1=====================
2Audio Pops and Clicks
3=====================
4
5Pops and clicks are unwanted audio artifacts caused by the powering up and down
6of components within the audio subsystem. This is noticeable on PCs when an
7audio module is either loaded or unloaded (at module load time the sound card is
8powered up and causes a popping noise on the speakers).
9
10Pops and clicks can be more frequent on portable systems with DAPM. This is
11because the components within the subsystem are being dynamically powered
12depending on the audio usage and this can subsequently cause a small pop or
13click every time a component power state is changed.
14
15
16Minimising Playback Pops and Clicks
17===================================
18
19Playback pops in portable audio subsystems cannot be completely eliminated
20currently, however future audio codec hardware will have better pop and click
21suppression.  Pops can be reduced within playback by powering the audio
22components in a specific order. This order is different for startup and
23shutdown and follows some basic rules:-
24::
25
26   Startup Order :- DAC --> Mixers --> Output PGA --> Digital Unmute
27
28   Shutdown Order :- Digital Mute --> Output PGA --> Mixers --> DAC
29
30This assumes that the codec PCM output path from the DAC is via a mixer and then
31a PGA (programmable gain amplifier) before being output to the speakers.
32
33
34Minimising Capture Pops and Clicks
35==================================
36
37Capture artifacts are somewhat easier to get rid as we can delay activating the
38ADC until all the pops have occurred. This follows similar power rules to
39playback in that components are powered in a sequence depending upon stream
40startup or shutdown.
41::
42
43   Startup Order - Input PGA --> Mixers --> ADC
44
45   Shutdown Order - ADC --> Mixers --> Input PGA
46
47
48Zipper Noise
49============
50An unwanted zipper noise can occur within the audio playback or capture stream
51when a volume control is changed near its maximum gain value. The zipper noise
52is heard when the gain increase or decrease changes the mean audio signal
53amplitude too quickly. It can be minimised by enabling the zero cross setting
54for each volume control. The ZC forces the gain change to occur when the signal
55crosses the zero amplitude line.
56