31.7.1 mips_fft16

Description

Computes the complex fast Fourier transform (FFT) of the input sequence din. The number of samples to be processed is specified by the parameter log2N: N = 2^log2N. The fftc array holds complex coefficients needed by the FFT algorithm. The scratch hold intermediate data; its contents are destroyed on each call to mips_fft16().

Mathematically,

o u t p u t [n] = \frac{1}{2^{\log 2 N}} \sum_{k = 0}^{N - 1} d i n [n] \times e^{- j \frac{2 π k n}{N}}

Include

dsplib_dsp.h

Prototype

void
mips_fft16
(
       int16c *dout,
       int16c *din,
       int16c *fftc,
       int16c *scratch,
       int log2N
);

Argument

dout: Output array with 16-bit complex fixed-point elements in Q15 format.

din: Input array with 16-bit complex fixed-point elements in Q15 format.

fftc: Input array with 16-bit complex fixed-point twiddle factors in Q15 format.

scratch: Intermediate results array holding 16-bit complex fixed-point data.

log2N: Logarithm base 2 of the number of samples: N = 2^log2N.

Return Value

None.

Remarks

The pointers dout, din, fftc, and scratch must be aligned on 4-byte boundaries.
log2N must be larger than or equal to 3.

Notes

The scratch array must be large enough to hold N 16-bit complex data samples having 16-bit real part and 16-bit imaginary part.

Copying fftc to RAM prior to calling this function can be used to improve performance.

Example

#include “fftc.h” // pre-computed coefficients
int log2N = 6; // log2(64) = 6
int N = 1 << log2N; // N = 2^6 = 64
int16c din[N];
int16c dout[N];
int16c scratch[N];
#define fftc fft16c64 // from fftc.h, for N = 64
while (true)
{
  // load complex input data into din
  ...
  mips_fft16(dout, din, fftc, scratch, log2N);
  // do something with dout
  ...
}