AudioUnitRender在iPad上的AudioBufferList中返回null

Question

我是通过AudioUnitRender从AVAudioEngineOutput创建音频文件。在iPhone上，这种实现工作正常，但在iPad上我得到了无效的音频文件。为什么会这样？

主要方法

NSTimeInterval duration = CMTimeGetSeconds(asset.duration);

NSUInteger lengthInFrames = (NSUInteger) (duration * audioDescription->mSampleRate);

    const NSUInteger kBufferLength = 1024; //3756;

    AudioBufferList *bufferList = AEAllocateAndInitAudioBufferList(*audioDescription, kBufferLength);
    AudioTimeStamp timeStamp;
    memset (&timeStamp, 0, sizeof(timeStamp));
    timeStamp.mFlags = kAudioTimeStampSampleTimeValid;
    OSStatus status = noErr;
    for (NSUInteger i = kBufferLength; i < lengthInFrames; i += kBufferLength) {
        status = [self renderToBufferList:bufferList writeToFile:audioFile bufferLength:kBufferLength timeStamp:&timeStamp];
        if (status != noErr)
            break;
    }
    if (status == noErr && timeStamp.mSampleTime < lengthInFrames) {
        NSUInteger restBufferLength = (NSUInteger) (lengthInFrames - timeStamp.mSampleTime);
        AudioBufferList *restBufferList = AEAllocateAndInitAudioBufferList(*audioDescription, (Float32)restBufferLength);
        status = [self renderToBufferList:restBufferList writeToFile:audioFile bufferLength:restBufferLength timeStamp:&timeStamp];
        AEFreeAudioBufferList(restBufferList);
    }


    SInt64 fileLengthInFrames;
    UInt32 size = sizeof(SInt64);
    ExtAudioFileGetProperty(audioFile, kExtAudioFileProperty_FileLengthFrames, &size, &fileLengthInFrames);
    AEFreeAudioBufferList(bufferList);
    ExtAudioFileDispose(audioFile);
    if (status != noErr)
        [self showAlertWithTitle:@"Error" message:@"See logs for details"];
    else {
        NSLog(@"Finished writing to file at path: %@ \n File size must be %f Mb", path,(tmpData.length/1024.0)/1024.0);
        [self showAlertWithTitle:@"Success!" message:@"Now you can play a result file"];
    }

分配缓冲区

AudioBufferList *AEAllocateAndInitAudioBufferList(AudioStreamBasicDescription audioFormat, int frameCount) {
    int numberOfBuffers = audioFormat.mFormatFlags & kAudioFormatFlagIsNonInterleaved ? audioFormat.mChannelsPerFrame : 1;
    int channelsPerBuffer = audioFormat.mFormatFlags & kAudioFormatFlagIsNonInterleaved ? 1 : audioFormat.mChannelsPerFrame;
    int bytesPerBuffer = audioFormat.mBytesPerFrame * frameCount;
    AudioBufferList *audio = malloc(sizeof(AudioBufferList) + (numberOfBuffers - 1) * sizeof(AudioBuffer));
    if (!audio) {
        return NULL;
    }
    audio->mNumberBuffers = numberOfBuffers;
    for (int i = 0; i < numberOfBuffers; i++) {
        if (bytesPerBuffer > 0) {
            audio->mBuffers[i].mData = calloc(bytesPerBuffer, 1);
            if (!audio->mBuffers[i].mData) {
                for (int j = 0; j < i; j++) free(audio->mBuffers[j].mData);
                free(audio);
                return NULL;
            }
        } else {
            audio->mBuffers[i].mData = NULL;
        }
        audio->mBuffers[i].mDataByteSize = bytesPerBuffer;
        audio->mBuffers[i].mNumberChannels = channelsPerBuffer;
    }
    return audio;
}

渲染方法

- (OSStatus)renderToBufferList:(AudioBufferList *)bufferList
                   writeToFile:(ExtAudioFileRef)audioFile
                  bufferLength:(NSUInteger)bufferLength
                     timeStamp:(AudioTimeStamp *)timeStamp {
    [self clearBufferList:bufferList];
    AudioUnit outputUnit = self.engine.outputNode.audioUnit;
    OSStatus status =AudioUnitRender(outputUnit, 0, timeStamp, 0, (UInt32)bufferLength, bufferList);

    [tmpData appendBytes:bufferList->mBuffers[0].mData length:bufferLength];

    float *data1 = bufferList->mBuffers[0].mData;
    float *data2 = bufferList->mBuffers[1].mData;;

    for(int i=0; i<bufferLength/4; i++)
    {
//On iPad data[i]==0 and data2[i] == 0 
        if(data1[i]!=0||data2[i]!=0)
        NSLog(@"%f - %f",data1[i],data2[i]);
    }

    if (status != noErr) {
        NSLog(@"Can not render audio unit");
        return status;
    }
    timeStamp->mSampleTime += bufferLength;
    status = ExtAudioFileWrite(audioFile, (UInt32)bufferLength, bufferList);
    if (status != noErr)
        NSLog(@"Can not write audio to file");
    return status;
}

渲染方法出现问题