RM0020 datasheet(18/69 Pages) STMICROELECTRONICS

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Function API

RM0020

18/69

6.5

N-point quad radix-2 complex to complex float in-place FFT

Function call:

void fft_quad_float(unsigned int N, float *inout_buffer,

float *twiddle_factor_table);

Arguments:

Description:

Computes the N-point quad radix-2 complex to complex float in-place fast Fourier

transform (FFT). Input bit reversed data are stored in inout_buffer, output data are

written over the input data into inout_buffer. There is used radix-2 algorithm in which

two stages are calculated in one loop (quad butterfly).

Algorithm:

Equation 5

for each n from 0 to N-1

Equation 6

where i is the imaginary unit with the property that:

Note: There must be at least 32 bytes of readable memory behind inout_buffer.

Performance: See Section 7 and Table 33.

Example 5. fft_quad_float

#include "libdsp2.h"

float inout_buffer[2*256];

Table 5.

fft_quad_float arguments

Nin

FFT length, must be even power of two, tested for 64, 256, 1024,

4096

inout_buffer

in/out

pointer to the input/output vector of length 2*N, vector must be

double word aligned

memory layout:

x_Re(0) x_Im(0) x_Re(1) x_Im(1) ... x_Re(N-1) x_Im(N-1)

(Re - real part, Im - imaginary part)

twiddle_factor_table in

pointer to the vector of twiddle factors of length N, vector must

be double word aligned

memory layout:

w_Re(0) w_Im(0) w_Re(1) w_Im(1) ... w_Re(N/2-1) w_Im(N/2-1)

Use predefined w_table_radix2_float_N arrays.

The w_table_radix2_float_N is computed according to this

formula:

for k = 0, 1, ... N/2-1

w_Re k

 iw_Im k







 w



–



i

--------

–

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