This behavior is due to the characteristics of the lowpass FIR used in the FIR rate converter (antialiasing in this case), which is controlled by the polyphase half-length parameter, P of the “designMultirateFIR” function. The default value of P is 12, and it may be too small for the input signals you are working with.
This issue can be fixed with any of the three approaches mentioned below:
- Approach 1: An easy way to fix this issue is to increase the value of P to a larger number, i.e., P = 64. This can be achieved by setting the third positional argument of the “designMultirateFIR” function to control the value of P.
>> P = 64; % default was 12
>> firrc = designMultirateFIR(FirInterpolation(i), FirDecimation(i), P, SystemObject=true);
You should expect a much better result after changing the value of P.
Refer to the documentation here,
- Approach 2: You can set the transition width directly instead of the FIR length. The third positional argument is also used in this case, but the value should be a fraction between 0 and 1.
>> TW = 0.005;
>> firrc = designMultirateFIR(FirInterpolation(i), FirDecimation(i), TW, SystemObject=true);
Refer to the documentation here,
- Approach 3: Another option is to use the “dsp.SampleRateConverter” object. This will allow a direct control of the bandwidth rather than transition width. This usually performs better than a “single-stage FIRRateConverter.”
>> firrc = dsp.SampleRateConverter(InputSampleRate = SampleRateIn(i), OutputSampleRate = SampleRateOut(i));
Refer to the documentation link for a detailed description of the “dsp.SampleRateConverter”.
