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.