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<p>Ok, I ended up working at a lower level than <code>AVFoundation</code> -- not sure if that was necessary. I read up to Chapter 5 of <em>Learning Core Audio</em> and went with an implementation using Audio Queues. This code is translated from being used for recording to a file/playing back a file, so there are surely some unnecessary bits I've accidentally left in. Additionally, I'm not actually re-enqueuing buffers onto the Output Queue (I should be), but just as a proof of concept this works. The only file is listed here, and is also on <a href="https://github.com/MaxGabriel/AudioPlaythrough" rel="nofollow">Github</a>.</p> <pre><code>// // main.m // Recorder // // Created by Maximilian Tagher on 8/7/13. // Copyright (c) 2013 Tagher. All rights reserved. // #import <Foundation/Foundation.h> #import <AudioToolbox/AudioToolbox.h> #define kNumberRecordBuffers 3 //#define kNumberPlaybackBuffers 3 #define kPlaybackFileLocation CFSTR("/Users/Max/Music/iTunes/iTunes Media/Music/Taylor Swift/Red/02 Red.m4a") #pragma mark - User Data Struct // listing 4.3 struct MyRecorder; typedef struct MyPlayer { AudioQueueRef playerQueue; SInt64 packetPosition; UInt32 numPacketsToRead; AudioStreamPacketDescription *packetDescs; Boolean isDone; struct MyRecorder *recorder; } MyPlayer; typedef struct MyRecorder { AudioQueueRef recordQueue; SInt64 recordPacket; Boolean running; MyPlayer *player; } MyRecorder; #pragma mark - Utility functions // Listing 4.2 static void CheckError(OSStatus error, const char *operation) { if (error == noErr) return; char errorString[20]; // See if it appears to be a 4-char-code *(UInt32 *)(errorString + 1) = CFSwapInt32HostToBig(error); if (isprint(errorString[1]) && isprint(errorString[2]) && isprint(errorString[3]) && isprint(errorString[4])) { errorString[0] = errorString[5] = '\''; errorString[6] = '\0'; } else { // No, format it as an integer NSLog(@"Was integer"); sprintf(errorString, "%d",(int)error); } fprintf(stderr, "Error: %s (%s)\n",operation,errorString); exit(1); } OSStatus MyGetDefaultInputDeviceSampleRate(Float64 *outSampleRate) { OSStatus error; AudioDeviceID deviceID = 0; AudioObjectPropertyAddress propertyAddress; UInt32 propertySize; propertyAddress.mSelector = kAudioHardwarePropertyDefaultInputDevice; propertyAddress.mScope = kAudioObjectPropertyScopeGlobal; propertyAddress.mElement = 0; propertySize = sizeof(AudioDeviceID); error = AudioHardwareServiceGetPropertyData(kAudioObjectSystemObject, &propertyAddress, 0, NULL, &propertySize, &deviceID); if (error) return error; propertyAddress.mSelector = kAudioDevicePropertyNominalSampleRate; propertyAddress.mScope = kAudioObjectPropertyScopeGlobal; propertyAddress.mElement = 0; propertySize = sizeof(Float64); error = AudioHardwareServiceGetPropertyData(deviceID, &propertyAddress, 0, NULL, &propertySize, outSampleRate); return error; } // Recorder static void MyCopyEncoderCookieToFile(AudioQueueRef queue, AudioFileID theFile) { OSStatus error; UInt32 propertySize; error = AudioQueueGetPropertySize(queue, kAudioConverterCompressionMagicCookie, &propertySize); if (error == noErr && propertySize > 0) { Byte *magicCookie = (Byte *)malloc(propertySize); CheckError(AudioQueueGetProperty(queue, kAudioQueueProperty_MagicCookie, magicCookie, &propertySize), "Couldn't get audio queue's magic cookie"); CheckError(AudioFileSetProperty(theFile, kAudioFilePropertyMagicCookieData, propertySize, magicCookie), "Couldn't set audio file's magic cookie"); free(magicCookie); } } // Player static void MyCopyEncoderCookieToQueue(AudioFileID