FFT Library

mikroC PRO for PIC32 includes a library for FFT calculation. All routines work with fractional Q15 format.

Library Dependency Tree

FFT Library Dependency Tree

Library Routines



void FFT(int *outdata, const int *Wn, unsigned int *log2N);


Function applies FFT transformation to input samples, input samples must be in Y data space.

  • f(n): array of complex input samples
  • WN: TwiddleFactors
  • N = 2m, m Î Z
The amplitude of current FFT sample is calculated as:
  • outdata: Pointer to an array of input samples. Upon completion, complex array of FFT samples is placed in the this parameter.
  • Wn: Address of constant array which contains complex Twiddle factors. See Twiddle Factors for adequate array values.
  • log2N: Logarithm base 2 of the number of samples, N = 2log2N, which represents "Butterfly" algorithm level number.




// 8 pairs of [Re, Im] values
int inputSamples[16] = { 0x4000, 0, 0x4000, 0, 0x4000, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,0};

// Twiddle factors for 8 pairs complex input values.
const int Wn[8]      = {0x7fff,0, 0x5A82, 0xA57E, 0, 0x8000, 0xA57E, 0xA57E};
unsigned int log2N   =3;

void main() {

  FFT(inputSamples, Wn, log2N);
  BitReverseComplex(inputSamples, log2N);


Complex array of FFT samples is placed in outdata parameter. Number of complex input samples must be a power of 2. Input samples are arranged in manner Re,Im,Re,Im... (where Im is always zero). Output samples are arranged in the same manner but Im parts are different from zero. Output samples are symmetrical (First half of output samples (index from 0 to N/2) is identical as second half of output samples(index from N/2 to N).

Note that the output values are scaled by factor of 1/N (due to the saturation avoidance), with N the length of the FFT. Input is expected in natural ordering, while output is produced in Bit Reverse ordering.

Twiddle Factors :


const int TwiddleCoeff_64[64] = {
  0x7FFF, 0x0000, 0x7F62, 0xF374, 0x7D8A, 0xE707, 0x7A7D, 0xDAD8,
  0x7642, 0xCF04, 0x70E3, 0xC3A9, 0x6A6E, 0xB8E3, 0x62F2, 0xAECC,
  0x5A82, 0xA57E, 0x5134, 0x9D0E, 0x471D, 0x9592, 0x3C57, 0x8F1D,
  0x30FC, 0x89BE, 0x2528, 0x8583, 0x18F9, 0x8276, 0x0C8C, 0x809E,
  0x0000, 0x8000, 0xF374, 0x809E, 0xE707, 0x8276, 0xDAD8, 0x8583,
  0xCF04, 0x89BE, 0xC3A9, 0x8F1D, 0xB8E3, 0x9592, 0xAECC, 0x9D0E,
  0xA57E, 0xA57E, 0x9D0E, 0xAECC, 0x9592, 0xB8E3, 0x8F1D, 0xC3A9,
  0x89BE, 0xCF04, 0x8583, 0xDAD8, 0x8276, 0xE707, 0x809E, 0xF374};


const int TwiddleCoeff_128[128] = {
  0x7FFF, 0x0000, 0x7FD9, 0xF9B8, 0x7F62, 0xF374, 0x7E9D, 0xED38,
  0x7D8A, 0xE707, 0x7C2A, 0xE0E6, 0x7A7D, 0xDAD8, 0x7885, 0xD4E1,
  0x7642, 0xCF04, 0x73B6, 0xC946, 0x70E3, 0xC3A9, 0x6DCA, 0xBE32,
  0x6A6E, 0xB8E3, 0x66D0, 0xB3C0, 0x62F2, 0xAECC, 0x5ED7, 0xAA0A,
  0x5A82, 0xA57E, 0x55F6, 0xA129, 0x5134, 0x9D0E, 0x4C40, 0x9930,
  0x471D, 0x9592, 0x41CE, 0x9236, 0x3C57, 0x8F1D, 0x36BA, 0x8C4A,
  0x30FC, 0x89BE, 0x2B1F, 0x877B, 0x2528, 0x8583, 0x1F1A, 0x83D6,
  0x18F9, 0x8276, 0x12C8, 0x8163, 0x0C8C, 0x809E, 0x0648, 0x8027,
  0x0000, 0x8000, 0xF9B8, 0x8027, 0xF374, 0x809E, 0xED38, 0x8163,
  0xE707, 0x8276, 0xE0E6, 0x83D6, 0xDAD8, 0x8583, 0xD4E1, 0x877B,
  0xCF04, 0x89BE, 0xC946, 0x8C4A, 0xC3A9, 0x8F1D, 0xBE32, 0x9236,
  0xB8E3, 0x9592, 0xB3C0, 0x9930, 0xAECC, 0x9D0E, 0xAA0A, 0xA129,
  0xA57E, 0xA57E, 0xA129, 0xAA0A, 0x9D0E, 0xAECC, 0x9930, 0xB3C0,
  0x9592, 0xB8E3, 0x9236, 0xBE32, 0x8F1D, 0xC3A9, 0x8C4A, 0xC946,
  0x89BE, 0xCF04, 0x877B, 0xD4E1, 0x8583, 0xDAD8, 0x83D6, 0xE0E6,
  0x8276, 0xE707, 0x8163, 0xED38, 0x809E, 0xF374, 0x8027, 0xF9B8};


