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Documentation / sound / alsa / soc / clocking.txt




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Based on kernel version 3.13. Page generated on 2014-01-20 22:04 EST.

1	Audio Clocking
2	==============
3	
4	This text describes the audio clocking terms in ASoC and digital audio in
5	general. Note: Audio clocking can be complex!
6	
7	
8	Master Clock
9	------------
10	
11	Every audio subsystem is driven by a master clock (sometimes referred to as MCLK
12	or SYSCLK). This audio master clock can be derived from a number of sources
13	(e.g. crystal, PLL, CPU clock) and is responsible for producing the correct
14	audio playback and capture sample rates.
15	
16	Some master clocks (e.g. PLLs and CPU based clocks) are configurable in that
17	their speed can be altered by software (depending on the system use and to save
18	power). Other master clocks are fixed at a set frequency (i.e. crystals).
19	
20	
21	DAI Clocks
22	----------
23	The Digital Audio Interface is usually driven by a Bit Clock (often referred to
24	as BCLK). This clock is used to drive the digital audio data across the link
25	between the codec and CPU.
26	
27	The DAI also has a frame clock to signal the start of each audio frame. This
28	clock is sometimes referred to as LRC (left right clock) or FRAME. This clock
29	runs at exactly the sample rate (LRC = Rate).
30	
31	Bit Clock can be generated as follows:-
32	
33	BCLK = MCLK / x
34	
35	 or
36	
37	BCLK = LRC * x
38	
39	 or
40	
41	BCLK = LRC * Channels * Word Size
42	
43	This relationship depends on the codec or SoC CPU in particular. In general
44	it is best to configure BCLK to the lowest possible speed (depending on your
45	rate, number of channels and word size) to save on power.
46	
47	It is also desirable to use the codec (if possible) to drive (or master) the
48	audio clocks as it usually gives more accurate sample rates than the CPU.
49	
50	
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