Hello maxidcx,
To investigate the phase issue I built a FIR filter with a known response and plotted it:
This FIR kernel has all zero coefficients except a "1" in the middle. Thus it's no surprise that it has unity gain, flat frequency response and a constantly increasing phase lag. It's functionally identical to the delay just below it, and the two have identical gain and phase plots:
Knowing the expected phase response, I can see two issues with the Stimulus-Probe phase plot in SigmaStudio:
- The phase axis is shown as "radians" but its actually in degrees. The filter has a 500-sample group delay, which works out to 10.4 mS at a 48K sample rate. This produces a frequency-dependent phase shift -- for example, at 1000 Hz, the delay amounts to 10.4 complete cycles of phase shift, equivalent to 3750 degrees or 65 radians. The graph shows about 3750, so it must be in degrees. That's about 58 times higher than the radian figure.
- Once the phase lag exceeds 6018 degrees, something overflows and you get the cyclic results shown. The actual phase shift, on the other hand, just keeps increasing.
So apparently something is wrong with the phase display in this case. I don't suspect it affects the actual FIR performance, because the coefficients you worked out go into the DSP while the Stimulus-Probe response is figured separately. Neither hardware nor compiling is needed to use Stimulus-Probe. Also, I tried the same experiment above with a ADAU1452 and a AD1940, and received the same results even though the chips use different compilers.
Best regards,
Bob