音频AAC解码

在相亲交友源码中，为了提升音频数据的传输效率，我们需要对音频数据进行编码处理，但是编码后的音频文件是无法直接播放的，需要进行解码处理，今天我们就来看一下在相亲交友源码开发中，实现音频AAC解码的相关代码。

在解码时 👉🏻 我们需要封装一个工具类CCAudioDecoder。

解码工具类头文件

#import <Foundation/Foundation.h>
#import <AVFoundation/AVFoundation.h>
@class CCAudioConfig;

/**AAC解码回调代理*/
@protocol CCAudioDecoderDelegate <NSObject>
- (void)audioDecodeCallback:(NSData *)pcmData;
@end

@interface CCAudioDecoder : NSObject
@property (nonatomic, strong) CCAudioConfig *config;
@property (nonatomic, weak) id<CCAudioDecoderDelegate> delegate;

//初始化 传入解码配置
- (instancetype)initWithConfig:(CCAudioConfig *)config;

/**解码aac*/
- (void)decodeAudioAACData: (NSData *)aacData;
@end

.m中的扩展类👇🏻

@interface CCAudioDecoder()
@property (nonatomic, strong) dispatch_queue_t decoderQueue;
@property (nonatomic, strong) dispatch_queue_t callbackQueue;

//对音频转换器对象
@property (nonatomic) AudioConverterRef audioConverter;
//AAC缓存区
@property (nonatomic) char *aacBuffer;
//AAC缓存区大小
@property (nonatomic) UInt32 aacBufferSize;
//音频流包的描述信息
@property (nonatomic) AudioStreamPacketDescription *packetDesc;

@end

初始化

接着看看初始化👇🏻

- (instancetype)initWithConfig:(CCAudioConfig *)config {
    self = [super init];
    if (self) {
        _decoderQueue = dispatch_queue_create("aac hard decoder queue", DISPATCH_QUEUE_SERIAL);
        _callbackQueue = dispatch_queue_create("aac hard decoder callback queue", DISPATCH_QUEUE_SERIAL);
        _audioConverter = NULL;
        _aacBufferSize = 0;
        _aacBuffer = NULL;
        _config = config;
        if (_config == nil) {
            _config = [[CCAudioConfig alloc] init];
        }
        AudioStreamPacketDescription desc = {0};
        _packetDesc = &desc;
        [self setupEncoder];
    }
    return self;
}

然后是setupEncoder👇🏻

- (void)setupEncoder {
    //输出参数pcm
    AudioStreamBasicDescription outputAudioDes = {0};
    outputAudioDes.mSampleRate = (Float64)_config.sampleRate;       //采样率
    outputAudioDes.mChannelsPerFrame = (UInt32)_config.channelCount; //输出声道数(左声道、右声道)
    outputAudioDes.mFormatID = kAudioFormatLinearPCM;                //输出格式
    outputAudioDes.mFormatFlags = (kAudioFormatFlagIsSignedInteger | kAudioFormatFlagIsPacked); //编码 12
    outputAudioDes.mFramesPerPacket = 1;                            //每一个packet帧数 ；
    outputAudioDes.mBitsPerChannel = 16;                             //数据帧中每个通道的采样位数。
    outputAudioDes.mBytesPerFrame = outputAudioDes.mBitsPerChannel / 8 *outputAudioDes.mChannelsPerFrame;                              //每一帧大小（采样位数 / 8 *声道数）
    outputAudioDes.mBytesPerPacket = outputAudioDes.mBytesPerFrame * outputAudioDes.mFramesPerPacket;                             //每个packet大小（帧大小 * 帧数）
    outputAudioDes.mReserved =  0;                                  //对其方式 0(8字节对齐)
    
    //输入参数aac
    AudioStreamBasicDescription inputAduioDes = {0};
    inputAduioDes.mSampleRate = (Float64)_config.sampleRate;
    inputAduioDes.mFormatID = kAudioFormatMPEG4AAC;
    inputAduioDes.mFormatFlags = kMPEG4Object_AAC_LC;
    inputAduioDes.mFramesPerPacket = 1024;
    inputAduioDes.mChannelsPerFrame = (UInt32)_config.channelCount;
    
    //填充输出相关信息
    UInt32 inDesSize = sizeof(inputAduioDes);
    AudioFormatGetProperty(kAudioFormatProperty_FormatInfo, 0, NULL, &inDesSize, &inputAduioDes);
    
    //获取解码器的描述信息(只能传入software)
    AudioClassDescription *audioClassDesc = [self getAudioCalssDescriptionWithType:outputAudioDes.mFormatID fromManufacture:kAppleSoftwareAudioCodecManufacturer];
    //创建converter
    OSStatus status = AudioConverterNewSpecific(&inputAduioDes, &outputAudioDes, 1, audioClassDesc, &_audioConverter);
    if (status != noErr) {
        NSLog(@"Error！：硬解码AAC创建失败, status= %d", (int)status);
        return;
    }
}

和编码的- (void)setupEncoderWithSampleBuffer: (CMSampleBufferRef)sampleBuffer流程基本上一模一样。区别在于相亲交友源码创建解码器的方法getAudioCalssDescriptionWithType:fromManufacture:👇🏻

