在Android系統4.1上面目前aac谷歌軟解已經給了兩套方案，一套是沿用以前的解碼庫，一套是使用Fraunhofer Institute開發出來的aac解碼庫，目前谷歌已經切換到了後者，Fraunhofer提供了一套標準的調用接口，谷歌實現的aac軟解component邏輯類SoftAAC2.cpp只要按照這個標準去調用，就能完成aac的解碼。
![Android平臺aac谷歌軟解框架和流程、解碼庫學習]( "Android平臺aac谷歌軟解框架和流程、解碼庫學習")
openMAX中定義的component的狀態機
![Android平臺aac谷歌軟解框架和流程、解碼庫學習]( "Android平臺aac谷歌軟解框架和流程、解碼庫學習")
![Android平臺aac谷歌軟解框架和流程、解碼庫學習]( "Android平臺aac谷歌軟解框架和流程、解碼庫學習")
aacDecoder_DecodeFrame用來解碼internal buffer中的數據，如果數據不足以解碼，則返回AAC_DEC_NOT_ENOUGH_BITS，繼續調用aacDecoder_Fill填充數據。
![Android平臺aac谷歌軟解框架和流程、解碼庫學習]( "Android平臺aac谷歌軟解框架和流程、解碼庫學習")
正文
一：SoftAAC2解碼流程
前面已經有博客分析了android中是如何加載解碼組件的，這裏不在重點看android framework中的omxcodec類是如何控制解碼組件完成各種解碼中的工作，這裏默認已經加載好了omx.google.aac.decode這個組件，看看SoftAAC2.cpp中是怎樣完成所有邏輯的，跟着打出來的log進行跟蹤
01-07 01:39:14.250 E/SoftAAC2( 151): in SoftAAC2 constructor....
首先會進入SoftAAC2的構造函數
01-07 01:39:14.250 E/SoftAAC2( 151): in SoftAAC2 initPorts....
爲這個解碼器初始化兩個端口（輸入和輸出，portIndex分別爲0和1），看下port對應的結構體：
struct PortInfo {
OMX_PARAM_PORTDEFINITIONTYPE mDef;
Vector mBuffers;
List mQueue;
enum {
NONE,
DISABLING,
ENABLING,
} mTransition;
};
port對應的結構體中包含了三部分的內容
1：關於port所有信息的結構體OMX_PARAM_PORTDEFINITIONTYPE，定義在omx_core.h標準中，需要好好看下；
2：每個port都會對應分配幾塊buffer（對應的結構體爲bufferInfo，後面詳解），用於放置解碼前後的數據
3：port也維護了一個狀態
初始化過程就是初始化OMX_PARAM_PORTDEFINITIONTYPEdef變量，最後調用addPort加入到一個容器中去管理
void SimpleSoftOMXComponent::addPort(const OMX_PARAM_PORTDEFINITIONTYPE &def) {
CHECK_EQ(def.nPortIndex, mPorts.size());
mPorts.push();
PortInfo *info = &mPorts.editItemAt(mPorts.size() - 1);
info->mDef = def;
info->mTransition = PortInfo::NONE;
}
01-07 01:39:14.250 E/SoftAAC2( 151): in SoftAAC2 initDecoder before call aacDecoder_open...
01-07 01:39:14.250 E/SoftAAC2( 151): in SoftAAC2 initDecoder before call aacDecoder_GetStreamInfo...
其次是在initDecoder函數初始化解碼器，調用aacDecoder_Open接口獲得解碼庫的句柄mAACDecoder，以及調用aacDecoder_GetStreamInfo獲取aac的一些信息，保存在結構體CStreamInfo中。
mAACDecoder = aacDecoder_Open(TT_MP4_ADIF, 1);
mStreamInfo = aacDecoder_GetStreamInfo(mAACDecoder);
01-07 01:39:14.270 V/OMXCodec( 151): [OMX.google.aac.decoder] allocating 4 buffers of size 8192 on input port
01-07 01:39:14.270 E/SoftOMXComponent( 151): huangbangbang in SoftOMXcomponent useBufferWrapper..
01-07 01:39:14.270 E/SimpleSoftOMXComponent( 151): huangbangbang in SimpleSoftOMXComponent useBuffer....
