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Recording Compressed Sounds

Q I can only record sounds compressed by 'MAC3' and 'MAC6. Why can't I record sounds compressed by 'ima4' or 'ulaw'?

A The Sound Input Manager doesn't do the compression when you record: the sound input driver does, and the Apple input drivers don't know how to do any compression except for MACE (furthermore, the MACE compression they use isn't the component one, it's a version that is built into the driver).

The reason for this is that you can call SPBRecord at interrupt time, but components can't be opened at interrupt time, so the sound input driver can't use the Component Manager to access compression codes. Therefore, the driver's compression options are limited to those implemented in the driver.

You can use the siCompressionAvailable selector with SPBGetDeviceInfo to find out which compressions are supported by a particular driver.

Note:
QuickTime can record with any available compression format because it compresses the sound after it has been recorded: it does not rely on the driver to do the compression for it. You can use this technique yourself by using Sound Manager 3.2.1 (or later) and the SoundConverter routines to convert each input buffer as you record it (or the whole sound once it has been recorded).

[Apr 12 1998]

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