- (AudioClassDescription *)getAudioCalssDescriptionWithType:(AudioFormatID)type fromManufacture:(uint32_t)manufacture {
    static AudioClassDescription desc;
    UInt32 decoderSpecific = type;
    //获取满足AAC解码器的总大小
    UInt32 size;
    OSStatus status = AudioFormatGetPropertyInfo(kAudioFormatProperty_Decoders, sizeof(decoderSpecific), &decoderSpecific, &size);
    if (status != noErr) {
        NSLog(@"Error！：硬解码AAC get info 失败, status= %d", (int)status);
        return nil;
    }
    //计算aac解码器的个数
    unsigned int count = size / sizeof(AudioClassDescription);
    //创建一个包含count个解码器的数组
    AudioClassDescription description[count];
    //将满足aac解码的解码器的信息写入数组
    status = AudioFormatGetProperty(kAudioFormatProperty_Encoders, sizeof(decoderSpecific), &decoderSpecific, &size, &description);
    if (status != noErr) {
        NSLog(@"Error！：硬解码AAC get propery 失败, status= %d", (int)status);
        return nil;
    }
    for (unsigned int i = 0; i < count; i++) {
        if (type == description[i].mSubType && manufacture == description[i].mManufacturer) {
            desc = description[i];
            return &desc;
        }
    }

有以下几点区别👇🏻

• 输出参数不同：编码是AAC 解码是PCM
• 转换器不同，编码是kAudioFormatProperty_Encoders，解码是kAudioFormatProperty_Decoders

相亲交友源码音频解码前的准备

我们封装了个结构体CCAudioUserData，用于记录aac的信息 作为解码回调函数的参数👇🏻

typedef struct {
    char * data;
    UInt32 size;
    UInt32 channelCount;
    AudioStreamPacketDescription packetDesc;
} CCAudioUserData;

解码

接着就是相亲交友源码中的音频解码流程了👇🏻

- (void)decodeAudioAACData:(NSData *)aacData {
    if (!_audioConverter) { return; }
    
    dispatch_async(_decoderQueue, ^{
        //CCAudioUserData记录aac的信息 作为参数参入解码回调函数
        CCAudioUserData userData = {0};
        userData.channelCount = (UInt32)_config.channelCount;
        userData.data = (char *)[aacData bytes];
        userData.size = (UInt32)aacData.length;
        userData.packetDesc.mDataByteSize = (UInt32)aacData.length;
        userData.packetDesc.mStartOffset = 0;
        userData.packetDesc.mVariableFramesInPacket = 0;
        
        //输出大小和packet个数
        UInt32 pcmBufferSize = (UInt32)(2048 * _config.channelCount);
        UInt32 pcmDataPacketSize = 1024;
        
        //创建临时容器pcm
        uint8_t *pcmBuffer = malloc(pcmBufferSize);
        memset(pcmBuffer, 0, pcmBufferSize);
        
        //输出buffer
        AudioBufferList outAudioBufferList = {0};
        outAudioBufferList.mNumberBuffers = 1;
        outAudioBufferList.mBuffers[0].mNumberChannels = (uint32_t)_config.channelCount;
        outAudioBufferList.mBuffers[0].mDataByteSize = (UInt32)pcmBufferSize;
        outAudioBufferList.mBuffers[0].mData = pcmBuffer;
        
        //输出描述
        AudioStreamPacketDescription outputPacketDesc = {0};
        
        //配置填充函数，获取输出数据
        OSStatus status = AudioConverterFillComplexBuffer(_audioConverter, &AudioDecoderConverterComplexInputDataProc, &userData, &pcmDataPacketSize, &outAudioBufferList, &outputPacketDesc);
        if (status != noErr) {
            NSLog(@"Error: AAC Decoder error, status=%d",(int)status);
            return;
        }
        //如果获取到数据
        if (outAudioBufferList.mBuffers[0].mDataByteSize > 0) {
            NSData *rawData = [NSData dataWithBytes:outAudioBufferList.mBuffers[0].mData length:outAudioBufferList.mBuffers[0].mDataByteSize];
            dispatch_async(_callbackQueue, ^{
                [_delegate audioDecodeCallback:rawData];
            });
        }
        free(pcmBuffer);
    });
}

其中解码的回调函数是AudioDecoderConverterComplexInputDataProc。

解码回调

static OSStatus AudioDecoderConverterComplexInputDataProc(  AudioConverterRef inAudioConverter, UInt32 *ioNumberDataPackets, AudioBufferList *ioData,  AudioStreamPacketDescription **outDataPacketDescription,  void *inUserData) {
    CCAudioUserData *audioDecoder = (CCAudioUserData *)(inUserData);
    if (audioDecoder->size <= 0) {
        ioNumberDataPackets = 0;
        return -1;
    }
   
    //填充数据
    *outDataPacketDescription = &audioDecoder->packetDesc;
    (*outDataPacketDescription)[0].mStartOffset = 0;
    (*outDataPacketDescription)[0].mDataByteSize = audioDecoder->size;
    (*outDataPacketDescription)[0].mVariableFramesInPacket = 0;
    
    ioData->mBuffers[0].mData = audioDecoder->data;
    ioData->mBuffers[0].mDataByteSize = audioDecoder->size;
    ioData->mBuffers[0].mNumberChannels = audioDecoder->channelCount;
    
    return noErr;
}

在相亲交友源码实现音频解码的过程中你会发现，不需要桥接self对象，因为我们把数据缓存在自定义的结构体CCAudioUserData之中。这点也是和编码的区别。
至此，解码的工具类已封装完毕！