01-07 01:39:14.270 V/OMXCodec( 151): [OMX.google.aac.decoder] allocated buffer 0x40069f48 on input port
01-07 01:39:14.270 E/SoftOMXComponent( 151): huangbangbang in SoftOMXcomponent useBufferWrapper..
01-07 01:39:14.270 E/SimpleSoftOMXComponent( 151): huangbangbang in SimpleSoftOMXComponent useBuffer....
01-07 01:39:14.270 V/OMXCodec( 151): [OMX.google.aac.decoder] allocated buffer 0x401fd3b8 on input port
01-07 01:39:14.270 E/SoftOMXComponent( 151): huangbangbang in SoftOMXcomponent useBufferWrapper..
01-07 01:39:14.270 E/SimpleSoftOMXComponent( 151): huangbangbang in SimpleSoftOMXComponent useBuffer....
01-07 01:39:14.270 V/OMXCodec( 151): [OMX.google.aac.decoder] allocated buffer 0x400a7008 on input port
01-07 01:39:14.270 E/SoftOMXComponent( 151): huangbangbang in SoftOMXcomponent useBufferWrapper..
01-07 01:39:14.270 E/SimpleSoftOMXComponent( 151): huangbangbang in SimpleSoftOMXComponent useBuffer....
01-07 01:39:14.270 V/OMXCodec( 151): [OMX.google.aac.decoder] allocated buffer 0x400a70c0 on input port
爲輸入端口分配四個buffer（個數是在初始化port過程中kNumInputBuffers指定的），這四個buffer的作用就是讓omx client往裏面放原始的aac數據，並且調用emptyBuffer，告訴解碼器，將這些buffer拿去解碼，這些buffer都是通過共享內存實現的，也就是omx client和omx component共享這段內存，client往裏面寫，component讀取裏面的數據，看下buffer對應的數據結構：
struct BufferInfo {
OMX_BUFFERHEADERTYPE *mHeader;
bool mOwnedByUs;
};
包含了兩部分內容：
1：buffer中包含的數據的結構信息OMX_BUFFERHEADERTYPE，定義在openmax頭文件中；
2：buffer當前被誰佔有，client(mOwnedByUs爲false)或者component(mOwnedByUs爲true)。
01-07 01:39:14.270 V/OMXCodec( 151): [OMX.google.aac.decoder] allocating 4 buffers of size 16384 on output port
01-07 01:39:14.270 E/SoftOMXComponent( 151): huangbangbang in SoftOMXcomponent useBufferWrapper..
01-07 01:39:14.270 E/SimpleSoftOMXComponent( 151): huangbangbang in SimpleSoftOMXComponent useBuffer....
01-07 01:39:14.270 V/OMXCodec( 151): [OMX.google.aac.decoder] allocated buffer 0x401fe6a8 on output port
01-07 01:39:14.270 E/SoftOMXComponent( 151): huangbangbang in SoftOMXcomponent useBufferWrapper..
01-07 01:39:14.270 E/SimpleSoftOMXComponent( 151): huangbangbang in SimpleSoftOMXComponent useBuffer....
01-07 01:39:14.270 V/OMXCodec( 151): [OMX.google.aac.decoder] allocated buffer 0x404d09c8 on output port
01-07 01:39:14.270 E/SoftOMXComponent( 151): huangbangbang in SoftOMXcomponent useBufferWrapper..
01-07 01:39:14.270 E/SimpleSoftOMXComponent( 151): huangbangbang in SimpleSoftOMXComponent useBuffer....
01-07 01:39:14.270 V/OMXCodec( 151): [OMX.google.aac.decoder] allocated buffer 0x408bedd8 on output port
01-07 01:39:14.270 E/SoftOMXComponent( 151): huangbangbang in SoftOMXcomponent useBufferWrapper..
01-07 01:39:14.270 E/SimpleSoftOMXComponent( 151): huangbangbang in SimpleSoftOMXComponent useBuffer....
