Objective-c - How to serialize audio file into small packets that can be played?

Question

So, I would like to get a sound file and convert it in packets, and send it to another computer. I would like that the other computer be able to play the packets as they arr

Answer 1

It seems you are solving wrong task, because AVAudioPlayer capable play only whole audiofile. You should use Audio Queue Service from AudioToolbox framework instead, to play audio on packet-by-packet basis. In fact you need not divide audiofile into real sound packets, you can use any data block like in your own example above, but then you should read received data chuncks using Audiofile Service or Audio File Stream Services functions (from AudioToolbox) and feed them to audioqueue buffers.

If you nevertheless want to divide audiofile into sound packets, you can easily do it with Audiofile Service functions. Audiofile consist of header where its properties like number of packets, samplerate, number of channels etc. are stored, and raw sound data.

Use AudioFileOpenURL to open audiofile and take all its properties with AudioFileGetProperty function. Basicaly you need only kAudioFilePropertyDataFormat and kAudioFilePropertyAudioDataPacketCount properties:

AudioFileID  fileID;    // the identifier for the audio file
CFURLRef     fileURL = ...; // file URL
AudioStreamBasicDescription format; // structure containing audio header info
    UInt64  packetsCount;

AudioFileOpenURL(fileURL, 
    0x01, //fsRdPerm,                       // read only
    0, //no hint
    &fileID
);

UInt32 sizeOfPlaybackFormatASBDStruct = sizeof format;
AudioFileGetProperty (
    fileID, 
    kAudioFilePropertyDataFormat,
    &sizeOfPlaybackFormatASBDStruct,
    &format
);

propertySize = sizeof(packetsCount);
AudioFileGetProperty(fileID, kAudioFilePropertyAudioDataPacketCount, &propertySize, &packetsCount);

Then you can take any range of audiopackets data with:

   OSStatus AudioFileReadPackets (
       AudioFileID                  inAudioFile,
       Boolean                      inUseCache,
       UInt32                       *outNumBytes,
       AudioStreamPacketDescription *outPacketDescriptions,
       SInt64                       inStartingPacket,
       UInt32                       *ioNumPackets,
       void                         *outBuffer
    );

Answer 2

Apple already has written something that can do this: AUNetSend and AUNetReceive. AUNetSend is an effect AudioUnit that sends audio to an AUNetReceive AudioUnit on another computer.

Unfortunately these AUs are not available on the iPhone, though.

Answer 3

Here is simplest class to play files with AQ Note that you can play it from any point (just set currentPacketNumber)

#import <Foundation/Foundation.h>
#import <AudioToolbox/AudioToolbox.h>

@interface AudioFile : NSObject {
    AudioFileID                     fileID;     // the identifier for the audio file to play
    AudioStreamBasicDescription     format;
    UInt64                          packetsCount;           
    UInt32                          maxPacketSize;  
}

@property (readwrite)           AudioFileID                 fileID;
@property (readwrite)           UInt64                      packetsCount;
@property (readwrite)           UInt32                      maxPacketSize;

- (id) initWithURL: (CFURLRef) url;
- (AudioStreamBasicDescription *)audioFormatRef;

@end


//  AudioFile.m

#import "AudioFile.h"


@implementation AudioFile

@synthesize fileID;
@synthesize format;
@synthesize maxPacketSize;
@synthesize packetsCount;

- (id)initWithURL:(CFURLRef)url{
    if (self = [super init]){       
        AudioFileOpenURL(
                         url,
                         0x01, //fsRdPerm, read only
                         0, //no hint
                         &fileID
                         );

        UInt32 sizeOfPlaybackFormatASBDStruct = sizeof format;
        AudioFileGetProperty (
                              fileID, 
                              kAudioFilePropertyDataFormat,
                              &sizeOfPlaybackFormatASBDStruct,
                              &format
                              );

        UInt32 propertySize = sizeof (maxPacketSize);

