RE: iPhone audio decompression
RE: iPhone audio decompression
- Subject: RE: iPhone audio decompression
- From: Luke Hollingworth <email@hidden>
- Date: Fri, 24 Oct 2008 09:20:21 +0000
- Importance: Normal
Hi Bill,
Thank you very much for the reply. I didn't fully understand what you meant by,
"but you can't decode more than one compressed stream at a time, so mp3 or aac is not going to help."
What
I understood from this is that I can only decode one file at at time.
If this is correct then this approach is fine because it's longer
background songs/tracks that take up all the space and only one of
these would be played at once. Therefore am trying to implement the
AQOffline render function but the documentation is limited.
I've
looked through the posts on this matter and none seem to have found a
solution of how to get it to work. From my best guesses the approach
would be to set up an output Audio Queue in the manner shown in the
Audio Queue Services Reference playback example, but instead of
starting the audio queue the AudioQueueSetOfflineRenderFormat should be
called to set the required decoded format of the audio and then the
AudioQueueOfflineRender function should be called with a different
AudioQueueBuffer than those used for supplying compressed audio to the
queue.
However, all functions return no error status but when
AudioQueueOfflineRender is called repeatedly to render audio my
HandleOutputBuffer function is never called to request more compressed
data for the audio queue.
Some further directions on how this
process should be performed or whether in fact my implementation is
correct but there is a problem with the API would be greatly
appreciated. The code added to the Playback Audio Queue example instead
of the AudioQueueStart is as follows;
//Setup the rendered output format of the AudioQueue - interleaved as only that is allowed
AudioStreamBasicDescription outputFormat;
outputFormat.mSampleRate = 44100;
outputFormat.mFormatID = kAudioFormatLinearPCM;
outputFormat.mFormatFlags = kLinearPCMFormatFlagIsSignedInteger | kAudioFormatFlagIsPacked;
outputFormat.mFramesPerPacket = 1;
outputFormat.mChannelsPerFrame = mAQData.mDataFormat.mChannelsPerFrame;
outputFormat.mBytesPerFrame = sizeof(SInt16) * outputFormat.mChannelsPerFrame;
outputFormat.mBytesPerPacket = outputFormat.mBytesPerFrame * outputFormat.mFramesPerPacket;
outputFormat.mBitsPerChannel = (outputFormat.mBytesPerFrame / outputFormat.mChannelsPerFrame) * 8;
//Setup the channel layout of the rendered output format of the AudioQueue
AudioChannelLayout layout;
layout.mChannelLayoutTag = kAudioChannelLayoutTag_Stereo;
layout.mChannelBitmap = 0;
layout.mNumberChannelDescriptions = 0;
//sets the queue to offline rendering
status = AudioQueueSetOfflineRenderFormat ( mAQData.mQueue, &outputFormat, &layout );
//Create a time stamp set the start of the queue using the sample time part of the structure
AudioTimeStamp timeStamp;
timeStamp.mFlags = kAudioTimeStampSampleTimeValid;
timeStamp.mSampleTime = 0;
//Allocate an audio queue buffer to redcieve the rendered audio
AudioQueueBufferRef outBuff;
status = AudioQueueAllocateBuffer(mAQData.mQueue, outputFormat.mBytesPerFrame * 1152, &outBuff);
//Setup a wav file to write the audio - this is a self made class that is full tested and functional
WavWrite* writer = [[WavWrite alloc] init];
[writer initWithFilePath:"/output.wav" numChannels:outputFormat.mChannelsPerFrame sampleType:kPCMStyle_SInt16];
//Render the audio file until it reaches the end of the file or times out -
UInt32 count = 0;
while( !mAQData.mEOF && count++ < 1000000)
{
status = AudioQueueOfflineRender(mAQData.mQueue, &timeStamp, outBuff, 1152);
timeStamp.mSampleTime += 1152;
[writer writeAudio:outBuff->mAudioData numFrames:1152];
}
[writer closeFile];
[writer release];
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