[Sonar] 关于dither（抖动）（转）

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cindyliang

发表于 2012-11-20 10:43:58 | 显示全部楼层 |阅读模式

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关于Dither（抖动）
关于dither，很多教科书和老师都说过在高bit降到低bit音频文件时候要用dither，似乎只要挂一个dither插件就可以，不过谁听的出来？似乎可以感知，但是不那么确定，前几天无意间看到pro tools 的说明书中的dither 部分，发现还是有很多使用dither 的讲究，先看看sonar说明书摘录出来的dither的讲解，定义就不翻译了，很多教科书都有，意思都一样。
Dithering means to add a certain audio signal to 16 bit audio to make it sound more like it did as a higher-bit signal。

Dithering
Dithering. Whenever an audio signal is converted from a higher-bit resolution to a lower resolution, it is necessary to apply dither to avoid introducing undesirable quantization noise or harmonic distortion into the signal. The purpose of dither is to reduce the resulting distortion by adding low-level random noise or “dither” to the audio signal. Different mathematical calculations are used to generate dither, each method has advantages and disadvantages depending on the particular operation. SONAR features the Pow-r dithering process, short for Psycho-acoustically Optimized Wordlength Reduction, which can produce lower-bit files that sound indistinguishable from higher-bit source files. When this option is turned on, SONAR uses dithering when you export a higher-bit file at a lower resolution, or lower the bit depth of a project’s audio files by using the Utilities > Change Audio Format command, or when you “render” audio (bounce, freeze, or apply effects).
下面书sonar producer版本提供的五种dither方式SONAR Producer offers five kinds of dithering:
○ Rectangular. Essentially white noise, no noise shaping. Advantages: least CPU-intensive, lowest signal-to-noise ratio, preferable to shaped dither when successive dithering can occur (e.g. bouncing, freezing). Disadvantages: suffers from intermodulation distortion, higher perceived loudness than Pow-r dither. 大致说白噪声，没有噪声塑性，低cpu，低信噪比，可以在频繁的bouncing或者freezing时候用，估计是比较快，但是有这个intermodulation 失真。

○ Triangular. Higher level than rectangular, no noise shaping. Advantages: low CPU-intensive dither, superior to Rectangular as it does not suffer from modulation noise effects. Preferable to shaped (Pow-r) dither when successive dithering can occur (e.g. bouncing, freezing). Disadvantages: higher perceived loudness than Pow-r dither.这个和上面的差不多，比它好一点，也没有噪声塑性，也是低cpu，比下面的pow-r算法差。

○ Pow-r 1. Noise-shaped dither. Advantages: less CPU-intensive than Pow-r types 2 and 3, lower perceived loudness than Rectangular or Triangular. Disadvantages: less noise shaping than Pow-r types 2 and 3, not recommended for operations where dither will be applied successively (e.g. bounce and freeze).噪声塑性的dither，cpu比下面两个使用少，不推荐在连续的bounce和freeze。

○ Pow-r 2. Noise-shaped dither. Advantages: lowest perceived loudness, highest quality settings, recommended for audio export. Disadvantages: highest CPU-intensive settings, not recommended for operations where dither will be applied successively (e.g. bounce and freeze).可以感知到的这种噪声的程度更小，好质量，建议音频导出！高cpu，（其实也估计高不到哪里去，都是offline的算的，除非cpu真的太差了）

○ Pow-r 3. Same as Pow-r 2 except most CPU-intensive and transparent of all choices和pow-r2差不多，但是cpu更多，是不是这个会更好？看起来应该建议这个作为音频导出。

以上是sonar 的dither说明，在看下pro tools 的说明。因为说明书在单位，我就把大致意思说明一下，pro tools 需要在master 音轨上加载自带的或者是第三方的dither插件来完成dither，和sonar 不太一样，pro tools 在说明书建议了几点，dither 应该在音频总输出的master音轨上，因为master 轨上的插件是推子后的，dither 不能挂在推子前的轨道，并且应该在效果器链条的最后一个；录音和回放的时候不要开dither，应该bypass或者在导出的时候在挂载，因为protools内部运算是48bit定点，10是32bitfloating，sonar也是，所以即使是24bit 文件bounce to disk 也推荐要开dither，dither上也有相应的bit选择。
大致总结一下，sonar内部回放时候的dither 也可以关掉，那么针对不同bit 音频文件共存在同一个工程时候，可以使用change audio bit 来off-line 的实现。bounce to track 可以不开dither，选择32floating，导出时候可以根据需要选择dither 或者最高bit率的导出则不用dither，留给母带的时候用硬件去降bit。

学编曲，就上编曲186.

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