linux音頻alsa-uda134x驅動文檔閱讀之九（咔咔聲)(不完整)

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

Audio Pops and Clicks

音頻咔咔聲

=====================

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).

咔咔聲是因為音頻子系統中的元件的電源開啟和關閉産生的意處雜音。在PC中，當一個音頻裝置被加載和禦載時是很明顯的（加載時聲霸卡上電，揚聲器産生躁聲）。

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:

您現在所閱讀的，是君子閱讀Linux音頻SoC驅動時，寫下的文檔譯文。

君子寫些譯文，一方面是作為自己的筆記，幫助記憶，另一方面也希望能對他人有所幫助。

如果您能于君子的譯文中有所收獲，則吾心甚慰。

所有這幾個文檔原文，是見于linux-2.6.30/Documentation/sound/alsa/soc目錄下的全部内容。

由于君子對音頻方面的内容了解也不多，很多地方我自己也不了解，隻是照着原文生硬的譯出來。

其中錯誤願來來訪者之賓不吝指正。

張君再拜.