音频编码相关知识概念

AAC是新一代音频有损压缩技术，它通过一些附加的编码技术(PS、SBR)衍生出LC-AAC、HE-AAC、HE-AAC v2三种主要的编码格式。“LC-AAC是比较传统的AAC，相对而言，其主要应用于中高码率场景的编码（≥80Kbit/s）；HE-AAC（相当于AAC+SBR）主要应用于中低码率场景的编码（≤80Kbit/s）；而新近推出的HE-AAC v2（相当于AAC+SBR+PS）主要应用于低码率场景的编码（≤48Kbit/s）。事实上大部分编码器都设置为≤48Kbit/s自动启用PS技术，而>48Kbit/s则不加PS，相当于普通的HE-AAC。

特点：在小于128Kbit/s的码率下表现优异，并且多用于视频中的音频编码。

pcm：脉冲编码调制 = 量化格式(samoleFormat) + 采样率(sampleRate) + 声道数(channel)
比特率 = 采样率 * 比特 * 声道数

音频采样率：指录音设备在一秒钟对声音信号的采样次数，采用频率越高声音的还原就越真实越自然，采用频率一般分为22.05KHz、44.1KHz、48KHz三个等级。
比特率：每秒传递的比特数(bit)。单位为bbs，比特率越高，传送数据速度越快。

ADTS(Audio Data Transport Stream),是AAC的一种十分常见的传输格式。
AAC解码器都需要把AAC的ES流打包成ADTS的格式，一般是在AAC ES流前添加7个字节的ADTS header.
ADTS头中包括采用率、声道数、帧长度等。一般情况下ADTS的头信息都是7个字节，分为两部分:
adts_fixed_header()、adts_variable_header()

adts_fixed_header() { 
syncword;// 同步头，总是0xFFF,代表ADTS帧的开始 
ID; // mpeg版本 0：MPEG-4 1:MPEG-2 
layer; // 00 
protection_absent; 
profile; 
samping_frequency_index; // 采样率下标 
private_bit; 
channel_configuration; // 声道数 
original_copy; home; 
} 

adts_variable_header() { 
copyright_identification_bit; 
copyright_identification_start; 
aac_frame_length; // 一个ADTS帧的长度包括ADTS和AAC原始流 
adts_buffer_fullness; // 0x7FF说明是码率可变的码流 
number_of_raw_data_blocks_in_frame; 
}

音频硬编码流程(AudioToolbox)

图片.png

AudioToolbox示例代码

- (NSFileHandle *)audioFileHandle { 
if (!_audioFileHandle) { 
NSString * filePath = [NSHomeDirectory() stringByAppendingPathComponent:@"/Documents/demo.aac"];
 [[NSFileManager defaultManager] removeItemAtPath:filePath error:nil]; 
BOOL createFile = [[NSFileManager defaultManager] createFileAtPath:filePath contents:nil attributes:nil]; 
NSAssert(createFile, @"create audio path error"); 
_audioFileHandle = [NSFileHandle fileHandleForWritingAtPath:filePath]; 
} 
return _audioFileHandle; 
} 
- (void)dealloc { 
AudioConverterDispose(_audioConverter); 
free(_aacBuffer);
 } 
- (id)init {
 if (self = [super init]) { 
_encoderQueue = dispatch_queue_create("aac encode queue", DISPATCH_QUEUE_SERIAL); 
_audioConverter = NULL; 
_pcmBufferSize = 0; 
_pcmBuffer = NULL; 
_aacBufferSize = 1024;
 _aacBuffer = malloc(_aacBufferSize * sizeof(uint8_t)); 
memset(_aacBuffer, 0, _aacBufferSize); 
} 
return self; 
} 
- (void)stopEncodeAudio { 
[self.audioFileHandle closeFile]; 
self.audioFileHandle = NULL;
 } 
// 配置编码参数 
- (void)setupEncoderFromSampleBuffer:(CMSampleBufferRef)sampleBuffer { 
NSLog(@"开始配置编码参数。。。。"); 
// 获取原音频声音格式设置 
AudioStreamBasicDescription inAudioStreamBasicDescription = *CMAudioFormatDescriptionGetStreamBasicDescription((CMAudioFormatDescriptionRef)CMSampleBufferGetFormatDescription(sampleBuffer)); 
AudioStreamBasicDescription outAudioStreamBasicDescription = {0}; 

