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AVAudioPCMBuffer to CMSampleBufferRef

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需求

把AVAudioPCMBuffer转换层CMSampleBufferRef，同理也可以从AudioBufferList加上AudioStreamBasicDescription转换成CMSampleBufferRef

实现代码

实现代码如下：

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- (CMSampleBufferRef)createAudioSampleBufferFrom:(AVAudioPCMBuffer *) pcmBuffer {
	AudioBufferList *audioBufferList = [pcmBuffer mutableAudioBufferList];
	AudioStreamBasicDescription asbd = *pcmBuffer.format.streamDescription;

	CMSampleBufferRef sampleBuffer = NULL;
	CMFormatDescriptionRef format = NULL;

	OSStatus error = CMAudioFormatDescriptionCreate(kCFAllocatorDefault, &asbd, 0, NULL, 0, NULL, NULL, &format);
	if (error != noErr) {
		return nil;
	}

	CMSampleTimingInfo timing = { CMTimeMake(1, asbd.mSampleRate), kCMTimeZero, kCMTimeInvalid };
	error = CMSampleBufferCreate(kCFAllocatorDefault,
								 NULL, false, NULL, NULL, format,
								 pcmBuffer.frameLength,
								 1, &timing, 0, NULL, &sampleBuffer);
	if (error != noErr) {
		CFRelease(format);
		NSLog(@"CMSampleBufferCreate returned error: %d", (int)error);
		return nil;
	}

	error = CMSampleBufferSetDataBufferFromAudioBufferList(sampleBuffer, kCFAllocatorDefault, kCFAllocatorDefault, 0, audioBufferList);
	if (error != noErr) {
		CFRelease(format);
		NSLog(@"CMSampleBufferSetDataBufferFromAudioBufferList returned error: %d", (int)error);
		return nil;
	}

	CFRelease(format);
	return sampleBuffer;
}

内存释放

生成的CMSampleBufferRef引用计数为1，记得释放，类似：

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CMSampleBufferRef sampleBuffer = [self createAudioSampleBufferFrom:pcmBuffer];

// do somthing with sampleBuffer

CFRelease(sampleBuffer);

AudioBufferList

关于AudioBufferList，需要说的是PCM中的双通道，对应到AudioBufferList就是mNumberBuffers

假定存在

AudioBufferList *ioData = …
AudioStreamBasicDescription asbd = …

如果ioData->mNumberBuffers为2，那么有以下几个推导：

  • 1、ioData->mBuffers[0]有数据外，ioData->mBuffers[1]也有数据；
  • 2、跟AudioBufferList对应的asbd->mChannelsPerFrame == 2
  • 3、还要补充一点的是，asbd.mChannelsPerFrame == 2的时候，asbd.mBytesPerFrame并不会包含2个channel的数据
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asbd.mBytesPerFrame = asbd.mBitsPerChannel/8; // 正确
asbd.mBytesPerFrame = asbd.mBitsPerChannel/8 * mChannelsPerFrame; // 错误

参考文档

CMSampleBuffer
iOS 音频采集 AudioBufferList转CMSampleBufferRef
CMSampleBufferSetDataBufferFromAudioBufferList returning -12731
osstatus