dsp.FIRRateConverter has unexpected results

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Andy 2017-6-30

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https://ww2.mathworks.cn/matlabcentral/answers/346989-dsp-firrateconverter-has-unexpected-results

评论： Honglei Chen 2017-7-6

I am trying to resample a complex signal and the output of the filter is not what I expect. I expect the output of the following example to be a smooth complex sinusoid, but it seems to have misaligned samples.

filt = dsp.FIRRateConverter;
filt.InterpolationFactor = 8;
filt.DecimationFactor = 2;
x = exp(1i * 2 * pi * [1:100] * 1 / 10).';
y = step(filt,x);
figure(1);plot(real(x))
figure(2);plot(real(y(:)))

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Answer 1

Andy 2017-7-5

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https://ww2.mathworks.cn/matlabcentral/answers/346989-dsp-firrateconverter-has-unexpected-results#answer_272976

编辑：Andy 2017-7-6

在 MATLAB Online 中打开

It works for those particular values, but I need to work with a range of values. Here's a better example of the problem. Notice that the "resample" function works properly, but for my primary application I am required to use the "dsp" object, which has the problem.

filt = dsp.FIRRateConverter;
filt.InterpolationFactor = 8;
filt.DecimationFactor = 5;
x = exp(1i * 2 * pi * [1:100] * 1 / 10).';
y = step(filt,x);
z = resample(x,filt.InterpolationFactor,filt.DecimationFactor);
figure(1); 
subplot(3,1,1);plot(real(x))
subplot(3,1,2);plot(real(y(:)))
subplot(3,1,3);plot(real(z))

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Honglei Chen 2017-7-6

在 MATLAB Online 中打开

In this case, the default filter (see documentation) is not suitable since the cutoff is too high and it introduces the aliasing. Change the filter to

filt.Numerator = firpm(70,[0 0.125 0.2 1],[1 1 0 0]);

and you'll see the right result. However, it also has some group delay effect in it.

The resample designs the filter for you and compensate for the group delay, but it does not work if you have streaming data. That's probably the tradeoff.

HTH

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