// 采样率 
outAudioStreamBasicDescription.mSampleRate = inAudioStreamBasicDescription.mSampleRate; 
// 格式 
kAudioFormatMPEG4AAC = 'aac' , outAudioStreamBasicDescription.mFormatID = kAudioFormatMPEG4AAC; 
// 标签格式 无损编码 
outAudioStreamBasicDescription.mFormatFlags = kMPEG4Object_AAC_LC; 
// 每个Packet 的 Bytes 数量 0:动态大小格 outAudioStreamBasicDescription.mBytesPerPacket = 0; 
// 每个Packet的帧数量，设置一个较大的固定值 1024 
outAudioStreamBasicDescription.mFramesPerPacket = 1024; 
// 每帧的Bytes数量 
outAudioStreamBasicDescription.mBytesPerFrame = 0; 
// 1 单声道 2: 立体声 
outAudioStreamBasicDescription.mChannelsPerFrame = 1; 
// 语言每采样点占用位数 
outAudioStreamBasicDescription.mBitsPerChannel = 0; 
// 保留参数（对齐当时） 
outAudioStreamBasicDescription.mReserved = 0; 
// 获取编码器 AudioClassDescription * description = [self getAudioClassDescriptionWithType:kAudioFormatMPEG4AAC fromManufacturer:kAppleSoftwareAudioCodecManufacturer]; 
// 创建编码器 
/* inAudioStreamBasicDescription 传入源音频格式 outAudioStreamBasicDescription 目标音频格式 第三个参数：传入音频编码器的个数 description 传入音频编码器的描述 */ 
OSStatus status = AudioConverterNewSpecific(&inAudioStreamBasicDescription, &outAudioStreamBasicDescription, 1, description, &_audioConverter); 
if (status != 0) { 
NSLog(@"创建编码器失败"); 
} 
} 

// 获取编码器 
- (AudioClassDescription *)getAudioClassDescriptionWithType:(UInt32)type fromManufacturer:(UInt32)manufacturer { 
NSLog(@"开始获取编码器。。。。");
 // 选择aac编码 
static AudioClassDescription desc; 
UInt32 encoderS = type; 
OSStatus status; 
UInt32 size; 
/* kAudioFormatProperty_Encoders 编码ID 编码说明大小 编码说明 属性当前值的大小 */ 
status = AudioFormatGetPropertyInfo(kAudioFormatProperty_Encoders, sizeof(encoderS), &encoderS, &size); 
if (status) { NSLog(@"编码aac错误"); 
return nil;
 }
 // 计算编码器的个数 
unsigned int count = size / sizeof(AudioClassDescription); 
// 定义编码器数组 
AudioClassDescription description[count]; 
status = AudioFormatGetProperty(kAudioFormatProperty_Encoders, sizeof(encoderS), &encoderS, &size, description); 
for (unsigned int i = 0; i < count; i++) { 
if (type == description[i].mSubType && manufacturer == description[i].mManufacturer) { 
// 拷贝编码器到desc 
memcpy(&desc, &description[i], sizeof(desc)); 
NSLog(@"找到aac编码器"); 
return &desc; 
} 
} 
return nil;
} 

// 回调函数 
OSStatus inInputDataProc(AudioConverterRef inAudioConverter, UInt32 *ioNumberDataPackets, AudioBufferList *ioData, AudioStreamPacketDescription **outDataPacketDescription, void *inUserData) {
 // 编码器 
AACEncoder *encoder = (__bridge AACEncoder *) inUserData; 
// 编码包的数据 
UInt32 requestPackes = *ioNumberDataPackets; 
// 将ioData填充到缓冲区 
size_t cp = [encoder copyPCMSamplesIntoBuffer:ioData]; 
if (cp < requestPackes) { 
*ioNumberDataPackets = 0; 
// 清空 
return -1; 
} 
*ioNumberDataPackets = 1; 
return noErr; 
} 

// pcm -> 缓冲区 
- (size_t)copyPCMSamplesIntoBuffer:(AudioBufferList*)ioData { 
// 获取pcm大小 
size_t os = _pcmBufferSize; 
if (!_pcmBufferSize) { 
return 0; 
} 
ioData->mBuffers[0].mData = _pcmBuffer; 
ioData->mBuffers[0].mDataByteSize = (int)_pcmBufferSize; 
// 清空 
_pcmBuffer = NULL; 
_pcmBufferSize = 0; 
return os; 
} 

