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Documentation / sound / alsa / soc / pops_clicks.txt




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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:04 EST.

1	Audio Pops and Clicks
2	=====================
3	
4	Pops and clicks are unwanted audio artifacts caused by the powering up and down
5	of components within the audio subsystem. This is noticeable on PCs when an
6	audio module is either loaded or unloaded (at module load time the sound card is
7	powered up and causes a popping noise on the speakers).
8	
9	Pops and clicks can be more frequent on portable systems with DAPM. This is
10	because the components within the subsystem are being dynamically powered
11	depending on the audio usage and this can subsequently cause a small pop or
12	click every time a component power state is changed.
13	
14	
15	Minimising Playback Pops and Clicks
16	===================================
17	
18	Playback pops in portable audio subsystems cannot be completely eliminated
19	currently, however future audio codec hardware will have better pop and click
20	suppression.  Pops can be reduced within playback by powering the audio
21	components in a specific order. This order is different for startup and
22	shutdown and follows some basic rules:-
23	
24	 Startup Order :- DAC --> Mixers --> Output PGA --> Digital Unmute
25	
26	 Shutdown Order :- Digital Mute --> Output PGA --> Mixers --> DAC
27	
28	This assumes that the codec PCM output path from the DAC is via a mixer and then
29	a PGA (programmable gain amplifier) before being output to the speakers.
30	
31	
32	Minimising Capture Pops and Clicks
33	==================================
34	
35	Capture artifacts are somewhat easier to get rid as we can delay activating the
36	ADC until all the pops have occurred. This follows similar power rules to
37	playback in that components are powered in a sequence depending upon stream
38	startup or shutdown.
39	
40	 Startup Order - Input PGA --> Mixers --> ADC
41	
42	 Shutdown Order - ADC --> Mixers --> Input PGA
43	
44	
45	Zipper Noise
46	============
47	An unwanted zipper noise can occur within the audio playback or capture stream
48	when a volume control is changed near its maximum gain value. The zipper noise
49	is heard when the gain increase or decrease changes the mean audio signal
50	amplitude too quickly. It can be minimised by enabling the zero cross setting
51	for each volume control. The ZC forces the gain change to occur when the signal
52	crosses the zero amplitude line.
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