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1=====================2Audio Pops and Clicks3=====================4 5Pops and clicks are unwanted audio artifacts caused by the powering up and down6of components within the audio subsystem. This is noticeable on PCs when an7audio module is either loaded or unloaded (at module load time the sound card is8powered up and causes a popping noise on the speakers).9 10Pops and clicks can be more frequent on portable systems with DAPM. This is11because the components within the subsystem are being dynamically powered12depending on the audio usage and this can subsequently cause a small pop or13click every time a component power state is changed.14 15 16Minimising Playback Pops and Clicks17===================================18 19Playback pops in portable audio subsystems cannot be completely eliminated20currently, however future audio codec hardware will have better pop and click21suppression. Pops can be reduced within playback by powering the audio22components in a specific order. This order is different for startup and23shutdown and follows some basic rules:-24::25 26 Startup Order :- DAC --> Mixers --> Output PGA --> Digital Unmute27 28 Shutdown Order :- Digital Mute --> Output PGA --> Mixers --> DAC29 30This assumes that the codec PCM output path from the DAC is via a mixer and then31a PGA (programmable gain amplifier) before being output to the speakers.32 33 34Minimising Capture Pops and Clicks35==================================36 37Capture artifacts are somewhat easier to get rid as we can delay activating the38ADC until all the pops have occurred. This follows similar power rules to39playback in that components are powered in a sequence depending upon stream40startup or shutdown.41::42 43 Startup Order - Input PGA --> Mixers --> ADC44 45 Shutdown Order - ADC --> Mixers --> Input PGA46 47 48Zipper Noise49============50An unwanted zipper noise can occur within the audio playback or capture stream51when a volume control is changed near its maximum gain value. The zipper noise52is heard when the gain increase or decrease changes the mean audio signal53amplitude too quickly. It can be minimised by enabling the zero cross setting54for each volume control. The ZC forces the gain change to occur when the signal55crosses the zero amplitude line.56