You have chosen a difficult place to start. Audio drivers are complicated, and
the environment in which they operate has some unique issues.
In the early days of Windows, pre-Vista, many people wrote filter drivers to do
the cutesy things you’re talking about here. So many people wrote cutesy,
time-consuming filter drivers that it became impossible for professional audio
applications to get anything approaching the guaranteed latencies that they
need to do their thing. So, when the Audio Engine was totally rewritten in
Vista, the philosophy was changed. Audio drivers work with the Audio Engine.
For WaveRT drivers, which is almost all audio drivers, the audio data is never
touched by kernel code. The Audio Engine reads and writes from circular
buffers in the hardware directly. There’s no place for a kernel filter to hook
in.
Conceptually, what Microsoft wants you to do is write a Global Effects Audio
Processing Object (GFX APO), which is a user-mode DLL that runs inside the
Audio Engine process. There are 4 or 5 classes of FX APOs that tap in at
various points in the audio pipeline. However you can’t just distribute a
system-wide GFX APO, because that would lead to the same kind of latency muck
that we had before. Instead, an APO is considered part of the hardware, and
gets distributed and installed as part of the driver for a given audio device.
There are sample APOs in the massive SYSVAD driver sample.
The architecture you’re proposing is certainly possible, but it’s a difficult
path. The SYSVAD sample I mentioned above implements a fake microphone and a
fake speaker. The fake microphone is fed from a signal generator, the fake
speaker writes to a WAV file. The SYSVAD driver is close to what you want, but
the Microsoft audio team has chosen to use it as a platform to demonstrate
essentially every feature that an audio driver can support, so it is extremely
large. You should take a look to see what you’re facing:
https://github.com/Microsoft/Windows-driver-samples/tree/master/audio/sysvad
—
Tim Roberts, timr@xxxxxxxxx
Providenza & Boekelheide, Inc.
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