Inside Macintosh: Sound Manager

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Supporting Stereo Recording

Many sound input devices support recording stereo sounds (that is, sounds from two or more channels). If you are writing a device driver for a stereo device, you need to make sure that you support the siNumberChannels , siActiveChannels , and siActiveLevels selectors.

The siNumberChannels selector controls the number of sound input channels and thereby determines the format of the data stream your device driver produces. If the number of channels is 1, the driver should produce monophonic data in response to a Read request. If the number of channels is 2, the driver should produce interleaved stereo data in response to a Read request.

The siActiveChannels selector controls which of the available input channels are used for recording. The active channels are specified using a bitmap value. For example, the value $01 indicates that the first channel (the left channel) is to be used. The value $02 indicates that the second channel (the right channel) is to be used.

The siNumberChannels and siActiveChannels selectors together determine the exact format of the output data stream. If the current number of channels is 1 and the current active channel bitmap is $01, the driver should produce a stream of monophonic data containing samples only from the left input channel. If the current number of channels is 1 and the current active channel bitmap is $02, the driver should produce a stream of monophonic data containing samples only from the right input channel. If the current number of channels is 1 and the current active channel bitmap is $03, the driver should mix the right and left channels to produce a stream of monophonic data. If the current number of channels is 2 and the current active channel bitmap is $03, the driver should produce a stream of interleaved samples from the left and right input channels.

If the siActiveChannels selector is never passed to a sound input device driver, it's recommended that the active channel default bitmap for both monophonic and stereo recording should be $03. When the active channel bitmap conflicts with the number of channels (for example, there are two channels but the active channel bitmap is $01), you should use the default value of $03.

Inside Macintosh: Sound Manager

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