czt

Signal Processing Toolbox

Chirp z-transform.

y = czt(x,m,w,a)
y = czt(x)

Description

y = czt(x,m,w,a)

returns the chirp z-transform of signal x. The chirp z-transform is the z-transform of x along a spiral contour defined by w and a. m is a scalar that specifies the length of the transform, w is the ratio between points along the z-plane spiral contour of interest, and scalar a is the complex starting point on that contour. The contour, a spiral or "chirp" in the z-plane, is given by

z = a*(w.^-(0:m-1))

y = czt(x)

uses the following default values:

m = length(x)
w = exp(j*2*pi/m)
a = 1

With these defaults, czt returns the z-transform of x at m equally spaced points around the unit circle. This is equivalent to the discrete Fourier transform of x, or fft(x). The empty matrix [] specifies the default value for a parameter.

If x is a matrix, czt(x,m,w,a) transforms the columns of x.

Examples

Create a random vector x of length 1013 and compute its DFT using czt. This is faster than the fft function on the same sequence.

x = randn(1013,1);
y = czt(x);

Use czt to zoom in on a narrow-band section (100 to 150 Hz) of a filter's frequency response. First design the filter:

h = fir1(30,125/500,boxcar(31)); % filter

Establish frequency and CZT parameters:

Fs = 1000; f1 = 100; f2 = 150;  % in Hertz
m = 1024;
w = exp(-j*2*pi*(f2-f1)/(m*Fs));
a = exp(j*2*pi*f1/Fs);

Compute both the DFT and CZT of the filter:

y = fft(h,1000);
z = czt(h,m,w,a);

Create frequency vectors and compare the results:

fy = (0:length(y)-1)'*1000/length(y); 
fz = ((0:length(z)-1)'*(f2-f1)/length(z)) + f1;
plot(fy(1:500),abs(y(1:500))); axis([1 500 0 1.2])
plot(fz,abs(z)); axis([f1 f2 0 1.2])

Algorithm

czt uses the next power-of-2 length FFT to perform a fast convolution when computing the z-transform on a specified chirp contour [1]. czt can be significantly faster than fft for large, prime-length sequences.

Diagnostics

If m, w, or a is not a scalar, czt gives the following error message:

Inputs M, W, and A must be scalars.

`fft`	One-dimensional fast Fourier transform.
`freqz`	Frequency response of digital filters.