Using ADAU1701 with 32-bit I2S input from external ADCs
Hi,
I would like to use two TI PCM1809 ADCs to supply 4 channels of audio via I2S to an ADAU1701 so that it can process and output the resulting audio stream via two I2S ports to an amplifier.
The PCM1809 output format is I2S with 32-bit word length, can the ADAU1701 handle this 32-bit serial input?
Additionally, what does the datasheet mean that the audio processor is 28/56-bit capable?
Thanks!
Audio Signal Processors
hardware
SigmaDSP Audio Processors
adau1701
Hello fc0,
The ADAU1701 serial port will only take in 24 bit data but it can be setup for the standard 32 bit wide data slot on an I2S format. The data is left justified so what will happen is that the bottom 8 bits will be truncated. Usually this is not a problem because most "32-bit" converters only have about a 22 bit resolution. 22 bits is 132dB of SNR. I did not look up the specs on the ADC but I am guessing it might not even be that good. So the truncation will not be a problem. The serial port will bring in the data as a 24 bit value.
The 28/56 means this. When the 24-bit data comes in from the serial port we add four bits to the top and sign extend the sign bit to give a headroom of 24dB for processing. When the core does a multiply, the accumulator is 56 bits wide to not saturate when two 28 bit numbers are multiplied.
FYI, just make sure the clocks going to the ADCs are either from the 1701 or if the ADCs are the clock masters, then make sure the ADCs are running off of the same master clock that the 1701 is using. If you do not do this the DSP will just mute because it will see that the clocks are not correlated.
Dave T
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