Twidfftrad2 - Analog Devices VISUALDSP++ 3.5 Manual

DSP Run-Time Library Reference

twidfftrad2

generate FFT twiddle factors for radix-2 FFT
Synopsis
#include <filter.h>
void twidfftrad2_fr16 (complex_fract16 w[], int n)
Description
This function calculates complex twiddle coefficients for a radix-2 FFT
with points and returns the coefficients in the vector
n
known as the twiddle table, is normally calculated once and is then passed
to an FFT function as a separate argument. The size of the table must be
at least ½ of , the number of points in the FFT.
n
FFTs of different sizes can be accommodated with the same twiddle table.
Simply allocate the table at the maximum size. Each FFT has an addi-
tional parameter, the "stride" of the twiddle table. To use the whole table,
specify a stride of 1. If the FFT uses only half the points of the largest
FFT, the stride should be 2 (this takes only every other element).
Algorithm
This function takes FFT length
lookup table of complex twiddle coefficients. The samples are:
=
twid re k _ ( ) cos
= −
twid _ im ( k ) sin
where k = {0, 1, 2, ..., n/2 - 1}
4-102
n
π
 2 
k
 
 
n
π 2
 
 k 
n
 
VisualDSP++ 3.5 C/C++ Compiler and Library Manual
as an input parameter and generates the
. The vector ,
w w
for ADSP-219x DSPs