<p dir="auto">在音频处理中，压缩可以指数据压缩格式（如 mp3 格式），如何压缩可以减少文件大小。本文学习的则是动态范围（Dynamic range）的压缩。<br />
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<img src="https://images.hive.blog/0x0/http://artsites.ucsc.edu/ems/music/equipment/signal_processors/processors_intro/comp.gif" />
<p dir="auto">EQ（均衡器） 和 Compressor（压缩器）是 Sound engineering 里面最基础最常用的效果器。压缩器是动态处理器的一种。动态处理器最简单的形式就像一个<strong>自动的音量控制器（gain control），当音量太大时减低音量，当音量太小时调高音量。
<p dir="auto">想象一下，你家的电视有两个不同的频道。一个频道总是很小声，需要用遥控器调整到 20 才听得清楚。另外一个频道却很大声，必需用遥控器调到 12 才不会吵到邻居。
<p dir="auto">Compressor 的作用，就相当于每次切换这两个频道的时候，自动帮你调整声音大小，使之保持在合适的范围。<br />
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<img src="https://images.hive.blog/768x0/http://www.yamahaproaudio.com/global/en/Images/pa_beginners_comp12_2.jpg" srcset="https://images.hive.blog/768x0/http://www.yamahaproaudio.com/global/en/Images/pa_beginners_comp12_2.jpg 1x, https://images.hive.blog/1536x0/http://www.yamahaproaudio.com/global/en/Images/pa_beginners_comp12_2.jpg 2x" />
<p dir="auto">在音频处理中，<strong>压缩的目的是让乐器、人声或录音的动态范围保持在我们想要的范围：
<ul>
<li>控制动态性能
<li>填补漏洞
<li>声音包络控制/塑形
<li>调控音色
<li>调整峰值和失真
<li>Sidechain compression(ducking/dipping/pumping)
<p dir="auto"><img src="https://images.hive.blog/768x0/https://www.howtogeek.com/wp-content/uploads/2011/04/clipping_compared_to_limiting.png" srcset="https://images.hive.blog/768x0/https://www.howtogeek.com/wp-content/uploads/2011/04/clipping_compared_to_limiting.png 1x, https://images.hive.blog/1536x0/https://www.howtogeek.com/wp-content/uploads/2011/04/clipping_compared_to_limiting.png 2x" />
<p dir="auto">更直观来说，压缩处理可以：
<ul>
<li>使音频的动态范围更平整（even out）
<li>融合两种乐器的声音（如bass 和底鼓），听起来更为整体
<li>提亮音量比较小的声音
<li>控制太响的声音
<li>使声音获得特定的音色效果
<li>创造特别的声音
<p dir="auto">所有的动态处理器基本上都是指定一个开始工作的信号电平，然后对信号做出某种形式动态变化。
<p dir="auto"><img src="https://images.hive.blog/768x0/https://www.residentadvisor.net/images/features/2012/tech-compress-dynamic-range.jpg" srcset="https://images.hive.blog/768x0/https://www.residentadvisor.net/images/features/2012/tech-compress-dynamic-range.jpg 1x, https://images.hive.blog/1536x0/https://www.residentadvisor.net/images/features/2012/tech-compress-dynamic-range.jpg 2x" />
<p dir="auto">不同压缩方法对 dynamic range 的影响：<br />
<br />
<img src="https://images.hive.blog/768x0/https://dt7v1i9vyp3mf.cloudfront.net/styles/header/s3/imagelibrary/P/Pcomp-02-scales-5vGDFjBDltP2gtc.qx_VD3N2xWxwZHMy.jpg" srcset="https://images.hive.blog/768x0/https://dt7v1i9vyp3mf.cloudfront.net/styles/header/s3/imagelibrary/P/Pcomp-02-scales-5vGDFjBDltP2gtc.qx_VD3N2xWxwZHMy.jpg 1x, https://images.hive.blog/1536x0/https://dt7v1i9vyp3mf.cloudfront.net/styles/header/s3/imagelibrary/P/Pcomp-02-scales-5vGDFjBDltP2gtc.qx_VD3N2xWxwZHMy.jpg 2x" />
<p dir="auto">在 <a href="https://www.uegeek.com/180629-make-noise-06.html" target="_blank" rel="nofollow noreferrer noopener" title="This link will take you away from hive.blog" class="external_link">MakeNoise 06 中我们学习了均衡器(EQ)。那么在处理音频时，应该是先调 EQ 还是先调 Compressor 呢？
<blockquote>
