James Trevelyan wrote:

... However, what I did notice is that in all circumstances the windows driver was capturing at the same time as playback, even when I was not asking it to record. This suggests to me that the comment in the driver source about synchronising playback to capture has some relevance ...

Indeed. The driver would have to send the data at the exact speed of the device's internal clock, and the only way to determine that clock's speed is to capture data.

So far I haven't found the time to rewrite the driver to support this synchronization mechanism.

Best regards, Clemens