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