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55 lines
1.7 KiB
55 lines
1.7 KiB
/*
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* Copyright 2012 Google Inc.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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// Resolve frequency signal (1.0 in Q24 format = 1 octave) to phase delta.
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// The LUT is just a global, and we'll need the init function to be called before
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// use.
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#include <stdint.h>
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#include <math.h>
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#include "freqlut.h"
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#define LG_N_SAMPLES 10
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#define N_SAMPLES (1 << LG_N_SAMPLES)
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#define SAMPLE_SHIFT (24 - LG_N_SAMPLES)
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#define MAX_LOGFREQ_INT 20
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int32_t lut[N_SAMPLES + 1];
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void Freqlut::init(FRAC_NUM sample_rate) {
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FRAC_NUM y = (1LL << (24 + MAX_LOGFREQ_INT)) / sample_rate;
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FRAC_NUM inc = pow(2, 1.0 / N_SAMPLES);
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for (int i = 0; i < N_SAMPLES + 1; i++) {
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lut[i] = (int32_t)floor(y + 0.5);
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y *= inc;
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}
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}
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// Note: if logfreq is more than 20.0, the results will be inaccurate. However,
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// that will be many times the Nyquist rate.
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int32_t Freqlut::lookup(int32_t logfreq) {
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int ix = (logfreq & 0xffffff) >> SAMPLE_SHIFT;
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int32_t y0 = lut[ix];
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int32_t y1 = lut[ix + 1];
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int lowbits = logfreq & ((1 << SAMPLE_SHIFT) - 1);
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int32_t y = y0 + ((((int64_t)(y1 - y0) * (int64_t)lowbits)) >> SAMPLE_SHIFT);
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int hibits = logfreq >> 24;
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return y >> (MAX_LOGFREQ_INT - hibits);
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}
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