<p dir="auto">Placing an EQ before a compressor can have the effect of exaggerating the applied EQ. Placing an EQ after a compressor you can often attain more audible results with less EQ, producing results that often do not "sound EQ's EQ" -- Chuck Zwicky
<p dir="auto">一般来说，EQ 在 Compressor 之前，主要为了调整 Compressor 的反应；EQ 在 Compressor 之后，是为了调整波形达到更好的混音效果。当然也要<a href="http://a85115230.pixnet.net/blog/post/313698829-%E5%85%88eq%EF%BC%9F%E9%82%84%E6%98%AF%E5%85%88compressor%EF%BC%9F" target="_blank" rel="nofollow noreferrer noopener" title="This link will take you away from hive.blog" class="external_link">取决于声音素材和想要的效果。
<h2>在 Ableton Live 里压缩不同的音轨
<p dir="auto">进入 Ableton Live，在效果库里面找到 Compressor，拖到音轨上，就看到 Compressor 面板中调整压缩的参数：
<p dir="auto"><img src="https://images.hive.blog/768x0/https://cdn.steemitimages.com/DQmciPCL1gwHe8HZxZZ57v7U63dvagi6y829TcppdtoV5BS/[Ableton]compressor.jpg" alt="[Ableton]compressor.jpg" srcset="https://images.hive.blog/768x0/https://cdn.steemitimages.com/DQmciPCL1gwHe8HZxZZ57v7U63dvagi6y829TcppdtoV5BS/[Ableton]compressor.jpg 1x, https://images.hive.blog/1536x0/https://cdn.steemitimages.com/DQmciPCL1gwHe8HZxZZ57v7U63dvagi6y829TcppdtoV5BS/[Ableton]compressor.jpg 2x" />
<ul>
<li>Threshold/Input：控制何时触发压缩
<li>Gain reduction (GR)：有多少 dynamic range 被去掉
<li>Output/Makeup gain：补偿因压缩导致的音量减小，设定输出的最高电平
<li>Attack：当信号达到阈值时，压缩器在多少时间内开始处理。Fast attcak 0.01~10 ms, slow attack > 50 ms
<li>Release：当压缩时，原始信号被保留多久。通常以音频的 tempo 为基准
<li>Ratio：以输入为基础，按比例设定输出值
<li>Link：连接双声道/单声道压缩器到另外一个单声道，形成立体效果
<li>Side chain：使用信号而不是主输入来控制压缩
<li>Bypass：压缩过滤
<p dir="auto">如何调整不同乐器音轨的压缩器，请看下面视频里的详细讲解：
<p dir="auto"><a href="https://v.qq.com/x/page/d0711gj2yf2.html" target="_blank" rel="nofollow noreferrer noopener" title="This link will take you away from hive.blog" class="external_link">Compressor in Ableton Live_腾讯视频
<h2>用 Max/MSP 实现 Compressor
<p dir="auto">更进一步，我们到 MSP 里拆解 Compressor 是如何实现的。
<p dir="auto">官方文档里提供了详细的说明和例子。00 调整和补充了一些注释：
<p dir="auto"><img src="https://images.hive.blog/768x0/https://cdn.steemitimages.com/DQmbSQmLJHAmKd91nu2nRjchZSH8SqRmg47PtbMWgKDmxKc/[Max]compression.png" alt="[Max]compression.png" srcset="https://images.hive.blog/768x0/https://cdn.steemitimages.com/DQmbSQmLJHAmKd91nu2nRjchZSH8SqRmg47PtbMWgKDmxKc/[Max]compression.png 1x, https://images.hive.blog/1536x0/https://cdn.steemitimages.com/DQmbSQmLJHAmKd91nu2nRjchZSH8SqRmg47PtbMWgKDmxKc/[Max]compression.png 2x" />
<p dir="auto">压缩器主要有三部分构成：参数、压缩流程和处理算法。下面分别看看每一部分的实现。
<p dir="auto">可控制的参数主要有 Input gain, Threshold, Ratio, Attack time, Release time，之前我们在 Ableton Live 的 Compressor 面板里都看到了：
<p dir="auto"><img src="https://images.hive.blog/768x0/https://cdn.steemitimages.com/DQmVsmoAyM7WvoFTorJhnHFQNux6k7Hx211cQJiVxDSqVSQ/[Max]compression-para.png" alt="[Max]compression-para.png" srcset="https://images.hive.blog/768x0/https://cdn.steemitimages.com/DQmVsmoAyM7WvoFTorJhnHFQNux6k7Hx211cQJiVxDSqVSQ/[Max]compression-para.png 1x, https://images.hive.blog/1536x0/https://cdn.steemitimages.com/DQmVsmoAyM7WvoFTorJhnHFQNux6k7Hx211cQJiVxDSqVSQ/[Max]compression-para.png 2x" />