01-07 01:39:14.270 V/OMXCodec( 151): [OMX.google.aac.decoder] allocated buffer 0x408bef00 on output port
同理，爲輸出端口分配四塊buffer，用於client和component共享，這四個buffer用於存放解碼器解碼後的pcm數據，解碼器往裏面寫，omx client讀取裏面的數據，其他和input port對應的buffer一致。
上面八個buffer都是通過android中Shared Memory機制實現，內存是在OMXCodec中分配的
size_t totalSize = def.nBufferCountActual * def.nBufferSize;
mDealer[portIndex] = new MemoryDealer(totalSize, "OMXCodec");
for (OMX_U32 i = 0; i < def.nBufferCountActual; ++i) {
sp mem = mDealer[portIndex]->allocate(def.nBufferSize);
CHECK(mem.get() != NULL);
......
}
01-07 01:39:14.270 E/SoftOMXComponent( 151): huangbangbang in SoftOMXcomponent GetParameterWrapper..
01-07 01:39:14.270 E/SimpleSoftOMXComponent( 151): huangbangbang in SimpleSoftOMXComponent getParameter ....
01-07 01:39:14.270 E/SoftAAC2( 151): huangbangbang in SoftAAC2 internalGetParameter index is 33554433
01-07 01:39:14.270 E/SoftAAC2( 151): huangbangbang in SoftAAC2 internalGetParameter in default...
01-07 01:39:14.270 E/SimpleSoftOMXComponent( 151): huangbangbang in SimpleSoftOMXComponent internalGetParameter....
01-07 01:39:14.270 E/SimpleSoftOMXComponent( 151): huangbangbang in internalGetParameter in case OMX_IndexParamPortDefinition....
01-07 01:39:14.280 E/SimpleSoftOMXComponent( 151): huangbangbang in SimpleSoftOMXComponent msgType = 0 (kWhatSendCommand: 0, kWhatEmptyFillThisBuffer: 1, kWhatFillThisBuffer:2)
01-07 01:39:14.280 E/SimpleSoftOMXComponent( 151): huangbangbang in SimpleSoftOMXComponent onSendcommand.
01-07 01:39:14.280 E/SimpleSoftOMXComponent( 151): huangbangbang in SimpleSoftOMXComponent onSendcommand in onChangeState.
01-07 01:39:14.280 E/SimpleSoftOMXComponent( 151): huangbangbang in SimpleSoftOMXComponent onChangeStage state=2, component_cur_state=1
01-07 01:39:14.280 E/SoftOMXComponent( 151): huangbangbang in SoftOMXcomponent notify..
01-07 01:39:14.280 V/OMXCodec( 151): [OMX.google.aac.decoder] onStateChange 2
01-07 01:39:14.280 V/OMXCodec( 151): [OMX.google.aac.decoder] Now Idle.
01-07 01:39:14.280 E/SoftOMXComponent( 151): huangbangbang in SoftOMXcomponent SendCommandWrapper..
01-07 01:39:14.280 E/SimpleSoftOMXComponent( 151): huangbangbang in SimpleSoftOMXComponent sendCommand....
01-07 01:39:14.280 E/SimpleSoftOMXComponent( 151): huangbangbang in SimpleSoftOMXComponent msgType = 0 (kWhatSendCommand: 0, kWhatEmptyFillThisBuffer: 1, kWhatFillThisBuffer:2)
01-07 01:39:14.280 E/SimpleSoftOMXComponent( 151): huangbangbang in SimpleSoftOMXComponent onSendcommand.
01-07 01:39:14.280 E/SimpleSoftOMXComponent( 151): huangbangbang in SimpleSoftOMXComponent onSendcommand in onChangeState.
01-07 01:39:14.280 E/SimpleSoftOMXComponent( 151): huangbangbang in SimpleSoftOMXComponent onChangeStage state=3, component_cur_state=2
01-07 01:39:14.280 E/SoftOMXComponent( 151): huangbangbang in SoftOMXcomponent notify..
01-07 01:39:14.280 V/OMXCodec( 151): [OMX.google.aac.decoder] onStateChange 3
01-07 01:39:14.280 V/OMXCodec( 151): [OMX.google.aac.decoder] Now Executing.
