reading samples with AVAssetReader and timeRange in real time - objective-c

Question

Previously I read audio samples from a complete audio file using CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer. Right now I would like to do the same using ranges (ie i specify the range in time.. read a small chunk of audio as per the time, and then go back and read again). The reason why I want to use time range is b/c I want to control the size of each read (to fit in a packet with a max size).
for some reason, there is always a bump between each read. In my code you'll notice that I start the AVAssetReader and end it every time I set a time range, and that's b/c I cannot dynamically adjust the time range after the reader has started (see here for more details).
Could it be that starting and ending a reader is just too expensive to produce a continuous real time experience? Or are there other ways of doing this that I'm not aware of?
Also note that this jitter or lag happens at whatever point I set the time interval to be.. which makes me believe that starting and ending a reader the way I am is too expensive for real time audio playback.
- (void) setupReader
{
NSURL *assetURL = [NSURL URLWithString:#"ipod-library://item/item.m4a?id=1053020204400037178"];
songAsset = [AVURLAsset URLAssetWithURL:assetURL options:nil];
track = [songAsset.tracks objectAtIndex:0];
nativeTrackASBD = [self getTrackNativeSettings:track];
// set CM time parameters
assetCMTime = songAsset.duration;
CMTimeReadDurationInSeconds = CMTimeMakeWithSeconds(1, assetCMTime.timescale);
currentCMTime = CMTimeMake(0,assetCMTime.timescale);
}
-(void)readVBRPackets
{
// make sure assetCMTime is greater than currentCMTime
while (CMTimeCompare(assetCMTime,currentCMTime) == 1 )
{
NSError * error = nil;
reader = [[AVAssetReader alloc] initWithAsset:songAsset error:&error];
readerOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:track
outputSettings:nil];
[reader addOutput:readerOutput];
reader.timeRange = CMTimeRangeMake(currentCMTime, CMTimeReadDurationInSeconds);
[reader startReading];
while ((sample = [readerOutput copyNextSampleBuffer])) {
CMItemCount numSamples = CMSampleBufferGetNumSamples(sample);
if (numSamples == 0) {
continue;
}
NSLog(#"reading sample");
CMBlockBufferRef CMBuffer = CMSampleBufferGetDataBuffer( sample );
AudioBufferList audioBufferList;
OSStatus err = CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer(
sample,
NULL,
&audioBufferList,
sizeof(audioBufferList),
NULL,
NULL,
kCMSampleBufferFlag_AudioBufferList_Assure16ByteAlignment,
&CMBuffer
);
const AudioStreamPacketDescription * inPacketDescriptions;
size_t packetDescriptionsSizeOut;
size_t inNumberPackets;
CheckError(CMSampleBufferGetAudioStreamPacketDescriptionsPtr(sample,
&inPacketDescriptions,
&packetDescriptionsSizeOut),
"could not read sample packet descriptions");
inNumberPackets = packetDescriptionsSizeOut/sizeof(AudioStreamPacketDescription);
AudioBuffer audioBuffer = audioBufferList.mBuffers[0];
for (int i = 0; i < inNumberPackets; ++i)
{
SInt64 dataOffset = inPacketDescriptions[i].mStartOffset;
UInt32 packetSize = inPacketDescriptions[i].mDataByteSize;
size_t packetSpaceRemaining;
packetSpaceRemaining = bufferByteSize - bytesFilled;
// if the space remaining in the buffer is not
// enough for the data contained in this packet
// then just write it
if (packetSpaceRemaining < packetSize)
{
[self enqueueBuffer];
}
// copy data to the audio queue buffer
AudioQueueBufferRef fillBuf = audioQueueBuffers[fillBufferIndex];
memcpy((char*)fillBuf->mAudioData + bytesFilled,
(const char*)(audioBuffer.mData + dataOffset), packetSize);
// fill out packet description
packetDescs[packetsFilled] = inPacketDescriptions[i];
packetDescs[packetsFilled].mStartOffset = bytesFilled;
bytesFilled += packetSize;
packetsFilled += 1;
// if this is the last packet, then ship it
size_t packetsDescsRemaining = kAQMaxPacketDescs - packetsFilled;
if (packetsDescsRemaining == 0) {
[self enqueueBuffer];
}
}
CFRelease(CMBuffer);
CMSampleBufferInvalidate(sample);
CFRelease(sample);
}
[reader cancelReading];
reader = NULL;
readerOutput = NULL;
currentCMTime = CMTimeAdd(currentCMTime, CMTimeReadDurationInSeconds);
}
}

Answer 1

I know what happens :-D It took me near a whole day to figure it out.
In fact AVAssetReader fades the first 1024 samples (maybe a little more) in. That's why you hear the jitter effect.
I fixed it by reading 1024 samples before the position I really want to read, then skip that 1024 samples.
I hope it'll work for you also.