intfilt (Signal Processing Toolbox)

Interpolation FIR filter design.

Syntax

b = intfilt(r,l,alpha)
b = intfilt(r,n,'Lagrange')

Description

b = intfilt(r,l,alpha)

designs a linear phase FIR filter that performs ideal bandlimited interpolation using the nearest 2*l nonzero samples, when used on a sequence interleaved with r-1 consecutive zeros every r samples. It assumes an original bandlimitedness of alpha times the Nyquist frequency. The returned filter is identical to that used by interp.

b = intfilt(r,n,'Lagrange')

or b = intfilt(r,n,'l') designs an FIR filter that performs nth-order Lagrange polynomial interpolation on a sequence interleaved with r-1 consecutive zeros every r samples. b has length (n + 1)*r for n even, and length (n + 1)*r-1 for n odd.

Both types of filters are basically lowpass and are intended for interpolation and decimation.

Examples

Design a digital interpolation filter to upsample a signal by four, using the bandlimited method:

alpha = 0.5; % "bandlimitedness" factor
h1 = intfilt(4,2,alpha); % bandlimited interpolation

The filter h1 works best when the original signal is bandlimited to alpha times the Nyquist frequency. Create a bandlimited noise signal:

randn('seed',0)
x = filter(fir1(40,0.5),1,randn(200,1)); % bandlimit

Now zero pad the signal with three zeros between every sample. The resulting sequence is four times the length of x:

xr = reshape([x zeros(length(x),3)]',4*length(x),1);

Interpolate using the filter command:

y = filter(h1,1,xr);

y is an interpolated version of x, delayed by seven samples (the group-delay of the filter). Zoom in on a section to see this:

plot(100:200,y(100:200),7+(101:4:196),x(26:49),'o')

intfilt's other type of filter performs Lagrange polynomial interpolation of the original signal. For example, first-order polynomial interpolation is just linear interpolation, which is accomplished with a triangular filter:

h2 = intfilt(4,1,'l')   % Lagrange interpolation
h2 =
   0.2500   0.5000   0.7500   1.0000   0.7500   0.5000  0.2500

Algorithm

The bandlimited method uses firls to design an interpolation FIR equivalent to that presented in [1]. The polynomial method uses Lagrange's polynomial interpolation formula on equally spaced samples to construct the appropriate filter.

`decimate`	Decrease the sampling rate for a sequence (decimation).
`interp`	Increase sampling rate by an integer factor (interpolation).
`resample`	Change sampling rate by any rational factor.