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多媒体知识体系之音频格式

2016-07-28 10:04:59   2  举报

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多媒体知识体系之音频格式

多媒体技术

音频格式

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大纲/内容

AAC

常见格式

<span style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma; font-size: 14px; line-height: 25px;">LATM格式</span>

<span style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma; font-size: 14px; line-height: 25px;">Low-overhead MPEG-4 Audio TransportMultiplex</span>

<span style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma; font-size: 14px; line-height: 25px;">主要由</span><span style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma; font-size: 14px; line-height: 25px;">AudioSpecificConfig（音频特定配置单元）与音频负载</span>

带内传

每一个LATM 帧，都含有一个AudioSpecificConfig 信息

带外传

每一个LATM帧都不含有AudioSpecificConfig 信息<br>而通过其他方式把AudioSpecificConfig信息发送到解码端<br>由于AudioSpecificConfig 信息一般是不变的<br>所以只需发送一次即可

<span lang="EN-US" style="word-wrap: break-word; color: rgb(102, 102, 102); font-size: 12pt; line-height: 24px; font-family: 'Times New Roman', serif;">CMMB</span><span style="word-wrap: break-word; color: rgb(102, 102, 102); font-family: 宋体; font-size: 12pt; line-height: 24px;">中音频压缩标准为</span><span lang="EN-US" style="word-wrap: break-word; color: rgb(102, 102, 102); font-size: 12pt; line-height: 24px; font-family: 'Times New Roman', serif;">AAC</span><span style="word-wrap: break-word; color: rgb(102, 102, 102); font-family: 宋体; font-size: 12pt; line-height: 24px;">时，默认采用</span><span lang="EN-US" style="word-wrap: break-word; color: rgb(102, 102, 102); font-size: 12pt; line-height: 24px; font-family: 'Times New Roman', serif;">LATM</span><span style="word-wrap: break-word; color: rgb(102, 102, 102); font-family: 宋体; font-size: 12pt; line-height: 24px;">封装</span>

ADTS

<span style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma; font-size: 14px; line-height: 25px;">Audio Data Transport Stream</span>

<span style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma; font-size: 14px; line-height: 25px;">ADTS头</span>

<span style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma; font-size: 14px; line-height: 25px;">syncword</span>

<span style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma; font-size: 14px; line-height: 25px;">ID 总是设置为1</span>

<span style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma; font-size: 14px; line-height: 25px;">layer Indicates which layer is used. Set to ‘00’</span>

<span style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma; font-size: 14px; line-height: 25px;"> protection_absent 表示是否误码校验</span>

<span style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma; font-size: 14px; line-height: 25px;"> profile 表示使用哪个级别的AAC，如01 Low Complexity(LC)--- AACLC</span>

<span style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma; font-size: 14px; line-height: 25px;">sampling_frequency_index 表示使用的采样率下标</span>

<span style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma; font-size: 14px; line-height: 25px;">sampling_frequency_index sampling frequeny [Hz]</span>

<span style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma; font-size: 14px; line-height: 25px;">channel_configuration 表示声道数</span>

<span style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma; font-size: 14px; line-height: 25px;">frame_length 一个ADTS帧的长度包括ADTS头和raw data block.</span>

<span style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma; font-size: 14px; line-height: 25px;">adts_buffer_fullness  0x7FF 说明是码率可变的码流</span>

<span style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma; font-size: 14px; line-height: 25px;">number_of_raw_data_blocks_in_frame</span>

<span style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma; font-size: 14px; line-height: 25px;">表示ADTS帧中有number_of_raw_data_blocks_in_frame + 1个AAC原始帧</span>

<span style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma; font-size: 14px; line-height: 25px;">number_of_raw_data_blocks_in_frame == 0 表示说ADTS帧中有一个AAC数据块并不是说没有</span>

<span style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma; font-size: 14px; line-height: 25px;">一个AAC原始帧包含一段时间内1024个采样及相关数据</span>

<p style="font-family: Arial; font-size: 14px; line-height: 26px; color: rgb(51, 51, 51); margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px;"><strong>adts_fixed_header();</strong><br></p>

<div>  adts_fixed_header()</div><div> {</div><div>     syncword: 12 bslbf</div><div>     ID: 1 bslbf</div><div>     layer: 2 uimsbf</div><div>     protection_absent: 1 bslbf</div><div>     profile: 2 uimsbf</div><div>     sampling_frequency_index: 4 uimsbf</div><div>     private_bit: 1 bslbf</div><div>     channel_configuration: 3 uimsbf</div><div>     original/copy: 1 bslbf</div><div>     home: 1 bslbf</div><div> }</div>

