我正在使用AVCaptureSession使用AVCaptureVideoDataOutput抓取原始视频帧并在软件using x264中对其进行编码.这非常有用.
我想对音频做同样的事情,只是因为我不需要在音频方面那么多控制,所以我想使用内置的硬件编码器来产生AAC流.这意味着使用Audio ToolBox图层中的Audio Converter.为了做到这一点,我为AVCaptudeAudioDataOutput的音频帧添加了一个处理程序：
- (void)captureOutput:(AVCaptureOutput *)captureOutput
didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer
fromConnection:(AVCaptureConnection *)connection
{
// get the audio samples into a common buffer _pcmBuffer
CMBlockBufferRef blockBuffer = CMSampleBufferGetDataBuffer(sampleBuffer);
CMBlockBufferGetDataPointer(blockBuffer,NULL,&_pcmBufferSize,&_pcmBuffer);
// use AudioConverter to
UInt32 ouputPacketsCount = 1;
audiobufferlist bufferList;
bufferList.mNumberBuffers = 1;
bufferList.mBuffers[0].mNumberChannels = 1;
bufferList.mBuffers[0].mDataByteSize = sizeof(_aacBuffer);
bufferList.mBuffers[0].mData = _aacBuffer;
Osstatus st = AudioConverterFillComplexBuffer(_converter,converter_callback,(__bridge void *) self,&ouputPacketsCount,&bufferList,NULL);
if (0 == st) {
// ... send bufferList.mBuffers[0].mDataByteSize bytes from _aacBuffer...
}
}
在这种情况下,音频转换器的回调函数非常简单(假设数据包大小和计数设置正确)：
- (void) putPcmSamplesInBufferList:(audiobufferlist *)bufferList withCount:(UInt32 *)count
{
bufferList->mBuffers[0].mData = _pcmBuffer;
bufferList->mBuffers[0].mDataByteSize = _pcmBufferSize;
}
音频转换器的设置如下所示：
{
// ...
AudioStreamBasicDescription pcmASBD = {0};
pcmASBD.mSampleRate = ((AVAudioSession *) [AVAudioSession sharedInstance]).currentHardwareSampleRate;
pcmASBD.mFormatID = kAudioFormatLinearPCM;
pcmASBD.mFormatFlags = kAudioFormatFlagsCanonical;
pcmASBD.mChannelsPerFrame = 1;
pcmASBD.mBytesPerFrame = sizeof(AudioSampleType);
pcmASBD.mFramesPerPacket = 1;
pcmASBD.mBytesPerPacket = pcmASBD.mBytesPerFrame * pcmASBD.mFramesPerPacket;
pcmASBD.mBitsPerChannel = 8 * pcmASBD.mBytesPerFrame;
AudioStreamBasicDescription aacASBD = {0};
aacASBD.mFormatID = kAudioFormatMPEG4AAC;
aacASBD.mSampleRate = pcmASBD.mSampleRate;
aacASBD.mChannelsPerFrame = pcmASBD.mChannelsPerFrame;
size = sizeof(aacASBD);
AudioFormatGetProperty(kAudioFormatProperty_FormatInfo,&size,&aacASBD);
AudioConverterNew(&pcmASBD,&aacASBD,&_converter);
// ...
}
这似乎很简单,只有IT不工作. AVCaptureSession运行后,音频转换器(特别是AudioConverterFillComplexBuffer)返回’hwiu'(硬件使用中)错误.如果会话停止但是我无法捕获任何内容,转换工作正常…
我想知道是否有办法从AVCaptureSession中获取AAC流.我正在考虑的选项是：
>以某种方式使用AVAssetWriterInput将音频样本编码到AAC中,然后以某种方式获取编码的数据包(而不是通过AVAssetWriter,它只会写入文件).
>重新组织我的应用程序,使其仅在视频端使用AVCaptureSession,并在音频端使用Audio Queues.这将使流控制(开始和停止录制,响应中断)变得更加复杂,我担心它可能会导致音频和视频之间的同步问题.而且,它似乎不是一个好的设计.
有没有人知道是否可以从AVCaptureSession中获取AAC？我必须在这里使用音频队列吗？这可以让我进入同步或控制问题吗？
解决方法
似乎AVCaptureSession抓住了AAC硬件编码器,但只允许您使用它直接写入文件.
您可以使用软件编码器,但您必须专门询问它而不是使用AudioConverterNew：
AudioClassDescription *description = [self
getAudioClassDescriptionWithType:kAudioFormatMPEG4AAC
fromManufacturer:kAppleSoftwareAudioCodecManufacturer];
if (!description) {
return false;
}
// see the question as for setting up pcmASBD and arc ASBD
Osstatus st = AudioConverterNewSpecific(&pcmASBD,1,description,&_converter);
if (st) {
NSLog(@"error creating audio converter: %s",OsstATUS(st));
return false;
}
同
- (AudioClassDescription *)getAudioClassDescriptionWithType:(UInt32)type
fromManufacturer:(UInt32)manufacturer
{
static AudioClassDescription desc;
UInt32 encoderSpecifier = type;
Osstatus st;
UInt32 size;
st = AudioFormatGetPropertyInfo(kAudioFormatProperty_Encoders,sizeof(encoderSpecifier),&encoderSpecifier,&size);
if (st) {
NSLog(@"error getting audio format propery info: %s",OsstATUS(st));
return nil;
}
unsigned int count = size / sizeof(AudioClassDescription);
AudioClassDescription descriptions[count];
st = AudioFormatGetProperty(kAudioFormatProperty_Encoders,descriptions);
if (st) {
NSLog(@"error getting audio format propery: %s",OsstATUS(st));
return nil;
}
for (unsigned int i = 0; i < count; i++) {
if ((type == descriptions[i].mSubType) &&
(manufacturer == descriptions[i].mManufacturer)) {
memcpy(&desc,&(descriptions[i]),sizeof(desc));
return &desc;
}
}
return nil;
}
当然,软件编码器会占用cpu资源,但会完成工作.