DirectSound backend doesn't work with Synchronous Audio Router (SAR)

[dechamps]

When using FlexASIO 1.9 as the backend of Synchronous Audio Router 0.13.1, the default backend (DirectSound) doesn't seem to work if Windows audio devices are set up in SAR - the stream appears to deadlock.

It does seem to work fine if the WASAPI backend is used. It also seems to work if no audio devices are set up in SAR, but that's obviously not useful, and the audio is choppy.

This likely has something to do with SAR setting up its own WDM audio devices while DirectSound is starting.

[dechamps]

Investigation note: a (completely undocumented) way to make SAR log is to set the SAR_ASIO_LOG environment variable to 1: https://github.com/eiz/SynchronousAudioRouter/blob/d77615f811f9dd897c69996397d62406d4f0ea32/SarAsio/dllmain.cpp#L33-L43

The logs can then be found in %appdata%\SynchronousAudioRouter\logs.

[dechamps]

When the stream starts, it looks like buffer switches happen for the first ~1 second or so, and then they stop firing. (This seems oddly similar to the #29 failure mode, but the buffer size SAR uses - 10 ms - should not be large enough to trigger #29.) Meanwhile the entire Windows audio engine seems wedged, with other audio devices and other audio apps becoming nonfunctional until the stream is stopped.