        AudioFileGetProperty (
                              fileID, 
                              kAudioFilePropertyMaximumPacketSize,
                              &propertySize,
                              &maxPacketSize
                              );

        propertySize = sizeof(packetsCount);
        AudioFileGetProperty(fileID, kAudioFilePropertyAudioDataPacketCount, &propertySize, &packetsCount);
    }
    return self;
} 

-(AudioStreamBasicDescription *)audioFormatRef{
    return &format;
}

- (void) dealloc {
    AudioFileClose(fileID);
    [super dealloc];
}



//  AQPlayer.h

#import <Foundation/Foundation.h>
#import "AudioFile.h"

#define AUDIOBUFFERS_NUMBER     3
#define MAX_PACKET_COUNT    4096

@interface AQPlayer : NSObject {
@public
    AudioQueueRef                   queue;
    AudioQueueBufferRef             buffers[AUDIOBUFFERS_NUMBER];
    NSInteger                       bufferByteSize;
    AudioStreamPacketDescription    packetDescriptions[MAX_PACKET_COUNT];

    AudioFile * audioFile;
    SInt64  currentPacketNumber;
    UInt32  numPacketsToRead;
}

@property (nonatomic)               SInt64          currentPacketNumber;
@property (nonatomic, retain)       AudioFile       * audioFile;

-(id)initWithFile:(NSString *)file;
-(NSInteger)fillBuffer:(AudioQueueBufferRef)buffer;
-(void)play;

@end 

//  AQPlayer.m

#import "AQPlayer.h"

static void AQOutputCallback(void * inUserData, AudioQueueRef inAQ, AudioQueueBufferRef inBuffer) {
    AQPlayer * aqp = (AQPlayer *)inUserData;
    [aqp fillBuffer:(AudioQueueBufferRef)inBuffer];
}

@implementation AQPlayer

@synthesize currentPacketNumber;
@synthesize audioFile;

-(id)initWithFile:(NSString *)file{
    if ([self init]){
        audioFile = [[AudioFile alloc] initWithURL:[NSURL fileURLWithPath:file]];
        currentPacketNumber = 0;
        AudioQueueNewOutput ([audioFile audioFormatRef], AQOutputCallback, self, CFRunLoopGetCurrent (), kCFRunLoopCommonModes, 0, &queue);
        bufferByteSize = 4096;
        if (bufferByteSize < audioFile.maxPacketSize) bufferByteSize = audioFile.maxPacketSize; 
        numPacketsToRead = bufferByteSize/audioFile.maxPacketSize;
        for(int i=0; i<AUDIOBUFFERS_NUMBER; i++){
            AudioQueueAllocateBuffer (queue, bufferByteSize, &buffers[i]);
        }
    }
    return self;
}

-(void) dealloc{
    [audioFile release];
    if (queue){
        AudioQueueDispose(queue, YES);
        queue = nil;
    }
    [super dealloc];
}

- (void)play{
    for (int bufferIndex = 0; bufferIndex < AUDIOBUFFERS_NUMBER; ++bufferIndex){
        [self fillBuffer:buffers[bufferIndex]];
    }
    AudioQueueStart (queue, NULL);

}

-(NSInteger)fillBuffer:(AudioQueueBufferRef)buffer{
    UInt32 numBytes;
    UInt32 numPackets = numPacketsToRead;
    BOOL isVBR = [audioFile audioFormatRef]->mBytesPerPacket == 0 ? YES : NO;
    AudioFileReadPackets(
                         audioFile.fileID,
                         NO,
                         &numBytes,
                         isVBR ? packetDescriptions : 0,
                         currentPacketNumber,
                         &numPackets, 
                         buffer->mAudioData
                         );

    if (numPackets > 0) {
        buffer->mAudioDataByteSize = numBytes;      
        AudioQueueEnqueueBuffer (
                                 queue,
                                 buffer,
                                 isVBR ? numPackets : 0,
                                 isVBR ? packetDescriptions : 0
                                 );


    } 
    else{
        // end of present data, check if all packets are played
        // if yes, stop play and dispose queue
        // if no, pause queue till new data arrive then start it again
    }
    return  numPackets;
}