上面的log表示，client調用getParameter和setParameter獲取或者設置解碼器的一些參數，changeState改變解碼器當前狀態等函數，解碼器本身也維護了一個狀態機，每個狀態下面需要或者不能做哪些事情是有規定的，可以在openMAX文檔中查看，如下圖，
openMAX中定義的component的狀態機
從log中可以看出，我們的aac解碼器狀態機的變化是：
UNLOADED------->LOADED------->IDLE--------->EXECUTING
到達EXECUTING狀態之後，就能夠開始讀寫數據編解碼了
谷歌實現的軟解碼的邏輯類採用了多態機制，因爲谷歌繼承的不只是aac decoder一個component，還有很多其他類型文件的解碼器，所以爲了達到代碼的最大共用，谷歌實現先了兩個父類，對於和具體解碼器無關的一些操作都放在父類中完成，繼承關係如下：
01-07 01:39:14.460 E/SoftAAC2( 151): in SoftAAC2 onQueueFilled.....
01-07 01:39:14.460 E/SoftAAC2( 151): in SoftAAC2 onQueueFilled in portIndex is 0 mInputBufferCount == 0.....
01-07 01:39:14.460 E/SoftAAC2( 151): in SoftAAC2 onQueueFilled in portIndex is 0 before call aacDecoder_ConfigRaw.....
解碼器，輸入輸出端口，每個端口需要的buffer都分配好了，下面就是真正調用解碼庫進行數據的解碼了，真正音頻數據前面都會有一些configure數據，所以來到的第一個input buffer我們沒有去調用aacDecoder_DecodeFrame函數直接去解碼，而是調用aacDecoder_ConfigRaw 去重新獲取aac的一些信息，如sampleRate和numChannels，保存在CStreamInfo結構體中，如果sampleRate和numChannels參數有變化，可能還需要disable
output port，並且重新初始化output port和對應的四個buffer，對於CStreamInfo結構體將在第二部分講解解碼庫的時候進行
01-07 01:39:14.460 E/SoftAAC2( 151): in SoftAAC2 onQueueFilled.....
01-07 01:39:14.460 E/SoftAAC2( 151): in SoftAAC2 onQueueFilled in while looper.....
01-07 01:39:14.460 E/SoftAAC2( 151): in onQueueFilled in while before call aacDecoder_Fill and aacDecoder_DecodeFrame bytesvalid is 416
所有準備工作都做好了之後就開始真正aac數據的解碼了，解碼都是在onQueueFilled函數中進行，剔除一些參數的初始化，主要過程就是下面的while循環：
AAC_DECODER_ERROR decoderErr = AAC_DEC_NOT_ENOUGH_BITS;
while (bytesValid[0] > 0 && decoderErr == AAC_DEC_NOT_ENOUGH_BITS) {
aacDecoder_Fill(mAACDecoder,
inBuffer,
inBufferLength,
bytesValid);
decoderErr = aacDecoder_DecodeFrame(mAACDecoder,
outBuffer,
outHeader->nAllocLen,
0 );
if (decoderErr == AAC_DEC_NOT_ENOUGH_BITS) {
ALOGW("Not enough bits, bytesValid %d", bytesValid[0]);
}
}
要明白while循環裏面aacDecoder_Fill和aacDecoder_DecodeFrame的作用，需要明白解碼庫的buffer機制：
解碼過程中除了前面已經分配的input buffer和output buffer，解碼庫還會分配一個過渡性的decoder-internal input buffer，這個buffer大小又RANSPORTDEC_INBUF_SIZE規定，可以任意設定但必須滿足兩個條件：
1：each input channel requires 768 bytes
2：the whole buffer must be of size 2^n
So for example a stereo decoder:
TRANSPORTDEC_INBUF_SIZE = 2 768 = 1536 => 2048（選擇2048bytes）
aacDecoder_Fill就是從input buffer往ecoder-internal input buffer裏面拷貝數據，返回的是input buffer中還剩下多少沒有被拷貝的數據，如果input buffer中的數據全部拷貝完畢了，就需要re-fill，下圖是input buffer的變化圖（from aacDecoder.pdf）
aacDecoder_DecodeFrame用來解碼internal buffer中的數據，如果數據不足以解碼，則返回AAC_DEC_NOT_ENOUGH_BITS，繼續調用aacDecoder_Fill填充數據。
這樣解碼一直進行下去，知道OMXCodec中讀取aac原始數據過程中發現已經到該track的結尾，就會給這個mediabuffer打上一個標籤，在onQueueFilled函數中檢測的這個flag，解碼完這一幀之後，就停止解碼，並且做所有的release操作
OMXCodec::drainInputBuffer
if (signalEOS) {
flags |= OMX_BUFFERFLAG_EOS;
} else {
mNoMoreOutputData = false;
}
SoftAAC2::onQueueFilled
if (inHeader->nFlags & OMX_BUFFERFLAG_EOS) {
......
