I use the ADAU1761 evaluation board to receive serial data (I2S format) from SERIAL DATA INPUT/OUTPUT PORTS, and use some modules in SigmaStudio to process the input data, and then the data output from the analog side. In this process I encountered a few questions, and hope to get your answers.
question 1 When I use the I2S input, in addition to the data, I also need three clocks : ①MCLK=256*fs，②LRCLK=fs，③BCLK=2*sample_length*fs. The sampling rate of the input data is 32kHz, but the default sampling rate of ADAU1761 is 48kHz. Which sampling rate should I use to calculate the clock? I have used 32kHz to calculate the clock, and I have confirmed the received data is correct, but some problems will appear after I add certain module in sigmastudio. Please help me to check whether the sampling rate of the register settings are correct. The following is a screen shot of the register settings of sampling rate and clock.
In the ADAU1761 chip manual mentioned “It is recommended that the sampling rates for the converters, serial ports, and DSP be set to the same value” . However, after I set the DSP sampling rate of 32kHz, something is wrong: When I used a notch filter to remove the 3kHz signal, the 2 kHz signal is filtered out instead of 3kHz. After I set the DSP sampling rate of serial input data rate(as shown above), This problem does not exist.
question 2 I set all sample rates to 16kHz. I used 500Hz signal source and white noise , and then add noise reduction algorithm to reduce white noise(As shown below). However, I observed the output signal is 250Hz from the oscilloscope, even I have not enabled noise reduction algorithm. Why ??
before adding noise reduction algorithm=500Hz after adding noise reduction algorithm=250Hz
I do not know whether I use this module correctly. However, When it eliminates noise, the useful signal is also reduced a lot.
question 3 I want to use the FIR Filter in sigmastudio to design a band-pass filter (300Hz to 3400Hz). The filter coefficients are generated using FDATool in MATLAB. The filter spectrum after the coefficients are imported is shown in the following figure, which is right. When I input 200Hz/500Hz/1000Hz/2000Hz/3000Hz/4000Hz/5000Hz/6000Hz/7000Hz signal, the output signal is not what I want(As shown below). This is why???
Input signal spectrum(No FIR)
Output signal spectrum(with FIR)
To prevent the picture is not clear, I have uploaded the document about these problems
to the attachment.