| 1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253 |
- 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.
|
