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