const int TwiddleCoeff_256[256] = {
  0x7FFF, 0x0000, 0x7FF6, 0xFCDC, 0x7FD9, 0xF9B8, 0x7FA7, 0xF695,
  0x7F62, 0xF374, 0x7F0A, 0xF055, 0x7E9D, 0xED38, 0x7E1E, 0xEA1E,
  0x7D8A, 0xE707, 0x7CE4, 0xE3F4, 0x7C2A, 0xE0E6, 0x7B5D, 0xDDDC,
  0x7A7D, 0xDAD8, 0x798A, 0xD7D9, 0x7885, 0xD4E1, 0x776C, 0xD1EF,
  0x7642, 0xCF04, 0x7505, 0xCC21, 0x73B6, 0xC946, 0x7255, 0xC673,
  0x70E3, 0xC3A9, 0x6F5F, 0xC0E9, 0x6DCA, 0xBE32, 0x6C24, 0xBB85,
  0x6A6E, 0xB8E3, 0x68A7, 0xB64C, 0x66D0, 0xB3C0, 0x64E9, 0xB140,
  0x62F2, 0xAECC, 0x60EC, 0xAC65, 0x5ED7, 0xAA0A, 0x5CB4, 0xA7BD,
  0x5A82, 0xA57E, 0x5843, 0xA34C, 0x55F6, 0xA129, 0x539B, 0x9F14,
  0x5134, 0x9D0E, 0x4EC0, 0x9B17, 0x4C40, 0x9930, 0x49B4, 0x9759,
  0x471D, 0x9592, 0x447B, 0x93DC, 0x41CE, 0x9236, 0x3F17, 0x90A1,
  0x3C57, 0x8F1D, 0x398D, 0x8DAB, 0x36BA, 0x8C4A, 0x33DF, 0x8AFB,
  0x30FC, 0x89BE, 0x2E11, 0x8894, 0x2B1F, 0x877B, 0x2827, 0x8676,
  0x2528, 0x8583, 0x2224, 0x84A3, 0x1F1A, 0x83D6, 0x1C0C, 0x831C,
  0x18F9, 0x8276, 0x15E2, 0x81E2, 0x12C8, 0x8163, 0x0FAB, 0x80F6,
  0x0C8C, 0x809E, 0x096B, 0x8059, 0x0648, 0x8027, 0x0324, 0x800A,
  0x0000, 0x8000, 0xFCDC, 0x800A, 0xF9B8, 0x8027, 0xF695, 0x8059,
  0xF374, 0x809E, 0xF055, 0x80F6, 0xED38, 0x8163, 0xEA1E, 0x81E2,
  0xE707, 0x8276, 0xE3F4, 0x831C, 0xE0E6, 0x83D6, 0xDDDC, 0x84A3,
  0xDAD8, 0x8583, 0xD7D9, 0x8676, 0xD4E1, 0x877B, 0xD1EF, 0x8894,
  0xCF04, 0x89BE, 0xCC21, 0x8AFB, 0xC946, 0x8C4A, 0xC673, 0x8DAB,
  0xC3A9, 0x8F1D, 0xC0E9, 0x90A1, 0xBE32, 0x9236, 0xBB85, 0x93DC,
  0xB8E3, 0x9592, 0xB64C, 0x9759, 0xB3C0, 0x9930, 0xB140, 0x9B17,
  0xAECC, 0x9D0E, 0xAC65, 0x9F14, 0xAA0A, 0xA129, 0xA7BD, 0xA34C,
  0xA57E, 0xA57E, 0xA34C, 0xA7BD, 0xA129, 0xAA0A, 0x9F14, 0xAC65,
  0x9D0E, 0xAECC, 0x9B17, 0xB140, 0x9930, 0xB3C0, 0x9759, 0xB64C,
  0x9592, 0xB8E3, 0x93DC, 0xBB85, 0x9236, 0xBE32, 0x90A1, 0xC0E9,
  0x8F1D, 0xC3A9, 0x8DAB, 0xC673, 0x8C4A, 0xC946, 0x8AFB, 0xCC21,
  0x89BE, 0xCF04, 0x8894, 0xD1EF, 0x877B, 0xD4E1, 0x8676, 0xD7D9,
  0x8583, 0xDAD8, 0x84A3, 0xDDDC, 0x83D6, 0xE0E6, 0x831C, 0xE3F4,
  0x8276, 0xE707, 0x81E2, 0xEA1E, 0x8163, 0xED38, 0x80F6, 0xF055,
  0x809E, 0xF374, 0x8059, 0xF695, 0x8027, 0xF9B8, 0x800A, 0xFCDC};


