如何在 iOS 中将 AAC 音频缓冲区解码为 PCM 缓冲区? [英] How to decode AAC audio buffer to PCM buffer in iOS?

问题描述

我正在尝试在 iOS 中将 AAC 音频解码为 PCM 音频，最好的方法是什么?任何示例代码都会非常有帮助...是否有任何简单的 API 可以做到这一点..?

I am trying to decode AAC audio to PCM audio in iOS, what the best way to do this?Any sample code would be very helpful...Is there any simple APIs to do this..?

推荐答案

我有示例代码来做.

开始时，您应该配置输入/输出 ASBD(AudioStreamBasicDescription)并创建转换器:

At start you should configure in/out ASBD (AudioStreamBasicDescription) and create converter:

- (void)setupAudioConverter{
    AudioStreamBasicDescription outFormat;
    memset(&outFormat, 0, sizeof(outFormat));
    outFormat.mSampleRate       = 44100;
    outFormat.mFormatID         = kAudioFormatLinearPCM;
    outFormat.mFormatFlags      = kLinearPCMFormatFlagIsSignedInteger;
    outFormat.mBytesPerPacket   = 2;
    outFormat.mFramesPerPacket  = 1;
    outFormat.mBytesPerFrame    = 2;
    outFormat.mChannelsPerFrame = 1;
    outFormat.mBitsPerChannel   = 16;
    outFormat.mReserved         = 0;


    AudioStreamBasicDescription inFormat;
    memset(&inFormat, 0, sizeof(inFormat));
    inFormat.mSampleRate        = 44100;
    inFormat.mFormatID          = kAudioFormatMPEG4AAC;
    inFormat.mFormatFlags       = kMPEG4Object_AAC_LC;
    inFormat.mBytesPerPacket    = 0;
    inFormat.mFramesPerPacket   = 1024;
    inFormat.mBytesPerFrame     = 0;
    inFormat.mChannelsPerFrame  = 1;
    inFormat.mBitsPerChannel    = 0;
    inFormat.mReserved          = 0;

    OSStatus status =  AudioConverterNew(&inFormat, &outFormat, &_audioConverter);
    if (status != 0) {
        printf("setup converter error, status: %i
", (int)status);
    }
}

之后你应该为音频转换器制作回调函数:

After that you should make callback function for audio converter:

struct PassthroughUserData {
    UInt32 mChannels;
    UInt32 mDataSize;
    const void* mData;
    AudioStreamPacketDescription mPacket;
};


OSStatus inInputDataProc(AudioConverterRef aAudioConverter,
                         UInt32* aNumDataPackets /* in/out */,
                         AudioBufferList* aData /* in/out */,
                         AudioStreamPacketDescription** aPacketDesc,
                         void* aUserData)
{

    PassthroughUserData* userData = (PassthroughUserData*)aUserData;
    if (!userData->mDataSize) {
        *aNumDataPackets = 0;
        return kNoMoreDataErr;
    }

    if (aPacketDesc) {
        userData->mPacket.mStartOffset = 0;
        userData->mPacket.mVariableFramesInPacket = 0;
        userData->mPacket.mDataByteSize = userData->mDataSize;
        *aPacketDesc = &userData->mPacket;
    }

    aData->mBuffers[0].mNumberChannels = userData->mChannels;
    aData->mBuffers[0].mDataByteSize = userData->mDataSize;
    aData->mBuffers[0].mData = const_cast<void*>(userData->mData);

    // No more data to provide following this run.
    userData->mDataSize = 0;

    return noErr;
}

以及帧解码的方法:

- (void)decodeAudioFrame:(NSData *)frame withPts:(NSInteger)pts{
    if(!_audioConverter){
        [self setupAudioConverter];
    }

    PassthroughUserData userData = { 1, (UInt32)frame.length, [frame bytes]};
    NSMutableData *decodedData = [NSMutableData new];

    const uint32_t MAX_AUDIO_FRAMES = 128;
    const uint32_t maxDecodedSamples = MAX_AUDIO_FRAMES * 1;

    do{
        uint8_t *buffer = (uint8_t *)malloc(maxDecodedSamples * sizeof(short int));
        AudioBufferList decBuffer;
        decBuffer.mNumberBuffers = 1;
        decBuffer.mBuffers[0].mNumberChannels = 1;
        decBuffer.mBuffers[0].mDataByteSize = maxDecodedSamples * sizeof(short int);
        decBuffer.mBuffers[0].mData = buffer;

        UInt32 numFrames = MAX_AUDIO_FRAMES;

        AudioStreamPacketDescription outPacketDescription;
        memset(&outPacketDescription, 0, sizeof(AudioStreamPacketDescription));
        outPacketDescription.mDataByteSize = MAX_AUDIO_FRAMES;
        outPacketDescription.mStartOffset = 0;
        outPacketDescription.mVariableFramesInPacket = 0;

        OSStatus rv = AudioConverterFillComplexBuffer(_audioConverter,
                                                      inInputDataProc,
                                                      &userData,
                                                      &numFrames /* in/out */,
                                                      &decBuffer,
                                                      &outPacketDescription);

        if (rv && rv != kNoMoreDataErr) {
            NSLog(@"Error decoding audio stream: %d
", rv);
            break;
        }

        if (numFrames) {
            [decodedData appendBytes:decBuffer.mBuffers[0].mData length:decBuffer.mBuffers[0].mDataByteSize];
        }

        if (rv == kNoMoreDataErr) {
            break;
        }

    }while (true);

    //void *pData = (void *)[decodedData bytes];
    //audioRenderer->Render(&pData, decodedData.length, pts);
}

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