如何使用CoreAudio的AudioConverter实时编码AAC？

Question

我可以找到的所有示例代码都AudioConverterRef集中在我预先拥有所有数据的用例(例如在磁盘上转换文件).它们通常调用AudioConverterFillComplexBufferPCM进行转换,然后将其inInputDataProcUserData填入回调中.(这真的是它应该如何使用？为什么它需要回调呢？)对于我的用例,我试图从麦克风流式传输aac音频,所以我没有文件,我的PCM缓冲区正在实时填写.

由于我没有*ioNumberDataPackets = 0预先拥有所有数据,所以我在输入数据输出后尝试进行回调,但这只是将AudioConverter置于需要进行AudioConverterReset()ted 的死亡状态,而我不会从中获取任何数据.

我在网上看到的一种方法是,如果我存储的数据太小,就会从回调中返回错误,只要我有更多数据就再试一次,但这似乎是浪费资源,我不能让自己尝试一下.

我真的需要做"重试,直到我的输入缓冲区足够大",还是有更好的方法？

Answer 1

AudioConverterFillComplexBuffer实际上并不意味着"用我在这里的输入缓冲区填充编码器".这意味着" 在此处使用来自编码器的编码数据填充此输出缓冲区 ".从这个角度来看,回调突然变得有意义 - 它用于获取源数据以满足"为我填充此输出缓冲区"请求.也许这对其他人来说是显而易见的,但是我花了很长时间才明白这一点(从我看到的所有AudioConverter示例代码中我看到人们发送输入数据的地方inInputDataProcUserData,我猜我不是唯一一个).

该AudioConverterFillComplexBuffer调用是阻塞的,并期望您从回调中同步传递数据.如果您是实时编码,则需要调用FillComplexBuffer自己设置的单独线程.在回调中,您可以检查可用的输入数据,如果它不可用,则需要阻塞信号量.使用NSCondition,编码器线程看起来像这样:

- (void)startEncoder
{
    OSStatus creationStatus = AudioConverterNew(&_fromFormat, &_toFormat, &_converter);

    _running = YES;
    _condition = [[NSCondition alloc] init];
    [self performSelectorInBackground:@selector(_encoderThread) withObject:nil];
}

- (void)_encoderThread
{
    while(_running) {
        // Make quarter-second buffers.
        size_t bufferSize = (_outputBitrate/8) * 0.25;
        NSMutableData *outAudioBuffer = [NSMutableData dataWithLength:bufferSize];
        AudioBufferList outAudioBufferList;
        outAudioBufferList.mNumberBuffers = 1;
        outAudioBufferList.mBuffers[0].mNumberChannels = _toFormat.mChannelsPerFrame;
        outAudioBufferList.mBuffers[0].mDataByteSize = (UInt32)bufferSize;
        outAudioBufferList.mBuffers[0].mData = [outAudioBuffer mutableBytes];

        UInt32 ioOutputDataPacketSize = 1;

        _currentPresentationTime = kCMTimeInvalid; // you need to fill this in during FillComplexBuffer
        const OSStatus conversionResult = AudioConverterFillComplexBuffer(_converter, FillBufferTrampoline, (__bridge void*)self, &ioOutputDataPacketSize, &outAudioBufferList, NULL);

        // here I convert the AudioBufferList into a CMSampleBuffer, which I've omitted for brevity.
        // Ping me if you need it.
        [self.delegate encoder:self encodedSampleBuffer:outSampleBuffer];
    }
}

并且回调可能如下所示:(请注意,我通常使用此蹦床立即转发到我的实例上的方法(通过转发我的实例inUserData;为简洁省略此步骤)):

static OSStatus FillBufferTrampoline(AudioConverterRef               inAudioConverter,
                                        UInt32*                         ioNumberDataPackets,
                                        AudioBufferList*                ioData,
                                        AudioStreamPacketDescription**  outDataPacketDescription,
                                        void*                           inUserData)
{
    [_condition lock];

    UInt32 countOfPacketsWritten = 0;

    while (true) {
        // If the condition fires and we have shut down the encoder, just pretend like we have written 0 bytes and are done.
        if(!_running) break;

        // Out of input data? Wait on the condition.
        if(_inputBuffer.length == 0) {
            [_condition wait];
            continue;
        }

        // We have data! Fill ioData from your _inputBuffer here.
        // Also save the input buffer's start presentationTime here.

        // Exit out of the loop, since we're done waiting for data
        break;
    }

    [_condition unlock];

        // 2. Set ioNumberDataPackets to the amount of data remaining


    // if running is false, this will be 0, indicating EndOfStream
    *ioNumberDataPackets = countOfPacketsWritten;

    return noErr;
}

为了完整起见,您可以使用以下方式为此编码器提供数据,以及如何正确关闭它:

- (void)appendSampleBuffer:(CMSampleBufferRef)sampleBuffer
{
    [_condition lock];
    // Convert sampleBuffer and put it into _inputBuffer here
    [_condition broadcast];
    [_condition unlock];
}

- (void)stopEncoding
{
    [_condition lock];
    _running = NO;
    [_condition broadcast];
    [_condition unlock];
}