<strong style="color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px;">adts_variable_header();</strong>

<div>adts_variable_header()</div><div>{</div><div>    copyright_identification_bit: 1 bslbf</div><div>    copyright_identification_start: 1 bslbf</div><div>    frame_length: 13 bslbf</div><div>    adts_buffer_fullness: 11 bslbf</div><div>    number_of_raw_data_blocks_in_frame: 2 uimsfb</div><div>}</div>

<span style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma; font-size: 14px; line-height: 25px;">ADTS帧 </span>

<span style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma; font-size: 14px; line-height: 25px;">原始帧加上ADTS头进行ADTS 的封装</span>

默认状况下，编码参数如下：双声道，采样率24KHZ，帧长变长，码流可变码率的码流，<br>一般采用的AAC profile为AAC-LC。<br>

MPEG-2 TS中多以ADTS格式封装AAC

 开源解码库

FAAD2

<span style="color: rgb(64, 64, 64); font-family: verdana, tahoma, arial; font-size: 10.6667px; line-height: 21.3333px; background-color: rgb(248, 248, 248);">an open source MPEG-4 and MPEG-2 AAC decoder</span>

<div>NeAACDecHandle NEAACAPI NeAACDecOpen(void);</div><div>创建解码环境并返回一个句柄</div><div>void NEAACAPI NeAACDecClose(NeAACDecHandle hDecoder);</div><div>关闭解码环境</div><div>NeAACDecConfigurationPtr NEAACAPI NeAACDecGetCurrentConfiguration(NeAACDecHandle hDecoder);</div><div>获取当前解码器库的配置</div><div>unsigned char NEAACAPI NeAACDecSetConfiguration(NeAACDecHandle hDecoder, NeAACDecConfigurationPtr config);</div><div>为解码器库设置一个配置结构</div><div>long NEAACAPI NeAACDecInit(NeAACDecHandle hDecoder, unsigned char *buffer, unsigned long buffer_size, unsigned long *samplerate, unsigned char *channels);</div><div>初始化解码器库</div><div>void* NEAACAPI NeAACDecDecode(NeAACDecHandle hDecoder, NeAACDecFrameInfo *hInfo, unsigned char *buffer, unsigned long buffer_size);</div><div>解码AAC数据</div>

MP3

PCM

<span style="color: rgb(51, 51, 51); font-family: 'Courier New'; font-size: 16px; line-height: 27.7778px;">Pulse Code Modulation也被称为脉码编码调制</span>

<span style="color: rgb(51, 51, 51); font-family: 'Courier New'; font-size: 16px; line-height: 27.7778px;">基本组织单位是BYTE(8bit)或WORD(16bit)</span>

<span style="color: rgb(51, 51, 51); font-family: 'Courier New'; font-size: 16px; line-height: 27.7778px;">一般情况下，一帧PCM是由2048次采样组成的</span>

最常见的PCM格式

<span style="color: rgb(51, 51, 51); font-family: 'Microsoft Yahei'; font-size: 16px; line-height: 30px;">最常说的“无损音频”来说</span><span style="color: rgb(51, 51, 51); font-family: 'Microsoft Yahei'; font-size: 16px; line-height: 30px;">，一般都是指传统CD格式中的16bit/44.1kHz采样率的文件格式，而知所以称为无损压缩，也是因为其包含了20Hz-22.05kHz这个完全覆盖人耳可闻范围的频响频率而得名</span>

数据存放

对于ffmpeg来说

音频数据会保存在AVFrame中extended_data数组中，如果是打包模式(packed)，就只用extended_data[0],<br>如果是planar模式，则每个channel分别保存在extended_data[i]中。<br>对于音频，只有linesize[0]有效，打包模式保存整个音频帧的buff大小，planar模式保存每个channel的buff大小

<div>short *sample_buffer_L = pFrame->extended_data[0];//存放着左声道的数据</div><div>short *sample_buffer_R = pFrame->extended_data[1];//存放着右声道的数据</div><div><br></div><div>两者都是16bit，而裸的PCM文件里的数据是按照 LRLRLRLR  这样存储的，所以我们需要按照这种格式存储16bit的数据：</div><div><br></div><div>//Left channel</div><div>data[i] = (char)(sample_buffer_L[j] & 0xff);//左声道低8位</div><div>data[i+1] = (char)((sample_buffer_L[j]>>8) & 0xff);;//左声道高8位</div><div>//Right channel</div><div>data[i+2] = (char)(sample_buffer_R[j] & 0xff);//右声道低8位</div><div>data[i+3] = (char)((sample_buffer_R[j]>>8) & 0xff);;//右声道高8位</div>