const int TwiddleCoeff_512[512] = {
  0x7FFF, 0x0000, 0x7FFE, 0xFE6E, 0x7FF6, 0xFCDC, 0x7FEA, 0xFB4A,
  0x7FD9, 0xF9B8, 0x7FC2, 0xF827, 0x7FA7, 0xF695, 0x7F87, 0xF505,
  0x7F62, 0xF374, 0x7F38, 0xF1E4, 0x7F0A, 0xF055, 0x7ED6, 0xEEC6,
  0x7E9D, 0xED38, 0x7E60, 0xEBAB, 0x7E1E, 0xEA1E, 0x7DD6, 0xE892,
  0x7D8A, 0xE707, 0x7D3A, 0xE57D, 0x7CE4, 0xE3F4, 0x7C89, 0xE26D,
  0x7C2A, 0xE0E6, 0x7BC6, 0xDF61, 0x7B5D, 0xDDDC, 0x7AEF, 0xDC59,
  0x7A7D, 0xDAD8, 0x7A06, 0xD958, 0x798A, 0xD7D9, 0x790A, 0xD65C,
  0x7885, 0xD4E1, 0x77FB, 0xD367, 0x776C, 0xD1EF, 0x76D9, 0xD079,
  0x7642, 0xCF04, 0x75A6, 0xCD92, 0x7505, 0xCC21, 0x7460, 0xCAB2,
  0x73B6, 0xC946, 0x7308, 0xC7DB, 0x7255, 0xC673, 0x719E, 0xC50D,
  0x70E3, 0xC3A9, 0x7023, 0xC248, 0x6F5F, 0xC0E9, 0x6E97, 0xBF8C,
  0x6DCA, 0xBE32, 0x6CF9, 0xBCDA, 0x6C24, 0xBB85, 0x6B4B, 0xBA33,
  0x6A6E, 0xB8E3, 0x698C, 0xB796, 0x68A7, 0xB64C, 0x67BD, 0xB505,
  0x66D0, 0xB3C0, 0x65DE, 0xB27F, 0x64E9, 0xB140, 0x63EF, 0xB005,
  0x62F2, 0xAECC, 0x61F1, 0xAD97, 0x60EC, 0xAC65, 0x5FE4, 0xAB36,
  0x5ED7, 0xAA0A, 0x5DC8, 0xA8E2, 0x5CB4, 0xA7BD, 0x5B9D, 0xA69C,
  0x5A82, 0xA57E, 0x5964, 0xA463, 0x5843, 0xA34C, 0x571E, 0xA238,
  0x55F6, 0xA129, 0x54CA, 0xA01C, 0x539B, 0x9F14, 0x5269, 0x9E0F,
  0x5134, 0x9D0E, 0x4FFB, 0x9C11, 0x4EC0, 0x9B17, 0x4D81, 0x9A22,
  0x4C40, 0x9930, 0x4AFB, 0x9843, 0x49B4, 0x9759, 0x486A, 0x9674,
  0x471D, 0x9592, 0x45CD, 0x94B5, 0x447B, 0x93DC, 0x4326, 0x9307,
  0x41CE, 0x9236, 0x4074, 0x9169, 0x3F17, 0x90A1, 0x3DB8, 0x8FDD,
  0x3C57, 0x8F1D, 0x3AF3, 0x8E62, 0x398D, 0x8DAB, 0x3825, 0x8CF8,
  0x36BA, 0x8C4A, 0x354E, 0x8BA0, 0x33DF, 0x8AFB, 0x326E, 0x8A5A,
  0x30FC, 0x89BE, 0x2F87, 0x8927, 0x2E11, 0x8894, 0x2C99, 0x8805,
  0x2B1F, 0x877B, 0x29A4, 0x86F6, 0x2827, 0x8676, 0x26A8, 0x85FA,
  0x2528, 0x8583, 0x23A7, 0x8511, 0x2224, 0x84A3, 0x209F, 0x843A,
  0x1F1A, 0x83D6, 0x1D93, 0x8377, 0x1C0C, 0x831C, 0x1A83, 0x82C6,
  0x18F9, 0x8276, 0x176E, 0x822A, 0x15E2, 0x81E2, 0x1455, 0x81A0,
  0x12C8, 0x8163, 0x113A, 0x812A, 0x0FAB, 0x80F6, 0x0E1C, 0x80C8,
  0x0C8C, 0x809E, 0x0AFB, 0x8079, 0x096B, 0x8059, 0x07D9, 0x803E,
  0x0648, 0x8027, 0x04B6, 0x8016, 0x0324, 0x800A, 0x0192, 0x8002,
  0x0000, 0x8000, 0xFE6E, 0x8002, 0xFCDC, 0x800A, 0xFB4A, 0x8016,
