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- Audio Clocking
- ==============
- This text describes the audio clocking terms in ASoC and digital audio in
- general. Note: Audio clocking can be complex!
- Master Clock
- ------------
- Every audio subsystem is driven by a master clock (sometimes referred to as MCLK
- or SYSCLK). This audio master clock can be derived from a number of sources
- (e.g. crystal, PLL, CPU clock) and is responsible for producing the correct
- audio playback and capture sample rates.
- Some master clocks (e.g. PLLs and CPU based clocks) are configurable in that
- their speed can be altered by software (depending on the system use and to save
- power). Other master clocks are fixed at a set frequency (i.e. crystals).
- DAI Clocks
- ----------
- The Digital Audio Interface is usually driven by a Bit Clock (often referred to
- as BCLK). This clock is used to drive the digital audio data across the link
- between the codec and CPU.
- The DAI also has a frame clock to signal the start of each audio frame. This
- clock is sometimes referred to as LRC (left right clock) or FRAME. This clock
- runs at exactly the sample rate (LRC = Rate).
- Bit Clock can be generated as follows:-
- BCLK = MCLK / x
- or
- BCLK = LRC * x
- or
- BCLK = LRC * Channels * Word Size
- This relationship depends on the codec or SoC CPU in particular. In general
- it is best to configure BCLK to the lowest possible speed (depending on your
- rate, number of channels and word size) to save on power.
- It is also desirable to use the codec (if possible) to drive (or master) the
- audio clocks as it usually gives more accurate sample rates than the CPU.
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