0001 =====================
0002 Audio Pops and Clicks
0003 =====================
0004
0005 Pops and clicks are unwanted audio artifacts caused by the powering up and down
0006 of components within the audio subsystem. This is noticeable on PCs when an
0007 audio module is either loaded or unloaded (at module load time the sound card is
0008 powered up and causes a popping noise on the speakers).
0009
0010 Pops and clicks can be more frequent on portable systems with DAPM. This is
0011 because the components within the subsystem are being dynamically powered
0012 depending on the audio usage and this can subsequently cause a small pop or
0013 click every time a component power state is changed.
0014
0015
0016 Minimising Playback Pops and Clicks
0017 ===================================
0018
0019 Playback pops in portable audio subsystems cannot be completely eliminated
0020 currently, however future audio codec hardware will have better pop and click
0021 suppression. Pops can be reduced within playback by powering the audio
0022 components in a specific order. This order is different for startup and
0023 shutdown and follows some basic rules:-
0024 ::
0025
0026 Startup Order :- DAC --> Mixers --> Output PGA --> Digital Unmute
0027
0028 Shutdown Order :- Digital Mute --> Output PGA --> Mixers --> DAC
0029
0030 This assumes that the codec PCM output path from the DAC is via a mixer and then
0031 a PGA (programmable gain amplifier) before being output to the speakers.
0032
0033
0034 Minimising Capture Pops and Clicks
0035 ==================================
0036
0037 Capture artifacts are somewhat easier to get rid as we can delay activating the
0038 ADC until all the pops have occurred. This follows similar power rules to
0039 playback in that components are powered in a sequence depending upon stream
0040 startup or shutdown.
0041 ::
0042
0043 Startup Order - Input PGA --> Mixers --> ADC
0044
0045 Shutdown Order - ADC --> Mixers --> Input PGA
0046
0047
0048 Zipper Noise
0049 ============
0050 An unwanted zipper noise can occur within the audio playback or capture stream
0051 when a volume control is changed near its maximum gain value. The zipper noise
0052 is heard when the gain increase or decrease changes the mean audio signal
0053 amplitude too quickly. It can be minimised by enabling the zero cross setting
0054 for each volume control. The ZC forces the gain change to occur when the signal
0055 crosses the zero amplitude line.
