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-Audio Pops and Clicks
-Pops and clicks are unwanted audio artifacts caused by the powering up and down
-of components within the audio subsystem. This is noticeable on PCs when an
-audio module is either loaded or unloaded (at module load time the sound card is
-powered up and causes a popping noise on the speakers).
-Pops and clicks can be more frequent on portable systems with DAPM. This is
-because the components within the subsystem are being dynamically powered
-depending on the audio usage and this can subsequently cause a small pop or
-click every time a component power state is changed.
-Minimising Playback Pops and Clicks
-Playback pops in portable audio subsystems cannot be completely eliminated
-currently, however future audio codec hardware will have better pop and click
-suppression. Pops can be reduced within playback by powering the audio
-components in a specific order. This order is different for startup and
-shutdown and follows some basic rules:-
- Startup Order :- DAC --> Mixers --> Output PGA --> Digital Unmute
- Shutdown Order :- Digital Mute --> Output PGA --> Mixers --> DAC
-This assumes that the codec PCM output path from the DAC is via a mixer and then
-a 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 the
-ADC until all the pops have occurred. This follows similar power rules to
-playback in that components are powered in a sequence depending upon stream
-startup or shutdown.
- Startup Order - Input PGA --> Mixers --> ADC
- Shutdown Order - ADC --> Mixers --> Input PGA
-Zipper Noise
-An unwanted zipper noise can occur within the audio playback or capture stream
-when a volume control is changed near its maximum gain value. The zipper noise
-is heard when the gain increase or decrease changes the mean audio signal
-amplitude too quickly. It can be minimised by enabling the zero cross setting
-for each volume control. The ZC forces the gain change to occur when the signal
-crosses the zero amplitude line.