// 编码数据 
- (void)encodeAudioSampleBuffer:(CMSampleBufferRef)sampleBuffer { 
CFRetain(sampleBuffer); 
dispatch_sync(_encoderQueue, ^{ 
if (!self.audioConverter) { 
// 配置编码参数 
[self setupEncoderFromSampleBuffer:sampleBuffer];
 } 
// 获取CMBlockBufferRef 
CMBlockBufferRef blockBuffer = CMSampleBufferGetDataBuffer(sampleBuffer); 
CFRetain(blockBuffer); 
// 获取_pcmBufferSize 和 _pcmBuffer 
OSStatus status = CMBlockBufferGetDataPointer(blockBuffer, 0, NULL, &self->_pcmBufferSize, &self->_pcmBuffer); 
if (status != kCMBlockBufferNoErr) { 
NSLog(@"获取 pcmBuffer 数据错误"); 
return ; 
} 
// 清空 
memset(self->_aacBuffer, 0, self->_aacBufferSize); 
// 初始化缓冲列表
AudioBufferList outAudioBufferList = {0}; // 结构体 
// 缓冲区个数 outAudioBufferList.mNumberBuffers = 1; 
// 渠道个数 
outAudioBufferList.mBuffers[0].mNumberChannels = 1; 
// 缓存区大小 
outAudioBufferList.mBuffers[0].mDataByteSize = (int)self->_aacBufferSize; 
// 缓冲区内容 
outAudioBufferList.mBuffers[0].mData = self->_aacBuffer; 
// 编码 
AudioStreamPacketDescription * outPD = NULL; 
UInt32 inPutSize = 1; 
/* inInputDataProc 自己实现的编码数据的callback引用 self 获取的数据 inPutSize 输出数据的长度 outAudioBUfferList 输出的数据 outPD 输出数据的描述 */ 
status = AudioConverterFillComplexBuffer(self->_audioConverter, inInputDataProc, (__bridge void*)self, &inPutSize, &outAudioBufferList, outPD ); 
// 编码后完成 
NSData * data = nil; 
if (status == noErr) { 
// 获取缓冲区的原始数据acc数据 
NSData * rawAAC = [NSData dataWithBytes:outAudioBufferList.mBuffers[0].mData length:outAudioBufferList.mBuffers[0].mDataByteSize]; 
// 加头ADTS 
NSData * adtsHeader = [self adtsDataForPacketLength:rawAAC.length]; 
NSMutableData * fullData = [NSMutableData dataWithData:adtsHeader]; 
[fullData appendData:rawAAC]; 
data = fullData; 
} else { 
NSLog(@"数据错误"); 
return;
 } 

// 回调
 // if (completionBlock) { 
// dispatch_async(_callBackQueue, ^{ 
// completionBlock(data, nil); 
// }); 
// } 
// 写入数据 
[self.audioFileHandle writeData:data]; 
CFRelease(sampleBuffer); CFRelease(blockBuffer); 
}); 
} 

/** * Add ADTS header at the beginning of each and every AAC packet. * This is needed as MediaCodec encoder generates a packet of raw * AAC data. * * Note the packetLen must count in the ADTS header itself. 
注意：packetLen 必须在ADTS头身计算 * 
See: http://wiki.multimedia.cx/index.php?title=ADTS * Also: http://wiki.multimedia.cx/index.php?title=MPEG-4_Audio#Channel_Configurations **/
 - (NSData*)adtsDataForPacketLength:(NSUInteger)packetLength { 
int adtsLength = 7; 
char *packet = malloc(sizeof(char) * adtsLength); 
int profile = 2; 
int freqIdx = 4;
 int chanCfg = 1; 
NSUInteger fullLength = adtsLength + packetLength; 
packet[0] = (char)0xFF; 
packet[1] = (char)0xF9; 
packet[2] = (char)(((profile-1)<<6) + (freqIdx<<2) +(chanCfg>>2)); 
packet[3] = (char)(((chanCfg&3)<<6) + (fullLength>>11));
 packet[4] = (char)((fullLength&0x7FF) >> 3); 
packet[5] = (char)(((fullLength&7)<<5) + 0x1F); 
packet[6] = (char)0xFC; 
NSData *data = [NSData dataWithBytesNoCopy:packet length:adtsLength freeWhenDone:YES]; 
return data; 
}