Audio app › 音频接口/音视频线 › AAC音频编码 相关的原理和设置

banyuhu 发表于 2007-12-17 16:52:53

AAC音频编码 相关的原理和设置

AAC音频编码 相关的原理和设置

AAC(Advanced Audio Coding)，中文名:高级音频编码，出现于1997年，基于MPEG-2的音频编码技术。由Fraunhofer IIS、杜比实验室、AT&T、Sony等公司共同开发，目的是取代MP3格式。2000年，MPEG-4标准出现后，AAC重新集成了其特性，加入了SBR技术和PS技术，为了区别于传统的MPEG-2 AAC又称为MPEG-4 AAC。

iOS平台支持AAC编码器，主要使用AudioToolbox中的AudioConverter API。之所以做AAC编码器是因为在做一个HLS的功能，HLS要求的TS文件，需要视频采用H264编码，音频采用AAC编码。H264可以使用硬件或软件编码器，前面已经介绍。AAC也可以使用硬件或者软件编码，iOS全都支持。

首先需要创建一个Converter，也就是一个AAC Encoder，使用如下接口:

extern OSStatus

AudioConverterNew(      const AudioStreamBasicDescription*inSourceFormat,

const AudioStreamBasicDescription*inDestinationFormat,

AudioConverterRef*                  outAudioConverter)      __OSX_AVAILABLE_STARTING(__MAC_10_1,__IPHONE_2_0);

输入参数分别是源和目的的数据格式。

在AAC编码的场景下，源格式就是采集到的PCM数据，目的格式就是AAC。

AudioStreamBasicDescription inAudioStreamBasicDescription;

//    FillOutASBDForLPCM()

inAudioStreamBasicDescription.mFormatID = kAudioFormatLinearPCM;

inAudioStreamBasicDescription.mSampleRate = 44100;

inAudioStreamBasicDescription.mBitsPerChannel = 16;

inAudioStreamBasicDescription.mFramesPerPacket = 1;

inAudioStreamBasicDescription.mBytesPerFrame = 2;

inAudioStreamBasicDescription.mBytesPerPacket = inAudioStreamBasicDescription.mBytesPerFrame * inAudioStreamBasicDescription.mFramesPerPacket;

inAudioStreamBasicDescription.mChannelsPerFrame = 1;

inAudioStreamBasicDescription.mFormatFlags = kLinearPCMFormatFlagIsPacked | kLinearPCMFormatFlagIsSignedInteger | kLinearPCMFormatFlagIsNonInterleaved;

inAudioStreamBasicDescription.mReserved = 0;

AudioStreamBasicDescription outAudioStreamBasicDescription = {0}; // Always initialize the fields of a new audio stream basic description structure to zero, as shown here: ...

outAudioStreamBasicDescription.mChannelsPerFrame = 1;

outAudioStreamBasicDescription.mFormatID = kAudioFormatMPEG4AAC;

UInt32 size = sizeof(outAudioStreamBasicDescription);

AudioFormatGetProperty(kAudioFormatProperty_FormatInfo, 0, NULL, &size, &outAudioStreamBasicDescription);

OSStatus status = AudioConverterNew(&inAudioStreamBasicDescription, &outAudioStreamBasicDescription, &_audioConverter);

if(status != 0) {NSLog(@"setup converter failed: %d", (int)status);}

这样就创建了AAC编码器，默认情况下，Apple会创建一个硬件编码器，如果硬件不可用，会创建软件编码器。

经过我的测试，硬件AAC编码器的编码时延很高，需要buffer大约2秒的数据才会开始编码。而软件编码器的编码时延就是正常的，只要喂给1024个样点，就会开始编码。

banyuhu 发表于 2007-12-24 22:54:24

那么如何在创建的时候指定使用软件编码器呢?需要用到下面的接口:

-(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: %d", (int)(st));

return nil;

}

unsigned int count = size / sizeof(AudioClassDescription);

AudioClassDescription descriptions;

st = AudioFormatGetProperty(kAudioFormatProperty_Encoders,

sizeof(encoderSpecifier),

&encoderSpecifier,

&size,

descriptions);

if (st) {

NSLog(@"error getting audio format propery: %d", (int)(st));

return nil;

}

for (unsigned int i = 0; i <>

if ((type == descriptions.mSubType) &&

(manufacturer == descriptions.mManufacturer)) {

memcpy(&desc, &(descriptions), sizeof(desc));

return &desc;

}

}

return nil;

