Twidfftrad4 - Analog Devices VISUALDSP++ 3.5 Manual

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
factors for a radix-4 FFT. The number of points in the FFT are defined
by , and the coefficients are returned in the twiddle table
n
The size of the twiddle table must be at least ¾
input sequence. A table can accommodate several FFTs of different sizes
by allocating the table at maximum size, and then using the stride argu-
ment 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 pro-
gram initialization and is then supplied to the FFT routine as a separate
argument.
The
twidfft_fr16
twidfftrad4_fr16
Algorithm
This function takes FFT length
lookup table of complex twiddle coefficients.
4-104
function initializes a table with complex twiddle
routine is provided as an alternative to the
routine and performs the same function.
as an input parameter and generates the
n
VisualDSP++ 3.5 C/C++ Compiler and Library Manual
.
w
, the length of the FFT
n
for ADSP-219x DSPs