot of code from the musepack decoder, as libmpcdec always produces 32-bit native-endian (or maybe little-endian, I’m not yet sure) floating point samples; previously the decoder converted all the samples back to 16-bit format, and then gave it to the audio output loop to handle? now instead it sends them directly to the output, and as PulseAudio supports 32-bit float samples, they are not converted and play back fine.

Tomorrow I’ll see to work a way to handle upsamping and downsampling of streams, the problem is that it’s not trivial to decide what to do: if a plugin supports 32-bit integer samples, but not 24-bit integer samples, it should probably upscale the 24-bit to 32-bit to avoid losing precision; if it doesn’t it might upscale it to 32-bit float, or maybe downscale it to 16-bit integers. The same applies to channel mode, if the driver doesn’t support stereo output, should it be updated to 4.0 or should it be downgraded to mono?

For sure this time I’m very happy of being working on branches: leaving the code broken for weeks, maybe months, is not something you want to do on the main development branch. And I mean it, because with the changes I’m doing, not only I’ll be changing the ABI of the library itself (well, actually not much, just a couple of structures), but more importantly I’ll be changing the audio output plugins API, as I need to feed them a sample format rather than a bits-per-sample size.

Anyway, this is not going to be something easy to complete, but it will be a noticeable improvement for Amarok users once done, especially because I want to make sure that the capabilities for “mixer” volume and “PCM” volume are cleared up, probably by deprecating one of them, so that Amarok can be changed not to use xine’s software amplification (which also sucks and I also need to rewrite in good part in this branch) if the output plugin actually supports a per-stream volume (like PulseAudio).

Sponsoring, bribing, and comfort words are welcome, as xine-lib’s audio_out code is giving me creeps.