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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