1*c9746eeaSTakashi Iwai===================== 2*c9746eeaSTakashi IwaiAudio Pops and Clicks 3*c9746eeaSTakashi Iwai===================== 4*c9746eeaSTakashi Iwai 5*c9746eeaSTakashi IwaiPops and clicks are unwanted audio artifacts caused by the powering up and down 6*c9746eeaSTakashi Iwaiof components within the audio subsystem. This is noticeable on PCs when an 7*c9746eeaSTakashi Iwaiaudio module is either loaded or unloaded (at module load time the sound card is 8*c9746eeaSTakashi Iwaipowered up and causes a popping noise on the speakers). 9*c9746eeaSTakashi Iwai 10*c9746eeaSTakashi IwaiPops and clicks can be more frequent on portable systems with DAPM. This is 11*c9746eeaSTakashi Iwaibecause the components within the subsystem are being dynamically powered 12*c9746eeaSTakashi Iwaidepending on the audio usage and this can subsequently cause a small pop or 13*c9746eeaSTakashi Iwaiclick every time a component power state is changed. 14*c9746eeaSTakashi Iwai 15*c9746eeaSTakashi Iwai 16*c9746eeaSTakashi IwaiMinimising Playback Pops and Clicks 17*c9746eeaSTakashi Iwai=================================== 18*c9746eeaSTakashi Iwai 19*c9746eeaSTakashi IwaiPlayback pops in portable audio subsystems cannot be completely eliminated 20*c9746eeaSTakashi Iwaicurrently, however future audio codec hardware will have better pop and click 21*c9746eeaSTakashi Iwaisuppression. Pops can be reduced within playback by powering the audio 22*c9746eeaSTakashi Iwaicomponents in a specific order. This order is different for startup and 23*c9746eeaSTakashi Iwaishutdown and follows some basic rules:- 24*c9746eeaSTakashi Iwai:: 25*c9746eeaSTakashi Iwai 26*c9746eeaSTakashi Iwai Startup Order :- DAC --> Mixers --> Output PGA --> Digital Unmute 27*c9746eeaSTakashi Iwai 28*c9746eeaSTakashi Iwai Shutdown Order :- Digital Mute --> Output PGA --> Mixers --> DAC 29*c9746eeaSTakashi Iwai 30*c9746eeaSTakashi IwaiThis assumes that the codec PCM output path from the DAC is via a mixer and then 31*c9746eeaSTakashi Iwaia PGA (programmable gain amplifier) before being output to the speakers. 32*c9746eeaSTakashi Iwai 33*c9746eeaSTakashi Iwai 34*c9746eeaSTakashi IwaiMinimising Capture Pops and Clicks 35*c9746eeaSTakashi Iwai================================== 36*c9746eeaSTakashi Iwai 37*c9746eeaSTakashi IwaiCapture artifacts are somewhat easier to get rid as we can delay activating the 38*c9746eeaSTakashi IwaiADC until all the pops have occurred. This follows similar power rules to 39*c9746eeaSTakashi Iwaiplayback in that components are powered in a sequence depending upon stream 40*c9746eeaSTakashi Iwaistartup or shutdown. 41*c9746eeaSTakashi Iwai:: 42*c9746eeaSTakashi Iwai 43*c9746eeaSTakashi Iwai Startup Order - Input PGA --> Mixers --> ADC 44*c9746eeaSTakashi Iwai 45*c9746eeaSTakashi Iwai Shutdown Order - ADC --> Mixers --> Input PGA 46*c9746eeaSTakashi Iwai 47*c9746eeaSTakashi Iwai 48*c9746eeaSTakashi IwaiZipper Noise 49*c9746eeaSTakashi Iwai============ 50*c9746eeaSTakashi IwaiAn unwanted zipper noise can occur within the audio playback or capture stream 51*c9746eeaSTakashi Iwaiwhen a volume control is changed near its maximum gain value. The zipper noise 52*c9746eeaSTakashi Iwaiis heard when the gain increase or decrease changes the mean audio signal 53*c9746eeaSTakashi Iwaiamplitude too quickly. It can be minimised by enabling the zero cross setting 54*c9746eeaSTakashi Iwaifor each volume control. The ZC forces the gain change to occur when the signal 55*c9746eeaSTakashi Iwaicrosses the zero amplitude line. 56