<p dir="auto">压缩处理的主流程：先实时监测 input level，把信号从 amplitude 转换为 dB，然后跟阈值参数比较，判断是否进行压缩：
<p dir="auto"><img src="https://images.hive.blog/768x0/https://cdn.steemitimages.com/DQmcPq1Ct6u7d9qEiKVqpPyomrX9od48821DqrUNwuBDPZa/[Max]compression-compressor.png" alt="[Max]compression-compressor.png" srcset="https://images.hive.blog/768x0/https://cdn.steemitimages.com/DQmcPq1Ct6u7d9qEiKVqpPyomrX9od48821DqrUNwuBDPZa/[Max]compression-compressor.png 1x, https://images.hive.blog/1536x0/https://cdn.steemitimages.com/DQmcPq1Ct6u7d9qEiKVqpPyomrX9od48821DqrUNwuBDPZa/[Max]compression-compressor.png 2x" />
<p dir="auto">如果达到阈值后开始压缩，需要降低的 gain 由以下部分计算。公式是 <code>(Threshold - Level) * (1 - 1/ratio)：
<p dir="auto"><img src="https://images.hive.blog/768x0/https://cdn.steemitimages.com/DQmTGWYauwX7VA8ez19svLqn66Kbo754cBL7SrgoYM5ejPZ/[Max]compression-math.png" alt="[Max]compression-math.png" srcset="https://images.hive.blog/768x0/https://cdn.steemitimages.com/DQmTGWYauwX7VA8ez19svLqn66Kbo754cBL7SrgoYM5ejPZ/[Max]compression-math.png 1x, https://images.hive.blog/1536x0/https://cdn.steemitimages.com/DQmTGWYauwX7VA8ez19svLqn66Kbo754cBL7SrgoYM5ejPZ/[Max]compression-math.png 2x" />
<p dir="auto">然后将计算值再换算为 amplitude，输出到 compression 的主流程中，最后再加一道 <code>rampsmooth~ 来平滑 gain 的变化。
<h2>Ref
<ul>
<li><a href="https://book.douban.com/subject/7069885/" target="_blank" rel="nofollow noreferrer noopener" title="This link will take you away from hive.blog" class="external_link">Audio Engineering 101 - A Beginner's Guide to Music Production
<li><a href="https://zhuanlan.zhihu.com/p/32562033" target="_blank" rel="nofollow noreferrer noopener" title="This link will take you away from hive.blog" class="external_link">什么是动态处理器？
<li><a href="https://docs.cycling74.com/max7/tutorials/17_msp_compress_00" target="_blank" rel="nofollow noreferrer noopener" title="This link will take you away from hive.blog" class="external_link">MSP Compression Introduction: What is Compression?
<li><a href="https://www.soundonsound.com/techniques/parallel-compression" target="_blank" rel="nofollow noreferrer noopener" title="This link will take you away from hive.blog" class="external_link">Parallel Compression |
<li><a href="https://www.residentadvisor.net/features/1595" target="_blank" rel="nofollow noreferrer noopener" title="This link will take you away from hive.blog" class="external_link">RA: Understanding compression
<li><a href="http://a85115230.pixnet.net/blog/post/313698829-%E5%85%88eq%EF%BC%9F%E9%82%84%E6%98%AF%E5%85%88compressor%EF%BC%9F" target="_blank" rel="nofollow noreferrer noopener" title="This link will take you away from hive.blog" class="external_link">先EQ？還是先Compressor？ @ Balanced Audio Lab 平衡音訊實驗室 :: 痞客邦 ::
<li><a href="https://www.kadenze.com/courses/sound-production-in-ableton-live-for-musicians-and-artists-v/sessions/compression-and-dynamics" target="_blank" rel="nofollow noreferrer noopener" title="This link will take you away from hive.blog" class="external_link">Session 6: Compression and Dynamics
@kidult00, 生活，就是在平凡中变得不平凡！
对了，漂漂女生节貌似快到了，我是不是要给 @cn-cutie.pie 可可 准备一下礼物了？