}
最後就是free所有的buffer，disable端口等
二：解碼庫學習 Advanced Audio Coding DecoderLibrary
1，術語：
MPEG-2 and MPEG-4
AAC Low-Complexity (AAC-LC),
High-Eficiency AAC v2 (HE-AAC v2),
AAC Low-Delay (AAC-LD), andAAC Enhanced Low-Delay (AAC-ELD) decoder
2，主要頭文件：#include
3，常用的數據和數據結構：
struct CStreamInfo
This structure gives information about the currently decoded audio data. All fields are read-only, public attributes are as follows:
• INT sampleRate
• INT frameSize
• INT numChannels
• AUDIO_CHANNEL_TYPE pChannelType
• UCHAR pChannelIndices
• INT aacSampleRate
• INT profile
• AUDIO_OBJECT_TYPE aot
• INT channelConfig
• INT bitRate
• INT aacSamplesPerFrame
• AUDIO_OBJECT_TYPE extAot
• INT extSamplingRate
• UINT flags
• SCHAR epConfig
• INT numLostAccessUnits
• UINT numTotalBytes
• UINT numBadBytes
• UINT numTotalAccessUnits
• UINT numBadAccessUnits
Defines:
• #define IS_INIT_ERROR(err) ( (((err)>=aac_dec_init_error_start) && ((err)<=aac_dec_init_-
error_end)) ? 1 : 0)
• #define IS_DECODE_ERROR(err) ( (((err)>=aac_dec_decode_error_start) && ((err)<=aac_dec_-
decode_error_end)) ? 1 : 0)
• #define IS_OUTPUT_VALID(err) ( ((err) == AAC_DEC_OK) jj IS_DECODE_ERROR(err) )
• #define AACDEC_CONCEAL 1
• #define AACDEC_FLUSH 2
• #define AACDEC_INTR 4
• #define AACDEC_CLRHIST 8
Typedefs:
typedef struct AAC_DECODER_INSTANCE HANDLE_AACDECODER
Enumerations:
enum AAC_DECODER_ERROR
enum AACDEC_PARAM
Functions:
• LINKSPEC_H AAC_DECODER_ERROR aacDecoder_AncDataInit (HANDLE_AACDECODER self, UCHAR buffer, int
size)
Initialize ancillary data buffer.
• LINKSPEC_H AAC_DECODER_ERROR aacDecoder_AncDataGet (HANDLE_AACDECODER self, int index, UCHAR
ptr, int size)
Get one ancillary data element.
• LINKSPEC_H AAC_DECODER_ERROR aacDecoder_SetParam (const HANDLE_-AACDECODER self, const AACDEC_PARAM
param, const INT value)
Set one single decoder parameter.
• LINKSPEC_H AAC_DECODER_ERROR aacDecoder_GetFreeBytes (const HANDLE_-AACDECODER self, UINT pFreeBytes)
Get free bytes inside decoder internal buffer.
• LINKSPEC_H HANDLE_AACDECODER aacDecoder_Open (TRANSPORT_TYPE trans-portFmt, UINT nrOfLayers)
Open an AAC decoder instance.
• LINKSPEC_H AAC_DECODER_ERROR aacDecoder_ConfigRaw (HANDLE_AACDECODER self, UCHAR conf[ ], const
UINT length[ ])
Explicitly configure the decoder by passing a raw AudioSpecificConfig (ASC) or a StreamMuxConfig (SMC), contained in a binary buffer. This is required for MPEG-4 and Raw Packets file format bitstreams as well as for LATM bitstreams
with no in-band SMC. If the transport format is LATM with or without LOAS,
configuration is assumed to be an SMC, for all other file formats an ASC.