theFile, AudioQueueRef queue) { UInt32 propertySize; // Just check for presence of cookie OSStatus result = AudioFileGetProperty(theFile, kAudioFilePropertyMagicCookieData, &propertySize, NULL); if (result == noErr && propertySize != 0) { Byte *magicCookie = (UInt8*)malloc(sizeof(UInt8) * propertySize); CheckError(AudioFileGetProperty(theFile, kAudioFilePropertyMagicCookieData, &propertySize, magicCookie), "Get cookie from file failed"); CheckError(AudioQueueSetProperty(queue, kAudioQueueProperty_MagicCookie, magicCookie, propertySize), "Set cookie on file failed"); free(magicCookie); } } static int MyComputeRecordBufferSize(const AudioStreamBasicDescription *format, AudioQueueRef queue, float seconds) { int packets, frames, bytes; frames = (int)ceil(seconds * format->mSampleRate); if (format->mBytesPerFrame > 0) { // Not variable bytes = frames * format->mBytesPerFrame; } else { // variable bytes per frame UInt32 maxPacketSize; if (format->mBytesPerPacket > 0) { // Constant packet size maxPacketSize = format->mBytesPerPacket; } else { // Get the largest single packet size possible UInt32 propertySize = sizeof(maxPacketSize); CheckError(AudioQueueGetProperty(queue, kAudioConverterPropertyMaximumOutputPacketSize, &maxPacketSize, &propertySize), "Couldn't get queue's maximum output packet size"); } if (format->mFramesPerPacket > 0) { packets = frames / format->mFramesPerPacket; } else { // Worst case scenario: 1 frame in a packet packets = frames; } // Sanity check if (packets == 0) { packets = 1; } bytes = packets * maxPacketSize; } return bytes; } void CalculateBytesForPlaythrough(AudioQueueRef queue, AudioStreamBasicDescription inDesc, Float64 inSeconds, UInt32 *outBufferSize, UInt32 *outNumPackets) { UInt32 maxPacketSize; UInt32 propSize = sizeof(maxPacketSize); CheckError(AudioQueueGetProperty(queue, kAudioQueueProperty_MaximumOutputPacketSize, &maxPacketSize, &propSize), "Couldn't get file's max packet size"); static const int maxBufferSize = 0x10000; static const int minBufferSize = 0x4000; if (inDesc.mFramesPerPacket) { Float64 numPacketsForTime = inDesc.mSampleRate / inDesc.mFramesPerPacket * inSeconds; *outBufferSize = numPacketsForTime * maxPacketSize; } else { *outBufferSize = maxBufferSize > maxPacketSize ? maxBufferSize : maxPacketSize; } if (*outBufferSize > maxBufferSize && *outBufferSize > maxPacketSize) { *outBufferSize = maxBufferSize; } else { if (*outBufferSize < minBufferSize) { *outBufferSize = minBufferSize; } } *outNumPackets = *outBufferSize / maxPacketSize; } #pragma mark - Record callback function static void MyAQInputCallback(void *inUserData, AudioQueueRef inQueue, AudioQueueBufferRef inBuffer, const AudioTimeStamp *inStartTime, UInt32 inNumPackets, const AudioStreamPacketDescription *inPacketDesc) { // NSLog(@"Input callback"); // NSLog(@"Input thread = %@",[NSThread currentThread]); MyRecorder *recorder = (MyRecorder *)inUserData; MyPlayer *player = recorder->player; if (inNumPackets > 0) { // Enqueue on the output Queue! AudioQueueBufferRef outputBuffer; CheckError(AudioQueueAllocateBuffer(player->playerQueue, inBuffer->mAudioDataBytesCapacity, &outputBuffer), "Input callback failed to allocate new output buffer"); memcpy(outputBuffer->mAudioData, inBuffer->mAudioData, inBuffer->mAudioDataByteSize); outputBuffer->mAudioDataByteSize = inBuffer->mAudioDataByteSize; // [NSData dataWithBytes:inBuffer->mAudioData length:inBuffer->mAudioDataByteSize]; // Assuming LPCM so no packet descriptions CheckError(AudioQueueEnqueueBuffer(player->playerQueue, outputBuffer, 0, NULL), "Enqueing the buffer in input callback failed"); recorder->recordPacket += inNumPackets; } if (recorder->running) { CheckError(AudioQueueEnqueueBuffer(inQueue, inBuffer, 0, NULL), "AudioQueueEnqueueBuffer failed"); } } static void MyAQOutputCallback(void *inUserData, AudioQueueRef inAQ, AudioQueueBufferRef inCompleteAQBuffer) { // NSLog(@"Output thread = %@",[NSThread currentThread]); // NSLog(@"Output callback"); MyPlayer *aqp = (MyPlayer *)inUserData; MyRecorder *recorder = aqp->recorder; if (aqp->isDone) return; } int main(int argc, const char * argv[]) { @autoreleasepool { MyRecorder recorder = {0}; MyPlayer player = {0}; recorder.player = &player; player.recorder = &recorder; AudioStreamBasicDescription recordFormat; memset(&recordFormat, 0, sizeof(recordFormat)); recordFormat.mFormatID = kAudioFormatLinearPCM; recordFormat.mChannelsPerFrame = 2; //stereo // Begin my changes to make LPCM work recordFormat.mBitsPerChannel = 16; // Haven't checked if each of these flags is necessary, this is just what Chapter 2 used for LPCM. recordFormat.mFormatFlags = kAudioFormatFlagIsBigEndian | kAudioFormatFlagIsSignedInteger | kAudioFormatFlagIsPacked; // end my changes MyGetDefaultInputDeviceSampleRate(&recordFormat.mSampleRate); UInt32 propSize = sizeof(recordFormat); CheckError(AudioFormatGetProperty(kAudioFormatProperty_FormatInfo, 0, NULL, &propSize, &recordFormat), "AudioFormatGetProperty failed"); AudioQueueRef queue = {0}; CheckError(AudioQueueNewInput(&recordFormat, MyAQInputCallback, &recorder, NULL, NULL, 0, &queue), "AudioQueueNewInput failed"); recorder.recordQueue = queue; // Fills in ABSD a little more UInt32 size = sizeof(recordFormat); CheckError(AudioQueueGetProperty(queue, kAudioConverterCurrentOutputStreamDescription, &recordFormat, &size), "Couldn't get queue's format"); // MyCopyEncoderCookieToFile(queue, recorder.recordFile); int bufferByteSize = MyComputeRecordBufferSize(&recordFormat,queue,0.5); NSLog(@"%d",__LINE__); // Create and Enqueue buffers int bufferIndex; for (bufferIndex = 0; bufferIndex < kNumberRecordBuffers; ++bufferIndex) { AudioQueueBufferRef buffer; CheckError(AudioQueueAllocateBuffer(queue, bufferByteSize, &buffer), "AudioQueueBufferRef failed"); CheckError(AudioQueueEnqueueBuffer(queue, buffer, 0, NULL), "AudioQueueEnqueueBuffer failed"); } // PLAYBACK SETUP AudioQueueRef playbackQueue; CheckError(AudioQueueNewOutput(&recordFormat, MyAQOutputCallback, &player, NULL, NULL, 0, &playbackQueue), "AudioOutputNewQueue failed"); player.playerQueue = playbackQueue; UInt32 playBufferByteSize; CalculateBytesForPlaythrough(queue, recordFormat, 0.1, &playBufferByteSize, &player.numPacketsToRead); bool isFormatVBR = (recordFormat.mBytesPerPacket == 0 || recordFormat.mFramesPerPacket == 0); if (isFormatVBR) { NSLog(@"Not supporting VBR"); player.packetDescs = (AudioStreamPacketDescription*) malloc(sizeof(AudioStreamPacketDescription) * player.numPacketsToRead); } else { player.packetDescs = NULL; } // END PLAYBACK recorder.running = TRUE; player.isDone = false; CheckError(AudioQueueStart(playbackQueue, NULL), "AudioQueueStart failed"); CheckError(AudioQueueStart(queue, NULL), "AudioQueueStart failed"); CFRunLoopRunInMode(kCFRunLoopDefaultMode, 10, TRUE); printf("Playing through, press <return> to stop:\n"); getchar(); printf("* done *\n"); recorder.running = FALSE; player.isDone = true; CheckError(AudioQueueStop(playbackQueue, false), "Failed to stop playback queue"); CheckError(AudioQueueStop(queue, TRUE), "AudioQueueStop failed"); AudioQueueDispose(playbackQueue, FALSE); AudioQueueDispose(queue, TRUE); } return 0; } </code></pre>
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