My last post was mostly about adding multi-channel support to GStreamer.
That was only partially implemented though – many elements in GStreamer (and pretty much all of the ones that support more than stereo) require each channel to be explicitly labeled with its position. This information wasn’t bridged until now.
The two sides of this situation are:
The LV2 Port Groups extension: http://lv2plug.in/ns/dev/port-groups.lv2/port-groups.html
GStreamer multi-channel: http://gstreamer.freedesktop.org/data/doc/gstreamer/head/gst-plugins-base-libs/html/gst-plugins-base-libs-gstmultichannel.html
LV2 “roles” correspond to GstAudioChannelPosition. Unfortunately, the mapping was a bit strained with the Port Groups extension as it was at midterm time. Fortunately, that extension is not finalized yet, and in the process I found some other problems with it, so I decided to bring it closer to the way GStreamer does things, which seems to be common in other software that does this sort of stuff. Formerly, the port groups extension used the channel names from the relevant surround spec (see the Wikipedia page on Surround Sound). This resulted in a lot of roles that were really the same thing, e.g. the “Surround Left” channel in 5.1 is really the same as the “Left Back” channel in 7.1. This was pretty confusing in practice. GStreamer instead always uses positional names like “Left Back”, which is unambiguous regardless of context (you don’t have to know if it’s 17.2 or whatever to know what “Left Back” probably means…). So, I changed the LV2 port groups extension to be similar (though not identical; munging an LV2 extension to match GStreamer API wouldn’t be appropriate).
This information is now queried out from LV2 plugins that have it, mapped to GStreamer channel positions, and set as capabilities on mulit-channel pads. This means multi-channel surround LV2 plugins should now work fine with surround sound GStreamer set-ups, for example playing a Theora video through some audio processing and out through Alsa.
More to come…