RM0020 datasheet(21/69 Pages) STMICROELECTRONICS

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RM0020

Function API

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Description:

Computes the last stage of N-point radix-2 complex to complex float in-place fast

Fourier transform (FFT). Input data are stored in inout_buffer, output data are written

over the input data into inout_buffer. This function is used to calculate the FFT for

lengths which are odd power of two (128, 512, 2048).

Note: There must be at least 16 bytes of readable memory behind inout_buffer.

Performance: See Section 7 and Table 34.

Example 7. fft_radix2_last_stage_float

#include "libdsp2.h"

float inout_buffer[2*512];

void main() {

/* bit reverse permutation and calculation of 512-point FFT */

bitrev_table_32bit(16, inout_buffer, seed_radix2_32bit_512);

fft_quad_float(512, inout_buffer, w_table_radix2_float_512);

fft_radix2_last_stage_float(512, inout_buffer,

w_table_radix2_float_512);

}

6.8

Last stage of N-point radix-2 complex to complex frac32 in-

place FFT

Function call:

void fft_radix2_last_stage_frac32(unsigned int N, int

*inout_buffer,

int *twiddle_factor_table);

Arguments:

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

Table 7.

fft_radix2_last_stage_float arguments (continued)

w_Re k

 iw_Im k





Table 8.

fft_radix2_last_stage_frac32 arguments

FFT length, tested for 128, 512, 2048

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)

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