• LINKSPEC_H AAC_DECODER_ERROR aacDecoder_Fill (HANDLE_AACDECODER self,
UCHAR pBuffer[ ], const UINT bufferSize[ ], UINT bytesValid)
Fill AAC decoder’s internal input buffer with bitstream data from the external input buffer. The function only copies such data as long as the decoder-internal input buffer is not full. So it grabs whatever it can from pBuffer and
returns information (bytesValid) so that at a subsequent call of aacDecoder_Fill(), the right position in pBuffer can be determined to grab the next data.
• LINKSPEC_H AAC_DECODER_ERROR aacDecoder_DecodeFrame (HANDLE_-AACDECODER self, INT_PCM pTimeData, const INT timeDataSize, const
UINT flags)
decode one audio frame.
• LINKSPEC_H void aacDecoder_Close (HANDLE_AACDECODER self)
De-allocate all resources of an AAC decoder instance.
• LINKSPEC_H CStreamInfo aacDecoder_GetStreamInfo (HANDLE_AACDECODER self)
Get CStreamInfo handle from decoder.
• LINKSPEC_H INT aacDecoder_GetLibInfo (LIB_INFO info)
Get decoder library info
4：function call sequence
4.1. Call aacDecoder_Open() to open and retrieve a handle to a new AAC decoder instance.
aacDecoderInfo = aacDecoder_Open(mpegFileRead_GetTransportType(hDataSrc), nrOfLayers);
4.2 If out-of-band config data (Audio Specific Config (ASC) or Stream Mux Config (SMC)) is available, call aacDecoder_ConfigRaw() to pass it to the decoder and before the decoding process starts. If
this data is not available in advance, the decoder will get it from the bitstream and configure itself while decoding with aacDecoder_DecodeFrame().
4.3. Begin decoding loop. do {
4.4 Read data from bitstream file or stream into a client-supplied input buffer("inBuffer" in main.cpp). If it is very small like just 4, aacDecoder_DecodeFrame() will repeatedly return AAC_DEC_NOT_-ENOUGH_BITS until enough bits were fed by
aacDecoder_Fill(). Only read data when this buffer has completely been processed and is then empty. For file-based input execute mpegFileRead_Read() or any other implementation with similar functionality.
4.5 Call aacDecoder_Fill() to fill the decoder’s internal bitstream input buffer with the client-suppliedexternal bitstream input buffer.
aacDecoder_Fill(aacDecoderInfo, inBuffer, bytesRead, bytesValid);
4.6 Call aacDecoder_DecodeFrame() which writes decoded PCM audio data to a client-supplied buffer.It is the client’s responsibility to allocate a buffer which is large enough to hold this output data.
ErrorStatus = aacDecoder_DecodeFrame(aacDecoderInfo, TimeData, OUT_BUF_SIZE, flags);
If the bitstream’s configuration (number of channels, sample rate, frame size) is not known in ad-vance, you may call aacDecoder_GetStreamInfo() to retrieve a structure containing this information and then initialize an audio output device.
In the example main.cpp, if the number of channels or the sample rate has changed since program start or since the previously decoded frame, the audio output device will be re-initialized. If WAVE file output is chosen, a new WAVE file for each new configuration
will be created.
4.7 Repeat steps 5 to 7 until no data to decode is available anymore, or if an error occured.
4.8 Call aacDecoder_Close() to de-allocate all AAC decoder and transport layer structures.
5：buffer system
There are three main buffers in an AAC decoder application. One external input buffer to hold bitstream data from file I/O or elsewhere, one decoder-internal input buffer, and one to hold the decoded output PCM sample data, whereas
this output buffer may overlap with the external input buffer.
The external input buffer is set in the example framework main.cpp and its size is defined by IN_BUF_-SIZE. You may freely choose different sizes here. To feed the data to the decoder-internal input buffer, use the function aacDecoder_Fill().
This function returns important information about how many bytes in the external input buffer have not yet been copied into the internal input buffer (variable bytesValid). Once the external buffer has been fully copied, it can be re-filled again.
In case you want to re-fill it when there are
still unprocessed bytes (bytesValid is unequal 0), you would have to additionally perform a memcpy(), so that just means unnecessary computational overhead and therefore we recommend to re-fill the buffer only when bytesValid is 0.
over