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[20161123] In-kernel audio mixing ahead
NetBSD's sound device is currently only available for exclusive use. If one program uses it, another cannot. So if you want to play some music (mp3, audio stream) that's fine, but if you want to also have your web browser or mail client make some noise, this is not possible. Until now.

The solution is to mix multiple audio sources together, in effect allowing /dev/sound (etc.) access to be non-exclusive for a single process but several ones instead. To make this happen, audio from those sources needs to be mixed to come out of the same speaker, and since data writte to /dev/sound gets inside the kernel, that is a good place to do the mixing.

Challenges in the play are if audio sources are of different quality (bitrate, stereo/mono, bitrate), so some adjusting may be needed. All this is met by the latest patch by Nathanial Sloss, see his posting to tech-kern for more information.

Also, note his request for review and testing! :-)

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