Audio Pops and Clicks¶
Pops and clicks are unwanted audio artifacts caused by the powering up and downof components within the audio subsystem. This is noticeable on PCs when anaudio module is either loaded or unloaded (at module load time the sound card ispowered up and causes a popping noise on the speakers).
Pops and clicks can be more frequent on portable systems with DAPM. This isbecause the components within the subsystem are being dynamically powereddepending on the audio usage and this can subsequently cause a small pop orclick every time a component power state is changed.
Minimising Playback Pops and Clicks¶
Playback pops in portable audio subsystems cannot be completely eliminatedcurrently, however future audio codec hardware will have better pop and clicksuppression. Pops can be reduced within playback by powering the audiocomponents in a specific order. This order is different for startup andshutdown and follows some basic rules:-
Startup Order :- DAC --> Mixers --> Output PGA --> Digital UnmuteShutdown Order :- Digital Mute --> Output PGA --> Mixers --> DAC
This assumes that the codec PCM output path from the DAC is via a mixer and thena PGA (programmable gain amplifier) before being output to the speakers.
Minimising Capture Pops and Clicks¶
Capture artifacts are somewhat easier to get rid as we can delay activating theADC until all the pops have occurred. This follows similar power rules toplayback in that components are powered in a sequence depending upon streamstartup or shutdown.
Startup Order - Input PGA --> Mixers --> ADCShutdown Order - ADC --> Mixers --> Input PGA
Zipper Noise¶
An unwanted zipper noise can occur within the audio playback or capture streamwhen a volume control is changed near its maximum gain value. The zipper noiseis heard when the gain increase or decrease changes the mean audio signalamplitude too quickly. It can be minimised by enabling the zero cross settingfor each volume control. The ZC forces the gain change to occur when the signalcrosses the zero amplitude line.