Posts Tagged ‘linux audio’

Note: This is not really Arch-specific although I deal with Arch Linux files in this article; anyone can adapt it for their *NIX platform’s init process.

Quickstart
First, get your sound file ready. We are going to read it after the system has sourced our rc.conf MODULES array, so you are free to choose a player, format and location. To be safe, I have mine in boot, and use aplay instead of mplayer, so it is at /boot/audio.wav as (obviously) a WAVE instead of MP3 or OGG.

Edit /etc/rc.sysinit and look for the line:

# Load modules from the MODULES array defined in rc.conf

and then add:

# start some background audio
/usr/sbin/alsactl restore
/usr/bin/aplay /boot/audio.wav &

before the lines:

if [ -d /proc/acpi ]; then
  stat_busy "Loading standard ACPI modules"
  ACPI_MODULES="ac battery button fan processor thermal"

Or if you manually load modules (all or even a couple), then before:

# Wait for udev uevents

Else, as a last resort, before:

# bring up the loopback interface

Try from the earliest, and if you get low-level ALSA errors, it means the sound device didn’t initialise by the time you wanted it to run. In that case, try the other two positions. Any later than that, I don’t see a point. Also, remember to set a nice level beforehand with alsamixer and then alsactl store it. If you want a different set of levels separate from the post-boot system, simply use a different file with the –file option:

~# alsactl store -f /etc/asoundboot.state

Since this is still a little too late for my taste, I don’t kill it. Good for a non-invasive ambience track, especially if you run a login manager. If you’re very clever and into Rocket Science, you can also use a sound file which fits into the period of bootup from after modules have been loaded (Rocket Science Hint: time or bootchart). Or if you’re not very clever, you can just kill the process from somewhere, eg. /etc/rc.local:

killall -9 aplay

For what it’s worth, that is the earliest time we can play a sound file without modifying the system, and this is before any daemons are run. You can remove your alsa daemon from the system since we already restore the state during the above run (edit the alsactl line accordingly if you have made changes to /etc/conf.d/alsa). Like mentioned, the result is not good enough for me (and possibly a lot of others), because my bootup is 40 seconds til a fully-functional, action-ready and tiled KDE 4 desktop (30 to Awesome WM). Nevertheless, curiosity killed.

Warning: An update to initscripts will replace the sysinit file. Look for “NoUpgrade” in pacman.

Background
Sabayon Linux has been doing it ever since their LiveDVD releases of year 2006, and even though it’s not something everyone would like, the fact is that it gets the curious user excited no matter how nonsensical a purpose it serves. I used Sabayon from around late 2006 to early 2007, at which point I moved to Arch (and I never hopped again), so I can still remember that it had an option to “turn off” the music via the kernel line. Regardless of the on/off state, I never did hear the music. At one point of time, on Arch, I tried to implement something similar. I forgot what I did, whether it even worked, and lost the related prototype files if any at all (I do recall a custom daemon).

Anyway, someone on #archlinux asked about this yesterday, and it got me curious again. My stance is always the same:

There is no reason for any kind of “boot music” aside from when live media is used. Optical read speed is inherently slower than traditional disks or flash drives, and for LiveDVDs it equates to a significant amount of waiting time during the boot process. There, it makes ample sense. If you _are_ curious enough to try it nonetheless, on a sane, installed system, you have to ensure that the music does not start too early or too late, or else it becomes pointless to the extent I should have to punch you.

From this and this it appears Sabayon has something called a “Boot Music Infrastructure”, which is technically nothing more than an init script. The sound file itself is stored in /usr/share/sounds, and the script will read (i.e play) it while the rest of the daemons are being started. The following:

depend() {
  after modules
  before hostname
  before alsasound
}

Translates linguistically to:

Start only after all modules have loaded, before the hostname is set, and before alsa state is restored.

I have no idea why they’re creating their own values for the audio levels. Anyway, theoretically, it might be possible to start the sound subsystem even earlier. That would mean injecting the modules (along with dependencies) and binaries into the kernel image (see mkinitcpio), and reconstructing init itself.

Say..a low-level method to play music from POST onwards would be really nice.

Another Rocket Science Hint: iPod

=p

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My first brush with the new scheduler was when I came across http://lwn.net/Articles/350820/ via a mail to the LMMS lists (might have been LAU/LAD, can’t recall and lazy to search the archives or GMail trash). This prompted me to investigate it further, but not before I assumed some horrible things (mainly how the rt patch and the resulting realtime actually works). Suspecting my own judgement, I went over to #ck (irc.oftc.net) and posted my queries. True enough, part of my thinking was right – in that BFS can be a drop-in replacement for realtime audio work (traditionally so used to the -rt patch).

First of all, the “realtime” we have access to in an rt-patched kernel is “hard real, realtime”. Now, do recall that this patch was never designed with multimedia usage at the top of the priority list, but that it simply became a by-product of sorts. Primarily, we’re talking “embedded” and “lasers” here, then “audio”. Before this, us audio users only had this patch as our sole saviour. Well, not anymore.

Reason 1: The rt-tree has been slowly merged in mainline. Well, to an extent at least. As such, people have claimed to be able to run shitloads of VSTs and jconv IRs while recording a guitar sitting on top of Mt. Everest without -rt. The simple answer here is that “we no longer need a patch”.

Reason 2: Due to the revelation of BFS, users have found out that the “desktop experience” can apparently be better by a significant leap. Indirectly, this also means multimedia realtime becomes better proportionately. And this is not something hypothetical, you would (not should) immediately notice the difference if you’re a regular Linux Audio guy. What BFS introduces is a better “soft realtime” via SCHED_ISO, and this is more than enough unless we are audio rocket/laser scientists. Furthermore, in the BFS FAQ, Con Kolivas mentions realtime and jackd.

An extract of the conversation (by reading the following you agree not to hold anyone responsible for their comments and that it is just for your information):

[23:03] <Diablo-D3> bfs has realtime-compatible latencies
[23:03] <Diablo-D3> iso’ed processes on bfs generally defeats the entire point of having rt
[23:03] <mb_> Diablo-D3: It doesn’t change interrupt and spinlock latencies.
[23:03] <Diablo-D3> no it doesnt
[23:03] <Diablo-D3> but for something like, say, daft punk discovering linux, its good enough
[23:04] <Diablo-D3> if I was building a cruise missile, I wouldnt be using linux to begin with.

Ardour’s recent implementation of the revitalised FST initiative means that Linux sound engineers now have one more easy-access path to VST plug-ins. Among the tested DLLs, Native Instruments’ Guitar Rig 2 & 3 have been claimed to work well. If you’re a guitarist using Linux, it’s blasphemy to not have been moved by this revelation, so allow me to be your road to salvation.

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It’s not hard to start recording on Linux – it’s just hard to start migrating to record on Linux. The documentation is so fragmented that even I myself have no idea which would be the perfect FAQ, or how I learnt all that. That’s why a lot of times we end up providing information from what we know instead of citing sources to beginners. And that’s exactly why, yet again, there’s another level of fragmentation. This may not be true now, though. I’m probably assuming but things should pretty much be 95% 95% really 95% plug-and-play by now, aside from the occassional quirks, bugs, and glitches.

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