pops-clicks.rst 2.2 KB

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  1. =====================
  2. Audio Pops and Clicks
  3. =====================
  4. Pops and clicks are unwanted audio artifacts caused by the powering up and down
  5. of components within the audio subsystem. This is noticeable on PCs when an
  6. audio module is either loaded or unloaded (at module load time the sound card is
  7. powered up and causes a popping noise on the speakers).
  8. Pops and clicks can be more frequent on portable systems with DAPM. This is
  9. because the components within the subsystem are being dynamically powered
  10. depending on the audio usage and this can subsequently cause a small pop or
  11. click every time a component power state is changed.
  12. Minimising Playback Pops and Clicks
  13. ===================================
  14. Playback pops in portable audio subsystems cannot be completely eliminated
  15. currently, however future audio codec hardware will have better pop and click
  16. suppression. Pops can be reduced within playback by powering the audio
  17. components in a specific order. This order is different for startup and
  18. shutdown and follows some basic rules:-
  19. ::
  20. Startup Order :- DAC --> Mixers --> Output PGA --> Digital Unmute
  21. Shutdown Order :- Digital Mute --> Output PGA --> Mixers --> DAC
  22. This assumes that the codec PCM output path from the DAC is via a mixer and then
  23. a PGA (programmable gain amplifier) before being output to the speakers.
  24. Minimising Capture Pops and Clicks
  25. ==================================
  26. Capture artifacts are somewhat easier to get rid as we can delay activating the
  27. ADC until all the pops have occurred. This follows similar power rules to
  28. playback in that components are powered in a sequence depending upon stream
  29. startup or shutdown.
  30. ::
  31. Startup Order - Input PGA --> Mixers --> ADC
  32. Shutdown Order - ADC --> Mixers --> Input PGA
  33. Zipper Noise
  34. ============
  35. An unwanted zipper noise can occur within the audio playback or capture stream
  36. when a volume control is changed near its maximum gain value. The zipper noise
  37. is heard when the gain increase or decrease changes the mean audio signal
  38. amplitude too quickly. It can be minimised by enabling the zero cross setting
  39. for each volume control. The ZC forces the gain change to occur when the signal
  40. crosses the zero amplitude line.