Filter cascade with raised cosine filter and half-band filter

I've created a raised cosine filter and a half-band filter, respectively, and want to cascade them. However, I got "Numeric stages of size [1 11] are not supported. All numeric stages must be scalars."

Fs = 20e3;
Fp = 15e3;

rc = rcosdesign(0.35, 5, 2, 'sqrt');

Fs2 = 4*Fs;     % Sample rate
Fp2 = Fp;   % Passband edge
Fh2 = Fs2/4;      % Halfband frequency
TW2 = (Fh2-Fp2)/Fh2;
N2 = 48;

hb2 = designHalfbandFIR(FilterOrder=N2,...
                        TransitionWidth=TW2,...
                        Passband='lowpass',...
                        DesignMethod='kaiser',...
                        Structure='interp',...
                        SystemObject=true);


Fc = cascade(rc, hb2);

Error using dsp.internal.CompositeFilter/checkFilterAndClearMetadata (line 134)
Numeric stages of size [1 11] are not supported. All numeric stages must be scalars.

Error in dsp.FilterCascade/set.Stage1 (line 63)
obj.Stage1 = checkFilterAndClearMetadata(obj, val, obj.Stage1);

Error in dsp.FilterCascade/setStages (line 712)
obj.(stageIdx) = stages{ind};

Error in dsp.FilterCascade (line 153)
obj.setStages(varargin);

Error in dsp.internal.FilterAnalysis/cascade (line 1722)
FC = dsp.FilterCascade(obj,varargin{:});

I also created the raised cosine filter with comm.RaisedCosineTransmitFilter to see if this works. However, MATLAB returned another error, "Cascades must be made of discrete-time filter (DFILT) objects.".

Fs = 20e3;
Fp = 15e3;

rc = comm.RaisedCosineTransmitFilter;

Fs2 = 4*Fs;     % Sample rate
Fp2 = Fp;   % Passband edge
Fh2 = Fs2/4;      % Halfband frequency
TW2 = (Fh2-Fp2)/Fh2;
N2 = 48;

hb2 = designHalfbandFIR(FilterOrder=N2,...
                        TransitionWidth=TW2,...
                        Passband='lowpass',...
                        DesignMethod='kaiser',...
                        Structure='interp',...
                        SystemObject=true);


Fc = cascade(rc, hb2);

Why two codes show different errors and what am I supposed to cascade two filters?

Answer 1

[Paul]

Fs = 20e3;
Fp = 15e3;
rc = rcosdesign(0.35, 5, 2, 'sqrt');
Fs2 = 4*Fs;     % Sample rate
Fp2 = Fp;   % Passband edge
Fh2 = Fs2/4;      % Halfband frequency
TW2 = (Fh2-Fp2)/Fh2;
N2 = 48;
hb2 = designHalfbandFIR(FilterOrder=N2,...
                        TransitionWidth=TW2,...
                        Passband='lowpass',...
                        DesignMethod='kaiser',...
                        Structure='interp',...
                        SystemObject=true);
whos rc hb2

  Name      Size            Bytes  Class                          Attributes

  hb2       1x1                 8  dsp.FIRHalfbandInterpolator              
  rc        1x11               88  double                                   

rc is a vector of coefficients, but cascade requires its inputs to be filter system objects.

rc = dsp.FIRFilter(rc);
Fc = cascade(rc, hb2);

I'm not familiar with the DSP Toolbox. Why does hb2 have a sample rate (and where did it come from as it's not specified in the call to designHalfBandFIR?)

hb2.SampleRate

ans = 44100

but rc does not have a sample rate?

rc.SampleRate

Unrecognized method, property, or field 'SampleRate' for class 'dsp.FIRFilter'.

Regardless, I hope this at least points you in the right direction.

[Jay]

Thanks Paul! This works.