kAudioFileGlobalInfo_AvailableStreamDescriptionsForFormat returns ASBDs with mChannelsPerFrame != 0?
kAudioFileGlobalInfo_AvailableStreamDescriptionsForFormat returns ASBDs with mChannelsPerFrame != 0?
- Subject: kAudioFileGlobalInfo_AvailableStreamDescriptionsForFormat returns ASBDs with mChannelsPerFrame != 0?
- From: "Stephen F. Booth" <email@hidden>
- Date: Sat, 10 Nov 2007 21:54:30 -0800
I've run into a strange issue in Leopard that didn't occur in Tiger.
It seems that
kAudioFileGlobalInfo_AvailableStreamDescriptionsForFormat is returning
some ASBDs with the mChannelsPerFrame set to 1, instead of 0 as I
would expect (per the documentation only mFormatID, mFormatFlags,
mBitsPerChannel should be set).
I want to obtain a list of the available data formats for a given file
format to present to the user. In pseudo-code I do the following:
- Get the kAudioFileGlobalInfo_AvailableFormatIDs for the format in
question (in this case, kAudioFileM4AType)
- For each of these format IDs, construct an AudioFileTypeAndFormatID
and obtain a list of applicable ASBDs using
kAudioFileGlobalInfo_AvailableStreamDescriptionsForFormat
- For each of these ASBDs, get the display name using
kAudioFormatProperty_FormatName
Here is the output from my test program:
2007-11-10 21:41:31.538 CATest[688:10b] file type: 'm4af'
2007-11-10 21:41:31.562 CATest[688:10b] data format 0: 'aac '
2007-11-10 21:41:31.609 CATest[688:10b] variant 0: AAC, 1 channels
2007-11-10 21:41:31.630 CATest[688:10b] data format 1: 'aacl'
2007-11-10 21:41:31.639 CATest[688:10b] variant 0: MPEG 4 Low Delay
AAC, 1 channels
2007-11-10 21:41:31.659 CATest[688:10b] data format 2: 'ac-3'
2007-11-10 21:41:31.818 CATest[688:10b] variant 0: AC3
2007-11-10 21:41:31.935 CATest[688:10b] data format 3: 'alac'
2007-11-10 21:41:31.941 CATest[688:10b] variant 0: Apple Lossless
The Debugger has exited with status 0.
Have I made an error or should 'aac ' and 'aacl' set mChannelsPerFrame
to 0? Obviously I can manually zero the channels before calling
kAudioFormatProperty_FormatName, but I don't believe I should have to.
Stephen
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