4
This text describes the audio clocking terms in ASoC and digital audio in
5
general. Note: Audio clocking can be complex!
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.
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).
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.
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).
31
Bit Clock can be generated as follows:-
41
BCLK = LRC * Channels * Word Size
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.
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.