Twidfft2D - Analog Devices VISUALDSP++ 3.5 Manual

DSP Run-Time Library Reference

twidfft2d

generate FFT twiddle factors for 2-D FFT
Synopsis
#include <filter.h>
void twidfft2d_fr16 (complex_fract16 w[], int n)
Description
The
twidfft2d_fr16
FFT. The size of the FFT input sequence is given by the argument
the function writes the twiddle factors to the vector
dle table.
The size of the twiddle table must be
FFT. Normally, the table is only calculated once and is then passed to an
FFT function as an argument. A twiddle table may be used to generate
several FFTs of different sizes by initializing the table for the largest FFT
and then using the stride argument of the FFT function to specify the step
size through the table. For example, to generate the largest FFT, the stride
would be set to 1; and to generate an FFT of half this size, the stride
would be set to 2.
Algorithm
This function takes FFT length
lookup table of complex twiddle coefficients.
The samples generated are:
=
twid re k _ ( ) cos
4-106
function generates complex twiddle factors for a 2-D
n
π
 2 
k
 
 
n
VisualDSP++ 3.5 C/C++ Compiler and Library Manual
, the number of points in the
n/2
as an input parameter and generates the
and
n
, known as the twid-
w
for ADSP-219x DSPs