【FFmpeg】使用AudioToolbox播放PCM

跟YUV数据一样，PCM数据也是裸数据流，里面不包含存储格式、通道数、采样率等信息，我们只有知道这些信息，才能正常播放PCM数据，在AudioToolbox中使用AudioStreamBasicDescription结构体来描述这些参数。AudioStreamBasicDescription结构体定义如下：

struct AudioStreamBasicDescription
{
    Float64             mSampleRate;//采样率
    AudioFormatID       mFormatID;//数据格式
    AudioFormatFlags    mFormatFlags;//数据的存储方式
    UInt32              mBytesPerPacket;//每个packet中有几个字节
    UInt32              mFramesPerPacket;//每个packer中有几个frame
    UInt32              mBytesPerFrame;//每个frame中有几个字节
    UInt32              mChannelsPerFrame;//每个frame中有几个通道
    UInt32              mBitsPerChannel;//每个通道有多少位
    UInt32              mReserved;
};

我用到的PCM数据格式是s16le，通道数为2，采样率是44.1K。因此我构建的AudioStreamBasicDescription结构体如下：

int channels = 2;
AudioStreamBasicDescription audioStreamBasicDescription;
audioStreamBasicDescription.mSampleRate = 44100;
audioStreamBasicDescription.mFormatID = kAudioFormatLinearPCM;
audioStreamBasicDescription.mFormatFlags = kAudioFormatFlagIsSignedInteger | kAudioFormatFlagIsPacked;
audioStreamBasicDescription.mFramesPerPacket = 1;
audioStreamBasicDescription.mChannelsPerFrame = channels;
audioStreamBasicDescription.mBitsPerChannel = 16;
audioStreamBasicDescription.mBytesPerPacket = channels*2;
audioStreamBasicDescription.mBytesPerFrame = channels*2;

构建好这些音频参数后，我们就来使用AudioUnit一步一步来播放PCM数据。

1、初始化AudioUnit

- (void)setupAudioUnit {
    AudioComponentDescription audioDesc;
    audioDesc.componentType = kAudioUnitType_Output;
    audioDesc.componentSubType = kAudioUnitSubType_RemoteIO;
    audioDesc.componentManufacturer = kAudioUnitManufacturer_Apple;
    audioDesc.componentFlags = 0;
    audioDesc.componentFlagsMask = 0;
    
    AudioComponent inputComponent = AudioComponentFindNext(NULL, &audioDesc);
    AudioComponentInstanceNew(inputComponent, &_audioUnit);
}

2、设置AudioUnit属性

- (void)setupAudioUnitProperty:(AudioStreamBasicDescription)audioStreamBasicDescription {
    AudioUnitSetProperty(self.audioUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Input, 0, &audioStreamBasicDescription, sizeof(audioStreamBasicDescription));
    AURenderCallbackStruct callbackStruct;
    callbackStruct.inputProc = (AURenderCallback)AudioUnitPlayer_AURenderCallback;
    callbackStruct.inputProcRefCon = (__bridge void * _Nullable)(self);
    AudioUnitSetProperty(self.audioUnit, kAudioUnitProperty_SetRenderCallback, kAudioUnitScope_Global, 0, &callbackStruct, sizeof(callbackStruct));
}

这一步，我们把之前构建好的AudioStreamBasicDescription音频参数告诉AudioUnit，这样它就知道怎么去解析播放PCM数据了。我们还设置了一个回调方法AudioUnitPlayer_AURenderCallback，播放音频时，音频设备需要数据了，设备就是通过这个回调方法告诉我们，它没有PCM数据了，需要我们传输数据给它了。是一个被动的传输过程，而不是我们主动把数据喂给设备。

3、定义AudioUnitPlayer_AURenderCallback方法

static OSStatus AudioUnitPlayer_AURenderCallback(void *inRefCon,
                            AudioUnitRenderActionFlags *ioActionFlags,
                            const AudioTimeStamp *inTimeStamp,
                            UInt32 inBusNumber,
                            UInt32 inNumberFrames,
                            AudioBufferList *ioData)
{
    memset(ioData->mBuffers[0].mData, 0, ioData->mBuffers[0].mDataByteSize);
    AudioUnitPlayer *player = (__bridge AudioUnitPlayer *)inRefCon;
    NSData *data = [player.fileHandle readDataOfLength:ioData->mBuffers[0].mDataByteSize];
    if (data.lengthmBuffers[0].mDataByteSize) {
        if (@available(iOS 13.0, *)) {
            [player.fileHandle seekToOffset:0 error:nil];
        } else {
            [player.fileHandle seekToFileOffset:0];
        }
        [player stopPlayer];
        return noErr;
    }
    memcpy(ioData->mBuffers[0].mData, data.bytes, data.length);
    return noErr;
}

