Download VisualDSP++ 3.5 C/C++ Compiler and Library

DSP Run-Time Library Reference
twidfftrad4
generate FFT twiddle factors for radix-4 FFT
Synopsis
#include <filter.h>
void twidfftrad4_fr16 (complex_fract16 w[], int n)
void twidfft_fr16(complex_fract16 w[], int n)
Description
The twidfftrad4_fr16 function initializes a table with complex twiddle
factors for a radix-4 FFT. The number of points in the FFT are defined
by n, and the coefficients are returned in the twiddle table w.
The size of the twiddle table must be at least ¾n, the length of the FFT
input sequence. A table can accommodate several FFTs of different sizes
by allocating the table at maximum size, and then using the stride argument of the FFT function to specify the step size through the table. If the
stride is set to 1, the FFT function uses all the table; if your FFT has only
a quarter of the number of points of the largest FFT, the stride should be
4.
For efficiency, the twiddle table is normally generated once during program initialization and is then supplied to the FFT routine as a separate
argument.
The twidfft_fr16 routine is provided as an alternative to the
twidfftrad4_fr16 routine and performs the same function.
Algorithm
This function takes FFT length n as an input parameter and generates the
lookup table of complex twiddle coefficients.
4-104
VisualDSP++ 3.5 C/C++ Compiler and Library Manual
for ADSP-219x DSPs