}

AudioClassDescription *desc = [self getAudioClassDescriptionWithType:kAudioFormatMPEG4AAC

fromManufacturer:kAppleSoftwareAudioCodecManufacturer];

OSStatus status = AudioConverterNewSpecific(&inAudioStreamBasicDescription, &outAudioStreamBasicDescription, 1, desc, &_audioConverter);

如果要正确的编码，编码码率参数是必须设置的。否则编码时会返回560226676错误码(!dat)。

UInt32 ulBitRate = 64000;

UInt32 ulSize = sizeof(ulBitRate);

status = AudioConverterSetProperty(_audioConverter, kAudioConverterEncodeBitRate, ulSize, &ulBitRate);

需要注意，AAC并不是随便的码率都可以支持。比如如果PCM采样率是44100KHz，那么码率可以设置64000bps，如果是16K，可以设置为32000bps。

创建完成Converter和设置完Bitrate之后，可以查询一下最大编码输出的大小，后续会用到。

UInt32 value = 0;

size = sizeof(value);

AudioConverterGetProperty(_audioConverter, kAudioConverterPropertyMaximumOutputPacketSize, &size, &value);

获取出来的Value表示编码器最大输出的包大小。

然后调用AudioConverterFillCOmplexBuffer进行编码:

AudioBufferList outAudioBufferList = {0};

outAudioBufferList.mNumberBuffers = 1;

outAudioBufferList.mBuffers.mNumberChannels = 1;

outAudioBufferList.mBuffers.mDataByteSize = value;//value是上面查询到的值

outAudioBufferList.mBuffers.mData = new int8;

UInt32 ioOutputDataPacketSize = 1;

status = AudioConverterFillComplexBuffer(_audioConverter, inInputDataProc, (__bridge void *)(self), &ioOutputDataPacketSize, &outAudioBufferList, NULL);

编码接口中，inInputDataProc是一个输入数据的回调函数。用来喂PCM数据给Converter，ioOutputDataPacketSize为1表示编码产生1帧数据即返回。outAudioBufferList用来存放编码后的数据。

inInputDataProc中的处理如下:

static OSStatus inInputDataProc(AudioConverterRef inAudioConverter, UInt32 *ioNumberDataPackets, AudioBufferList *ioData, AudioStreamPacketDescription **outDataPacketDescription, void *inUserData)

{

AACEncoder *encoder = (__bridge AACEncoder *)(inUserData);

UInt32 requestedPackets = *ioNumberDataPackets;

uint8_t *buffer;

uint32_t bufferLength = requestedPackets * 2;

uint32_t bufferRead;

bufferRead = ;

if (bufferRead == 0) {

*ioNumberDataPackets = 0;

return -1;

}

ioData->mBuffers.mData = buffer;

ioData->mBuffers.mDataByteSize = bufferRead;

ioData->mNumberBuffers = 1;

ioData->mBuffers.mNumberChannels = 1;

*ioNumberDataPackets = bufferRead >> 1;

return noErr;

}

pcmPool是一个用于存放PCM数据的环形缓冲区。

因为采集输入每次不一定有1024样点，所以可以将数据缓存起来，再满足1024样点时再调用编码。

另外，对于TS文件来说，每个AAC数据需要增加一个adts头，adts头是一个7bit的数据，通过adts可以得知AAC数据的编码参数，方便解码器进行解码。

adts头的计算方法如下:

-(NSData*) adtsDataForPacketLength:(NSUInteger)packetLength {

int adtsLength = 7;

char *packet = (char *)malloc(sizeof(char) * adtsLength);

// Variables Recycled by addADTStoPacket

int profile = 2;//AAC LC

//39=MediaCodecInfo.CodecProfileLevel.AACObjectELD;

int freqIdx = 8;//16KHz

int chanCfg = 1;//MPEG-4 Audio Channel Configuration. 1 Channel front-center

NSUInteger fullLength = adtsLength + packetLength;

// fill in ADTS data

packet = (char)0xFF; // 11111111= syncword

packet = (char)0xF9; // 1111 1 00 1= syncword MPEG-2 Layer CRC

packet = (char)(((profile-1)<<6)>>2));

packet = (char)(((chanCfg&3)<<6)>>11));

packet = (char)((fullLength&0x7FF) >> 3);

packet = (char)(((fullLength&7)<<5)>

packet = (char)0xFC;

NSData *data = ;

return data;

}

29067933 发表于 2007-12-28 12:54:57

有特se.   AAC音频编码

oxford 发表于 2007-12-28 15:17:33

真棒 相关的原理和设置

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