1Audio Pops and Clicks
2=====================
3
4Pops and clicks are unwanted audio artifacts caused by the powering up and down
5of components within the audio subsystem. This is noticeable on PCs when an
6audio module is either loaded or unloaded (at module load time the sound card is
7powered up and causes a popping noise on the speakers).
8
9Pops and clicks can be more frequent on portable systems with DAPM. This is
10because the components within the subsystem are being dynamically powered
11depending on the audio usage and this can subsequently cause a small pop or
12click every time a component power state is changed.
13
14
15Minimising Playback Pops and Clicks
16===================================
17
18Playback pops in portable audio subsystems cannot be completely eliminated
19currently, however future audio codec hardware will have better pop and click
20suppression.  Pops can be reduced within playback by powering the audio
21components in a specific order. This order is different for startup and
22shutdown 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
28This assumes that the codec PCM output path from the DAC is via a mixer and then
29a PGA (programmable gain amplifier) before being output to the speakers.
30
31
32Minimising Capture Pops and Clicks
33==================================
34
35Capture artifacts are somewhat easier to get rid as we can delay activating the
36ADC until all the pops have occurred. This follows similar power rules to
37playback in that components are powered in a sequence depending upon stream
38startup or shutdown.
39
40 Startup Order - Input PGA --> Mixers --> ADC
41
42 Shutdown Order - ADC --> Mixers --> Input PGA
43
44
45Zipper Noise
46============
47An unwanted zipper noise can occur within the audio playback or capture stream
48when a volume control is changed near its maximum gain value. The zipper noise
49is heard when the gain increase or decrease changes the mean audio signal
50amplitude too quickly. It can be minimised by enabling the zero cross setting
51for each volume control. The ZC forces the gain change to occur when the signal
52crosses the zero amplitude line.
53