| 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. |
