If you want to run jackd or pulseaudio in real-time mode with the latest kernels, then you'll need the latest RT-Preempt patch for the vanilla kernel. With the proper config options, this will allow you to allocate a specified portion of the default real-time scheduling priority to another user ID (see sched-rt-group.txt in the kernel Documentation directory). I made an rt-sources ebuild for the 2.6.29.2-rt patch, which I may or may not commit to the portage tree (we'll just have to see) so you'll either have to find or patch and build a kernel with the appropriate patch and read the docs at the RTwiki and the Linux Musicians sites. Good stuff, Maynard...
Among the recommended options, make sure you have these options enabled:
* CONFIG_GROUP_SCHED=y
* CONFIG_FAIR_GROUP_SCHED=y
* CONFIG_RT_GROUP_SCHED=y
* CONFIG_USER_SCHED=y
/sys/kernel/uids/0/cpu_rt_runtime and then increase the value for your
own UID. Something like 450000 for each one should suffice to get you started.
If you just had your alsa audio output disappear after a software update (or after a fresh install) and you're not seeing any obvious or useful software errors, then you can try kamix. In this case, after a software update which happened to include alsa-lib-1.0.19, sound stopped working. Everything appeared nominal from a software and hardware standpoint in that amarok and other applications functioned normally. The only visible error in my logs was coming from pulseaudio, because it couldn't create the default alsa source/sink. I tried all the usual tricks, such as removing the state file and configuring sound levels with the console alsamixer. The audigy mixer options have changed over time, usually with a driver update, but I know my options pretty well, and everything looked like it *should* work. It just didn't...
After some googling and a hint from a forum post, I remembered the only software mixer I hadn't tried yet: kamix. Kamix shows things in such a different layout, I suppose I saw something there that I didn't notice in the other mixer applications, but somehow the default for the optical output switch (the IEC958 Optical Raw Playback Switch) got set to "on", which kills the normal analog and digital audio outputs on the Audigy2 mixer. The config section in kamix also showed a new option called "Analog Capture Boost" which now appears in the Switches section of the normal Gnome mixer. The latter mixer had lost most of the options after the update, so I had to re-enable the ones I wanted in the config section. It just goes to show you, mixer settings are crucial, and it can be very frustrating when they change on their own due to a software update/interface change. Just make sure you have the mixer settings you think have...
If you have an electric guitar, bass, or keyboard (or even an electric violin or banjo :) and you want a convenient way to connect it up to your Linux box for recording and other fun things, then read on...
This documents a few things I discovered while trying to connect a Line 6 Bass Pod XT to my Linux workstation. The short answer is yes, there's a current driver (now called line6usb instead of podxtpro), but it's not in the mainline kernel source. It's available in rpm and source packages, along with some sample scripts and basic documentation. Download one or the other and install/build as appropriate (if you're already running Gentoo, just emerge line6usb). Line 6 makes many other PODs and related devices, several of which are supported.
For a cheaper and more generic solution, which actually works with the standard USB audio driver, any of the models from Behringer, such as the UCA202 or UCG102, seem to work and average about $30-40 USD at online stores such as NewEgg or B&H Photo. Several have been reported working here. They typically come bundled with software for Mac OS/X and Windows, so some of the fancier stuff won't work without some googling and/or experimentation (and maybe even better hardware). However, the basic USB connection *should* work (see below for applications).
Even for the more generic option, you'll need to have your kernel configured for USB and ALSA, along with several applications, depending on what you want to do. Many current applications will work for both recording and jamming, including MIDI, USB, and Digital Audio, again, depending on your particular hardware setup (at the least you should have USB and basic audio line in/out). Applications you may find helpful include:
Note that the main point of using one of the above USB interfaces, in addition to whatever processing capabilities they may have, is to get the audio interface outside your computer case instead of inside (as is the case with your audio card). Unless you have at least a 24-bit Audigy card or better, the internal A/D converter is most likely too noisy for even casual use. The cheap alternative above will sound *much* better, and the PODs provide a virtually endless supply of amp, cabinet, and effects models...
Being a Gentoo developer, my platform of choice for pretty much everything from desktops, to servers, to embedded devices of all architectures is, Gentoo Linux, however, there are at least a couple of Linux distributions specifically designed for low-latency audio applications. I used to use the first one below years ago when I still ran RedHat and several other distributions, and the second one I only discovered recently (and haven't actually tried myself). Overall, I find Gentoo to be the most flexible and customizable platform, and the one I've learned more from than any previous platform I've tried over the years (and there have been many). However, if you already use RedHat or Debian, or just prefer a Linux distribution that you don't need to build from source, feel free to try one of these: