Fooling Around

IDirectSoundNotify8 is an interface to get notified on playback on capture audio buffer reaching certain position in the buffer. It is a must thing when implementing ring buffers with new data continuously added to the buffer for seamless playback (continuously copied from in case of capture).

This project is a minimalistic C++ sample code to illustrate the API. To initialize the DirectSound subsystem it is required to provide a window handle, which is created using ATL’s CWindowImpl (CMessageOnlyWindowImpl).

...
ATLENSURE_SUCCEEDED(DirectSoundCreate8(NULL, &pDirectSound8, NULL));
...
ATLENSURE_SUCCEEDED(pDirectSound8->SetCooperativeLevel(Window, DSSCL_PRIORITY));
...
ATLENSURE_SUCCEEDED(pDirectSound8->CreateSoundBuffer(&BufferDescriptor, &pDirectSoundBuffer, NULL));
...
CComQIPtr<IDirectSoundNotify8, &IID_IDirectSoundNotify8> pDirectSoundNotify8 = pDirectSoundBuffer;
...
ATLENSURE_SUCCEEDED(pDirectSoundNotify8->SetNotificationPositions(g_nPositionCount, pPositionNotify));
ATLENSURE_SUCCEEDED(pDirectSoundBuffer->Play(0, 0, DSBPLAY_LOOPING));

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Surprisingly fast I got new problems having ffdshow installed as a part of K-Lite Codec Pack. No wonder though because let us take a look at registration information:

Display Name: @device:sw:{083863F1-70DE-11D0-BD40-00A0C911CE86}\{0F40E1E5-4F79-4988-B1A9-CC98794E6B55}
CLSID: {0F40E1E5-4F79-4988-B1A9-CC98794E6B55}
Friendly Name: ffdshow Audio Decoder
Path: C:\Program Files\K-Lite Codec Pack\ffdshow\ffdshow.ax
Merit: 0×3fffffff

Nice merit, ain’t it? What is merit anyway? Let us check at MSDN:

MERIT_PREFERRED = 0×800000,
MERIT_NORMAL = 0×600000,
…
MERIT_HW_COMPRESSOR = 0×100050

The highest defined value is 0×00800000, while ffdshow is registered with 0×3FFFFFFF, that is on top of everything. No doubt the developers read Guidelines for Registering Filters and decided to get rid of the rest of installed software as unnecessary crap.

Anyway back to the problem: I had an A-law wave file (WAVE_FORMAT_ALAW) to play and make sure its data is valid and quite unexpectedly there was a silence while playing. A quick check confirmed that the sustem has CCITT A-law codec installed, however GraphEdit shown ffdshow Audio Decoder intercepting decoding. Obviously it spoiled the thing!

Finally I decided it was a right time to take an advantage of IAMGraphBuilderCallback interface to detect and reject the bastard.

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