1Audio Pops and Clicks 2===================== 3 4Pops and clicks are unwanted audio artifacts caused by the powering up and down 5of components within the audio subsystem. This is noticable on PC's when an 6audio module is either loaded or unloaded (at module load time the sound card is 7powered up and causes a popping noise on the speakers). 8 9Pops and clicks can be more frequent on portable systems with DAPM. This is 10because the components within the subsystem are being dynamically powered 11depending on the audio usage and this can subsequently cause a small pop or 12click every time a component power state is changed. 13 14 15Minimising Playback Pops and Clicks 16=================================== 17 18Playback pops in portable audio subsystems cannot be completely eliminated atm, 19however future audio codec hardware will have better pop and click supression. 20Pops can be reduced within playback by powering the audio components in a 21specific order. This order is different for startup and shutdown and follows 22some basic rules:- 23 24 Startup Order :- DAC --> Mixers --> Output PGA --> Digital Unmute 25 26 Shutdown Order :- Digital Mute --> Output PGA --> Mixers --> DAC 27 28This assumes that the codec PCM output path from the DAC is via a mixer and then 29a PGA (programmable gain amplifier) before being output to the speakers. 30 31 32Minimising Capture Pops and Clicks 33================================== 34 35Capture artifacts are somewhat easier to get rid as we can delay activating the 36ADC until all the pops have occured. This follows similar power rules to 37playback in that components are powered in a sequence depending upon stream 38startup or shutdown. 39 40 Startup Order - Input PGA --> Mixers --> ADC 41 42 Shutdown Order - ADC --> Mixers --> Input PGA 43 44 45Zipper Noise 46============ 47An unwanted zipper noise can occur within the audio playback or capture stream 48when a volume control is changed near its maximum gain value. The zipper noise 49is heard when the gain increase or decrease changes the mean audio signal 50amplitude too quickly. It can be minimised by enabling the zero cross setting 51for each volume control. The ZC forces the gain change to occur when the signal 52crosses the zero amplitude line. 53