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