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Questions re: Core Audio, offline rendering
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Questions re: Core Audio, offline rendering


  • Subject: Questions re: Core Audio, offline rendering
  • From: dct <email@hidden>
  • Date: Thu, 21 May 2009 10:18:27 -0700

Is this the correct forum?

If yes I have several questions:

I need to convert formatted audio(mp3, etc) files to continuous, linear pcm data in a memory buffer--i.e., a basic process of reading from disc, converting, and transferring to buffer.

Using Core Audio, I infer that I can to do the following:

1. Read formatted audio data into an input buffer--NSHandle methods do this just fine.

2. Create a file stream parser-- i.e.,
AudioFileStreamOpen( self, propertyCallback, packetCallback, hint, &fileParserID )


3. Parse the buffered input--
AudioFileStreamParseBytes( fileParserID, inBufSize, inBuffer, 0 );


4. (in propertyCallback function) -- when inPropertyID is "..._ReadyToProducePackets":
a. Get AudioStreamBasicDescription--
AudioFileStreamGetProperty( inAudioFileStream,
kAudioFileStreamProperty_DataFormat, &asbdSize, &asbd );
-All seems ok to here.-
b. Establish an audio queue--.
AudioQueueNewOutput( &asbd, queueCallback, inBuffer,
NULL, NULL, 0, &queue);
?? What does the callback process need to do, or is it needed,
given offline rendering??
c. Allocate an audio queue buffer
AudioQueueAllocateBuffer( queue, nbytes, &queueBuffer );
d. Set queue to render offline
AudioQueueSetOfflineRenderFormat( queue, &asbd, &chLayout );
?? What setting for channel layout & why is it needed for offline??


5. (in packetCallback) -- at each step of looping through the parsed packets:
a. Copy packet data to queue buffer
memcpy( queueBuffer->mAudioData +bytesFilled, inInputData +pcktOffset, pcktSize)
b. Update bytesFilled and pcktOffset
-- then after loop completion (or perhaps within the loop):
c. Render the packet data to pcm
AudioQueueOfflineRender( queue, timeStamp, outputBuffer, mFramesPerPacket)
?? Why timestamp for offline? And can timeStamp be set as NULL??
?? I assume that the output pcm is SInt16 for each channel of each frame.
If so, this means that for an mp3 case, 1152 frames/pckt & 2 chan/ftm, the
pcm output for every packet should be 4608 bytes. However, for that case,
the variable queueBuffer->mAudioDataBytesCapacity = 4640. Why the
extra 32 bytes (or am I misunderstanding DataBytesCapacity variable)??
?? At this point, with a 0 time stamp I get an error code = -66626.
I cannot find any reference to this error--what doe it mean??


I'll appreciate any and all help and advice.  Thanks, Don Thompson
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