可以看到，我们就是通过这个方法把PCM数据传输给音频设备的。

4、开始播放或暂停

- (void)startPlayer {
    AudioOutputUnitStart(self.audioUnit);
}

- (void)stopPlayer {
    AudioOutputUnitStop(self.audioUnit);
}

完整代码如下：
.h文件

#import 
#import 

NS_ASSUME_NONNULL_BEGIN

@interface AudioUnitPlayer : NSObject

+ (instancetype)player:(AudioStreamBasicDescription)audioDesc pcmPath:(NSString *)pcmPath;
- (void)startPlayer;
- (void)stopPlayer;

@end

NS_ASSUME_NONNULL_END

.m文件

#import "AudioUnitPlayer.h"

@interface AudioUnitPlayer ()

@property (nonatomic, assign) AudioUnit audioUnit;
@property (nonatomic, strong) NSFileHandle *fileHandle;

@end

@implementation AudioUnitPlayer

+ (instancetype)player:(AudioStreamBasicDescription)audioStreamBasicDescription pcmPath:(NSString *)pcmPath
{
    AudioUnitPlayer *obj = [[AudioUnitPlayer alloc] init:audioStreamBasicDescription pcmPath:pcmPath];
    return obj;
}

- (instancetype)init:(AudioStreamBasicDescription)audioStreamBasicDescription pcmPath:(NSString *)pcmPath
{
    self = [super init];
    if (self) {
        [self setupAudioUnit];
        [self setupAudioUnitProperty:audioStreamBasicDescription];
        self.fileHandle = [NSFileHandle fileHandleForReadingAtPath:pcmPath];

    }
    return self;
}

- (void)setupAudioUnit {
    AudioComponentDescription audioDesc;
    audioDesc.componentType = kAudioUnitType_Output;
    audioDesc.componentSubType = kAudioUnitSubType_RemoteIO;
    audioDesc.componentManufacturer = kAudioUnitManufacturer_Apple;
    audioDesc.componentFlags = 0;
    audioDesc.componentFlagsMask = 0;
    
    AudioComponent inputComponent = AudioComponentFindNext(NULL, &audioDesc);
    AudioComponentInstanceNew(inputComponent, &_audioUnit);
}

- (void)setupAudioUnitProperty:(AudioStreamBasicDescription)audioStreamBasicDescription {
    
    AudioUnitSetProperty(self.audioUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Input, 0, &audioStreamBasicDescription, sizeof(audioStreamBasicDescription));
    
    AURenderCallbackStruct callbackStruct;
    callbackStruct.inputProc = (AURenderCallback)AudioUnitPlayer_AURenderCallback;
    callbackStruct.inputProcRefCon = (__bridge void * _Nullable)(self);
    AudioUnitSetProperty(self.audioUnit, kAudioUnitProperty_SetRenderCallback, kAudioUnitScope_Global, 0, &callbackStruct, sizeof(callbackStruct));

}

static OSStatus AudioUnitPlayer_AURenderCallback(void *inRefCon,
                            AudioUnitRenderActionFlags *ioActionFlags,
                            const AudioTimeStamp *inTimeStamp,
                            UInt32 inBusNumber,
                            UInt32 inNumberFrames,
                            AudioBufferList *ioData)
{
    memset(ioData->mBuffers[0].mData, 0, ioData->mBuffers[0].mDataByteSize);
    AudioUnitPlayer *player = (__bridge AudioUnitPlayer *)inRefCon;
    NSData *data = [player.fileHandle readDataOfLength:ioData->mBuffers[0].mDataByteSize];
    if (data.lengthmBuffers[0].mDataByteSize) {
        if (@available(iOS 13.0, *)) {
            [player.fileHandle seekToOffset:0 error:nil];
        } else {
            [player.fileHandle seekToFileOffset:0];
        }
        [player stopPlayer];
        return noErr;
    }
    memcpy(ioData->mBuffers[0].mData, data.bytes, data.length);
    return noErr;
}

- (void)startPlayer {
    AudioOutputUnitStart(self.audioUnit);
}

- (void)stopPlayer {
    AudioOutputUnitStop(self.audioUnit);
}

@end