Electronic – How to program DSP inside ADAU1772 codec

You can do your entire project in a desktop PC. In fact, if I had to do it, I would start with the desktop:

1. a .wav file is already sampled at a high frequency, often 44100 or 48000 Hz.
2. determining the highest frequency can be done with an FFT. For prototyping, I would link FFTW.
3. downsampling to an arbitrary frequency is a bit hard, because downsampling involves low-pass filtering. You need to set up a filter for each frequency. Look at libsamplerate and see how it sets up a SINC function to convolve against.
4. Converting back to the original sample rate will involve another low-pass filter. See again libsamplerate.

I believe I would implement this in several passes, for ease of debugging:

1. Get everything working in Matlab or Octave first. Octave has libraries to do all the filtering and Fourier analysis.
2. Get everything working in C on PC, linking FFTW and libsamplerate for the downsampling / upsampling.
3. Rewrite the C code with explicit-width variable types (e.g., int16_t instead of "short") and replace FFTW and libsamplerate with own code so that it compiles standalone.
4. In C for the MSP430 or whatever DSP you've got, write interrupt routines to sample data on the ADC and output it on the DAC. Test that this works, just going from input to output.
5. Take the working code from step 3 and compile it for the MSP430 or whatever. Then wedge it in to the working code from step 4 to operate on the sampled data between ADC and DAC.

This may seem like a lot of steps, but it is much more likely to make a working result than heroically coding everything up in one huge MSP430 application, then trying to debug it on the dev board.

The interface to an AC97 codec is a bit more complicated than straight digital audio. The serial data consists of 256-bit data frames; each frame contains several channels of 20-bit samples. The overall data rate is 12.288 MHz; dividing by 256 gives the sample rate of 48 kHz. Part of the 256-bit frame is dedicated to control messages, e.g. to set mixer registers. You may need to do this once after power up/reset to set the volume.

The AC97 spec is available from Intel. Writing your own master is not unreasonable but it will take some time. You may also be able to find one you can reuse. OpenCores has one. There's a very barebones AC97 controller and some general information about the protocol here.

I can't remember how many of the AC97 registers are standardized. The manual I found online for your board says it has an LM4550 codec. Assuming that's right, you may want to refer to the LM4550 datasheet for a complete list of configuration registers.