1*4882a593Smuzhiyun===================== 2*4882a593SmuzhiyunAudio Pops and Clicks 3*4882a593Smuzhiyun===================== 4*4882a593Smuzhiyun 5*4882a593SmuzhiyunPops and clicks are unwanted audio artifacts caused by the powering up and down 6*4882a593Smuzhiyunof components within the audio subsystem. This is noticeable on PCs when an 7*4882a593Smuzhiyunaudio module is either loaded or unloaded (at module load time the sound card is 8*4882a593Smuzhiyunpowered up and causes a popping noise on the speakers). 9*4882a593Smuzhiyun 10*4882a593SmuzhiyunPops and clicks can be more frequent on portable systems with DAPM. This is 11*4882a593Smuzhiyunbecause the components within the subsystem are being dynamically powered 12*4882a593Smuzhiyundepending on the audio usage and this can subsequently cause a small pop or 13*4882a593Smuzhiyunclick every time a component power state is changed. 14*4882a593Smuzhiyun 15*4882a593Smuzhiyun 16*4882a593SmuzhiyunMinimising Playback Pops and Clicks 17*4882a593Smuzhiyun=================================== 18*4882a593Smuzhiyun 19*4882a593SmuzhiyunPlayback pops in portable audio subsystems cannot be completely eliminated 20*4882a593Smuzhiyuncurrently, however future audio codec hardware will have better pop and click 21*4882a593Smuzhiyunsuppression. Pops can be reduced within playback by powering the audio 22*4882a593Smuzhiyuncomponents in a specific order. This order is different for startup and 23*4882a593Smuzhiyunshutdown and follows some basic rules:- 24*4882a593Smuzhiyun:: 25*4882a593Smuzhiyun 26*4882a593Smuzhiyun Startup Order :- DAC --> Mixers --> Output PGA --> Digital Unmute 27*4882a593Smuzhiyun 28*4882a593Smuzhiyun Shutdown Order :- Digital Mute --> Output PGA --> Mixers --> DAC 29*4882a593Smuzhiyun 30*4882a593SmuzhiyunThis assumes that the codec PCM output path from the DAC is via a mixer and then 31*4882a593Smuzhiyuna PGA (programmable gain amplifier) before being output to the speakers. 32*4882a593Smuzhiyun 33*4882a593Smuzhiyun 34*4882a593SmuzhiyunMinimising Capture Pops and Clicks 35*4882a593Smuzhiyun================================== 36*4882a593Smuzhiyun 37*4882a593SmuzhiyunCapture artifacts are somewhat easier to get rid as we can delay activating the 38*4882a593SmuzhiyunADC until all the pops have occurred. This follows similar power rules to 39*4882a593Smuzhiyunplayback in that components are powered in a sequence depending upon stream 40*4882a593Smuzhiyunstartup or shutdown. 41*4882a593Smuzhiyun:: 42*4882a593Smuzhiyun 43*4882a593Smuzhiyun Startup Order - Input PGA --> Mixers --> ADC 44*4882a593Smuzhiyun 45*4882a593Smuzhiyun Shutdown Order - ADC --> Mixers --> Input PGA 46*4882a593Smuzhiyun 47*4882a593Smuzhiyun 48*4882a593SmuzhiyunZipper Noise 49*4882a593Smuzhiyun============ 50*4882a593SmuzhiyunAn unwanted zipper noise can occur within the audio playback or capture stream 51*4882a593Smuzhiyunwhen a volume control is changed near its maximum gain value. The zipper noise 52*4882a593Smuzhiyunis heard when the gain increase or decrease changes the mean audio signal 53*4882a593Smuzhiyunamplitude too quickly. It can be minimised by enabling the zero cross setting 54*4882a593Smuzhiyunfor each volume control. The ZC forces the gain change to occur when the signal 55*4882a593Smuzhiyuncrosses the zero amplitude line. 56