PCM1754-Q1: How many bit can be transferred via one LRCK?

Hi Elec,

BCLK = 64*fs is the maximum ratio that is supported. So yes that is 32 bits per channel. The number of bits in a frame is decided by the BCLK/Fs ratio. Based on the data format selected, the device will know how many bits are expected in the frame and where to look for the data within the frame to decode it accordingly.

Best regards,

Jeff

Hi Elec,

The PCM1754 specifically only supports two formats controlled by the FMT pin: 16 or 24 bit I2S (LOW) or 16 bit Right Justified (HIGH). In I2S the DAC will start counting at the second BCLK of the subframe and continue to the end of the frame. Any extra/padding zeros are on the LSB side so the DAC will still decode the same.

Best regards,

Jeff. 

Hi Elec,

Let me clarify the diagram. 48fs is for 16 bit data (48 bits / 2 channels - 8 bit padding per channel = 16 data bits per channel). 64fs is intended for 24 bit data (64 bits / 2 channels - 8 bit padding per channel = 24 data bits per channel. Additionally notice that the data word overlaps the next frame by 1 bit, making the offset linear.

Yes, if you give 32 bit data I expect the device to disregard the last 8 bits. The audio should not be abnormal, but there will be a marginal performance loss in SNR, compared to the original 32 bit data. In practical applications there isn't significant difference between 32 and 24 bit audio.

Best regards,

Jeff

Hi Elec,

The 8 bit padding isn't a requirement, but is commonly used to allow the BCLK/Fs ratio to be some power of 2. It's only a result of the clocking configuration, not a detailed requirement. You could use 48fs and fill the entire frame with no padding and the result would be the same. I was using the padding bits to acknowledge that in the diagram you can see that bit N is not at the end of the frame.

Best regards,

Jeff