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Hard-code K=8 FFT twiddle constants for now.

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So low on baseband RAM! This eliminates the sin_f32() function and table, which is only used in the FFT, for the moment.
This commit is contained in:
Jared Boone 2016-01-03 20:08:26 -08:00
parent 65dd385c94
commit 54e59cc224
1 changed files with 14 additions and 7 deletions
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21
firmware/common/dsp_fft.hpp
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@ -31,7 +31,6 @@
#include "dsp_types.hpp"
#include "complex.hpp"
#include "sine_table.hpp"
#include "hal.h"
namespace std {
@ -110,16 +109,24 @@ void fft_swap_in_place(std::array<T, N>& data) {
template<typename T, size_t N>
void fft_c_preswapped(std::array<T, N>& data) {
static_assert(power_of_two(N), "only defined for N == power of two");
constexpr auto K = log_2(N);
static_assert(N == 256, "lame that I've hard-coded a table for K=8");
static constexpr std::array<std::complex<float>, K> wp_table { {
{ -2.00000000000000000000000000000000f, 0.00000000000000000000000000000000f },
{ -1.00000000000000000000000000000000f, -1.00000000000000000000000000000000f },
{ -0.29289321881345242726268907063059f, -0.70710678118654746171500846685376f },
{ -0.07612046748871323376128827931097f, -0.38268343236508978177923268049199f },
{ -0.01921471959676954860407604996908f, -0.19509032201612824808378832130984f },
{ -0.00481527332780311376897453001789f, -0.09801714032956060362877792613290f },
{ -0.00120454379482760713659939000308f, -0.04906767432741801493456534899451f },
{ -0.00030118130379577984830768988544f, -0.02454122852291228812360301958506f },
} };
/* Provide data to this function, pre-swapped. */
for(size_t k = 0; k < log_2(N); k++) {
const size_t mmax = 1 << k;
const float theta = -pi / mmax;
const float wtemp = sin_f32(0.5f * theta);
const T wp {
-2.0f * wtemp * wtemp,
sin_f32(theta)
};
const auto wp = wp_table[k];
T w { 1.0f, 0.0f };
for(size_t m = 0; m < mmax; ++m) {
for(size_t i = m; i < N; i += mmax * 2) {