linux音频alsa-uda134x驱动文档阅读之九（咔咔声)(不完整)

转自：http://blog.chinaunix.net/uid-22917448-id-1765510.html

Audio Pops and Clicks
音频咔咔声
=====================

Pops and clicks are unwanted audio artifacts caused by the powering up and down
of components within the audio subsystem. This is noticeable on PCs when an
audio module is either loaded or unloaded (at module load time the sound card is
powered up and causes a popping noise on the speakers).
咔咔声是因为音频子系统中的元件的电源开启和关闭产生的意处杂音。在PC中，当一个音频设备被加载和御载时是很明显的（加载时声卡上电，扬声器产生躁声）。

Pops and clicks can be more frequent on portable systems with DAPM. This is
because the components within the subsystem are being dynamically powered
depending on the audio usage and this can subsequently cause a small pop or
click every time a component power state is changed.



Minimising Playback Pops and Clicks
===================================

Playback pops in portable audio subsystems cannot be completely eliminated
currently, however future audio codec hardware will have better pop and click
suppression.  Pops can be reduced within playback by powering the audio
components in a specific order. This order is different for startup and
shutdown and follows some basic rules:-

Startup Order :- DAC --> Mixers --> Output PGA --> Digital Unmute

Shutdown Order :- Digital Mute --> Output PGA --> Mixers --> DAC

This assumes that the codec PCM output path from the DAC is via a mixer and then
a PGA (programmable gain amplifier) before being output to the speakers.


Minimising Capture Pops and Clicks
==================================

Capture artifacts are somewhat easier to get rid as we can delay activating the
ADC until all the pops have occurred. This follows similar power rules to
playback in that components are powered in a sequence depending upon stream
startup or shutdown.

Startup Order - Input PGA --> Mixers --> ADC

Shutdown Order - ADC --> Mixers --> Input PGA


Zipper Noise
============
An unwanted zipper noise can occur within the audio playback or capture stream
when a volume control is changed near its maximum gain value. The zipper noise
is heard when the gain increase or decrease changes the mean audio signal
amplitude too quickly. It can be minimised by enabling the zero cross setting
for each volume control. The ZC forces the gain change to occur when the signal
crosses the zero amplitude line.

君子注:
您现在所阅读的，是君子阅读Linux音频SoC驱动时，写下的文档译文。
君子写些译文，一方面是作为自己的笔记，帮助记忆，另一方面也希望能对他人有所帮助。
如果您能于君子的译文中有所收获，则吾心甚慰。
所有这几个文档原文，是见于linux-2.6.30/Documentation/sound/alsa/soc目录下的全部内容。
由于君子对音频方面的内容了解也不多，很多地方我自己也不理解，只是照着原文生硬的译出来。
其中错误愿来来访者之宾不吝指正。
张君再拜.