Why does the example in the MFILT.CICINTERP section of the Filter Design Toolbox 3.2 (R14SP2) documentation interpolate the signal incorrectly?

The example in the documentation for MFILT.CICINTERP interpolates the "int16" signal incorrectly. The interpolated signal is saturated at +1 and -1 instead of following the sinusoid input. Even the plot included in the documentation shows this discrepancy.

Answer 1

[MathWorks Support Team]

This bug has been fixed in Release 14 Service Pack 3 (R14SP3). For previous product releases, read below for any possible workarounds:

This has been verified as an error in the documentation for Filter Design Toolbox 3.2 (R14SP2) in the MFILT.CICINTERP section. The example code in this documentation should be the following:

R = 2;                      % Interpolation factor

hm = mfilt.cicinterp(R);    % Use default NumberOfSections and DifferentialDelay

set(hm,'InputFracLength',0);    % Integer input specified.

fs = 22.05e3;               % Original sample freq:22.05 kHz.

n = 0:5119;                 % 5120 samples, 0.232 second long signal

x  = sin(2*pi*1e3/fs*n);    % Original signal, sinusoid at 1 kHz

% Scale input to use the full dynamic range of the INT16 data type

x = int16((2^15-1)*x/gain(hm));

y_int = filter(hm,x); % 5120 samples, still 0.232 seconds

% Scale the input and output to overlay plots

x = double(x); x = x/max(abs(x)); 

y = double(y_int); y = y/max(abs(y));

stem(n(1:22)/fs,x(1:22),'filled'); % Plot original signal sampled 

                                     % at 22.05 kHz 

hold on;

stem(n(1:44)/(fs*R),y(4:47),'r');  % Plot interpolated signal 

                                     % (44.1 kHz) in red

xlabel('Time (sec)');ylabel('Signal Value');