  0xF9B8, 0x8027, 0xF827, 0x803E, 0xF695, 0x8059, 0xF505, 0x8079,
  0xF374, 0x809E, 0xF1E4, 0x80C8, 0xF055, 0x80F6, 0xEEC6, 0x812A,
  0xED38, 0x8163, 0xEBAB, 0x81A0, 0xEA1E, 0x81E2, 0xE892, 0x822A,
  0xE707, 0x8276, 0xE57D, 0x82C6, 0xE3F4, 0x831C, 0xE26D, 0x8377,
  0xE0E6, 0x83D6, 0xDF61, 0x843A, 0xDDDC, 0x84A3, 0xDC59, 0x8511,
  0xDAD8, 0x8583, 0xD958, 0x85FA, 0xD7D9, 0x8676, 0xD65C, 0x86F6,
  0xD4E1, 0x877B, 0xD367, 0x8805, 0xD1EF, 0x8894, 0xD079, 0x8927,
  0xCF04, 0x89BE, 0xCD92, 0x8A5A, 0xCC21, 0x8AFB, 0xCAB2, 0x8BA0,
  0xC946, 0x8C4A, 0xC7DB, 0x8CF8, 0xC673, 0x8DAB, 0xC50D, 0x8E62,
  0xC3A9, 0x8F1D, 0xC248, 0x8FDD, 0xC0E9, 0x90A1, 0xBF8C, 0x9169,
  0xBE32, 0x9236, 0xBCDA, 0x9307, 0xBB85, 0x93DC, 0xBA33, 0x94B5,
  0xB8E3, 0x9592, 0xB796, 0x9674, 0xB64C, 0x9759, 0xB505, 0x9843,
  0xB3C0, 0x9930, 0xB27F, 0x9A22, 0xB140, 0x9B17, 0xB005, 0x9C11,
  0xAECC, 0x9D0E, 0xAD97, 0x9E0F, 0xAC65, 0x9F14, 0xAB36, 0xA01C,
  0xAA0A, 0xA129, 0xA8E2, 0xA238, 0xA7BD, 0xA34C, 0xA69C, 0xA463,
  0xA57E, 0xA57E, 0xA463, 0xA69C, 0xA34C, 0xA7BD, 0xA238, 0xA8E2,
  0xA129, 0xAA0A, 0xA01C, 0xAB36, 0x9F14, 0xAC65, 0x9E0F, 0xAD97,
  0x9D0E, 0xAECC, 0x9C11, 0xB005, 0x9B17, 0xB140, 0x9A22, 0xB27F,
  0x9930, 0xB3C0, 0x9843, 0xB505, 0x9759, 0xB64C, 0x9674, 0xB796,
  0x9592, 0xB8E3, 0x94B5, 0xBA33, 0x93DC, 0xBB85, 0x9307, 0xBCDA,
  0x9236, 0xBE32, 0x9169, 0xBF8C, 0x90A1, 0xC0E9, 0x8FDD, 0xC248,
  0x8F1D, 0xC3A9, 0x8E62, 0xC50D, 0x8DAB, 0xC673, 0x8CF8, 0xC7DB,
  0x8C4A, 0xC946, 0x8BA0, 0xCAB2, 0x8AFB, 0xCC21, 0x8A5A, 0xCD92,
  0x89BE, 0xCF04, 0x8927, 0xD079, 0x8894, 0xD1EF, 0x8805, 0xD367,
  0x877B, 0xD4E1, 0x86F6, 0xD65C, 0x8676, 0xD7D9, 0x85FA, 0xD958,
  0x8583, 0xDAD8, 0x8511, 0xDC59, 0x84A3, 0xDDDC, 0x843A, 0xDF61,
  0x83D6, 0xE0E6, 0x8377, 0xE26D, 0x831C, 0xE3F4, 0x82C6, 0xE57D,
  0x8276, 0xE707, 0x822A, 0xE892, 0x81E2, 0xEA1E, 0x81A0, 0xEBAB,
  0x8163, 0xED38, 0x812A, 0xEEC6, 0x80F6, 0xF055, 0x80C8, 0xF1E4,
  0x809E, 0xF374, 0x8079, 0xF505, 0x8059, 0xF695, 0x803E, 0xF827,
  0x8027, 0xF9B8, 0x8016, 0xFB4A, 0x800A, 0xFCDC, 0x8002, 0xFE6E};
Copyright (c) 2002-2012 mikroElektronika. All rights reserved.
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