2.1 surround sound is (by a very unscientific measure) the third most popular surround speaker setup, after 5.1 and 7.1. Yet, ALSA and PulseAudio has since a long time back supported more unusual setups such as 4.0, 4.1 but not 2.1. It took until 2015 to get all pieces in the stack ready for 2.1 as well.
So what made adding 2.1 surround more difficult than other setups? Well, first and foremost, because ALSA used to have a fixed mapping of channels. The first six channels were decided to be:
1. Front Left
2. Front Right
3. Rear Left
4. Rear Right
5. Front Center
6. LFE / Subwoofer
Thus, a four channel stream would default to the first four, which would then be a 4.0 stream, and a three channel stream would default to the first three. The only way to send a 2.1 channel stream would then be to send a six channel stream with three channels being silence.
This was not good enough, because some cards, including laptops with internal subwoofers, would only support streaming four channels maximum.
(To add further confusion, it seemed some cards wanted the subwoofer signal on the third channel of four, and others wanted the same signal on the fourth channel of four instead.)
ALSA channel map API
The first part of the solution was a new alsa-lib API for channel mapping, allowing drivers to advertise what channel maps they support, and alsa-lib to expose this information to programs (see snd_pcm_query_chmaps, snd_pcm_get_chmap and snd_pcm_set_chmap).
The second step was for the alsa-lib route plugin to make use of this information. With that, alsa-lib could itself determine whether the hardware was 5.1 or 2.1, and change the number of channels automatically.
PulseAudio bass / treble filter
With the alsa-lib additions, just adding another channel map was easy.
However, there was another problem to deal with. When listening to stereo material, we would like the low frequencies, and only those, to be played back from the subwoofer. These frequencies should also be removed from the other channels. In some cases, the hardware would have a built-in filter to do this for us, so then it was just a matter of setting enable-lfe-remixing in daemon.conf. In other cases, this needed to be done in software.
If you have a laptop with an internal subwoofer, chances are that it – with all these changes to the stack – still does not work. Because the HDA standard (which is what your laptop very likely uses for analog audio), does not have much of a channel mapping standard either! So vendors might decide to do things differently, which means that every single hardware model might need a patch in the kernel.
If you don’t have an internal subwoofer, but a separate external one, you might be able to use hdajackretask to reconfigure your headphone jack to an “Internal Speaker (LFE)” instead. But the downside of that, is that you then can’t use the jack as a headphone jack…
Do I have it?
In Ubuntu, it’s been working since the 15.04 release (vivid). If you’re not running Ubuntu, you need alsa-lib 1.0.28, PulseAudio 7, and a kernel from, say, mid 2014 or later.
Takashi Iwai wrote the channel mapping API, and also provided help and fixes for the alsa-lib route plugin work.
The crossover filter code was imported from CRAS (but after refactoring and cleanup, there was not much left of that code).
Hui Wang helped me write and test the PulseAudio implementation.
PulseAudio upstream developers, especially Alexander Patrakov, did a thorough review of the PulseAudio patch set.