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  音视频学习笔记 1 - kaelinda

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          <h1>音视频学习笔记 1</h1>

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          <span class="date">2018/4/12</span>

          <span>posted in&nbsp;</span>

              <span class="posted-in"><a href='ios.html'>iOS</a></span>


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      <h2 id="toc_0">音视频概念</h2>

<h3 id="toc_1">声音的物理性质</h3>

<ul>
<li>声波三要素：频率、振幅、波形</li>
<li>人耳听到的频率范围 20Hz-20KHz</li>
<li><p>声音在空气中的传播速度 340m/s、蒸馏水中1497m/s、铁棒5200m/s</p></li>
<li><p>一般声音的采样频率为 44.1KHz---&gt; 一秒钟采样44100次，这样 20KHz的频率也能被数字化</p></li>
<li><p>音频的裸数据格式 脉冲编码调制  PCM （Pluse Code Modulation）</p></li>
<li><p>PCM 相关概念：量化格式、采样频率、声道数</p></li>
</ul>

<h3 id="toc_2">音频编码</h3>

<p>音频压缩编码算法有：PCM、WAV、AAC、MP3、Ogg。</p>

<ul>
<li>WAV：音频质量好，大多数软件都支持，但是占用空间大；适合当做多媒体开发的中间文件，保存音乐、音效素材</li>
<li>MP3：不错的压缩比，LAME编码中高码率文件。音质在128Kbit/s 以上表现不错，压缩比高，大多数软件都支持。适合高比特率下对兼容性有要求的音乐欣赏。</li>
<li>AAC：包括LC-AAC、HE-AAC、HE-AAC v2,小于128Kbit/s表现优异。</li>
</ul>

<table>
<thead>
<tr>
<th style="text-align: center">LC-AAC</th>
<th style="text-align: center">HE-AAC</th>
<th style="text-align: center">HE-AAC v2</th>
</tr>
</thead>

<tbody>
<tr>
<td style="text-align: center">中高码率&gt;80Kbit/s</td>
<td style="text-align: center">中低码率&lt;=80Kbit/s</td>
<td style="text-align: center">低码率&lt;48Kbit/s</td>
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