eb1a6af39b
This patch adds documentation describing the ASoC architecture and a maintainer entry for ASoC. The documentation includes the following files:- codec.txt: Codec driver internals. DAI.txt: Description of Digital Audio Interface standards and how to configure a DAI within your codec and CPU DAI drivers. dapm.txt: Dynamic Audio Power Management. platform.txt: Platform audio DMA and DAI. machine.txt: Machine driver internals. pop_clicks.txt: How to minimise audio artifacts. clocking.txt: ASoC clocking for best power performance. Signed-off-by: Liam Girdwood <liam.girdwood@wolfsonmicro.com> Signed-off-by: Takashi Iwai <tiwai@suse.de> Signed-off-by: Jaroslav Kysela <perex@suse.cz>
53 lines
2.1 KiB
Plaintext
53 lines
2.1 KiB
Plaintext
Audio Pops and Clicks
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=====================
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Pops and clicks are unwanted audio artifacts caused by the powering up and down
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of components within the audio subsystem. This is noticable on PC's when an audio
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module is either loaded or unloaded (at module load time the sound card is
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powered up and causes a popping noise on the speakers).
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Pops and clicks can be more frequent on portable systems with DAPM. This is because
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the components within the subsystem are being dynamically powered depending on
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the audio usage and this can subsequently cause a small pop or click every time a
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component power state is changed.
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Minimising Playback Pops and Clicks
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===================================
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Playback pops in portable audio subsystems cannot be completely eliminated atm,
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however future audio codec hardware will have better pop and click supression.
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Pops can be reduced within playback by powering the audio components in a
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specific order. This order is different for startup and shutdown and follows
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some basic rules:-
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Startup Order :- DAC --> Mixers --> Output PGA --> Digital Unmute
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Shutdown Order :- Digital Mute --> Output PGA --> Mixers --> DAC
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This assumes that the codec PCM output path from the DAC is via a mixer and then
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a PGA (programmable gain amplifier) before being output to the speakers.
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Minimising Capture Pops and Clicks
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==================================
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Capture artifacts are somewhat easier to get rid as we can delay activating the
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ADC until all the pops have occured. This follows similar power rules to
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playback in that components are powered in a sequence depending upon stream
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startup or shutdown.
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Startup Order - Input PGA --> Mixers --> ADC
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Shutdown Order - ADC --> Mixers --> Input PGA
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Zipper Noise
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============
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An unwanted zipper noise can occur within the audio playback or capture stream
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when a volume control is changed near its maximum gain value. The zipper noise
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is heard when the gain increase or decrease changes the mean audio signal
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amplitude too quickly. It can be minimised by enabling the zero cross setting
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for each volume control. The ZC forces the gain change to occur when the signal
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crosses the zero amplitude line.
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