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/*
* Copyright (c) 2011 The WebRTC project authors. All Rights Reserved.
*
* Use of this source code is governed by a BSD-style license
* that can be found in the LICENSE file in the root of the source
* tree. An additional intellectual property rights grant can be found
* in the file PATENTS. All contributing project authors may
* be found in the AUTHORS file in the root of the source tree.
*/
#include "common_audio/vad/vad_gmm.h"
#include "common_audio/signal_processing/include/signal_processing_library.h"
static const int32_t kCompVar = 22005;
static const int16_t kLog2Exp = 5909; // log2(exp(1)) in Q12.
// For a normal distribution, the probability of `input` is calculated and
// returned (in Q20). The formula for normal distributed probability is
//
// 1 / s * exp(-(x - m)^2 / (2 * s^2))
//
// where the parameters are given in the following Q domains:
// m = `mean` (Q7)
// s = `std` (Q7)
// x = `input` (Q4)
// in addition to the probability we output `delta` (in Q11) used when updating
// the noise/speech model.
int32_t WebRtcVad_GaussianProbability(int16_t input,
int16_t mean,
int16_t std,
int16_t* delta) {
int16_t tmp16, inv_std, inv_std2, exp_value = 0;
int32_t tmp32;
// Calculate `inv_std` = 1 / s, in Q10.
// 131072 = 1 in Q17, and (`std` >> 1) is for rounding instead of truncation.
// Q-domain: Q17 / Q7 = Q10.
tmp32 = (int32_t)131072 + (int32_t)(std >> 1);
inv_std = (int16_t)WebRtcSpl_DivW32W16(tmp32, std);
// Calculate `inv_std2` = 1 / s^2, in Q14.
tmp16 = (inv_std >> 2); // Q10 -> Q8.
// Q-domain: (Q8 * Q8) >> 2 = Q14.
inv_std2 = (int16_t)((tmp16 * tmp16) >> 2);
// TODO(bjornv): Investigate if changing to
// inv_std2 = (int16_t)((inv_std * inv_std) >> 6);
// gives better accuracy.
tmp16 = (input << 3); // Q4 -> Q7
tmp16 = tmp16 - mean; // Q7 - Q7 = Q7
// To be used later, when updating noise/speech model.
// `delta` = (x - m) / s^2, in Q11.
// Q-domain: (Q14 * Q7) >> 10 = Q11.
*delta = (int16_t)((inv_std2 * tmp16) >> 10);
// Calculate the exponent `tmp32` = (x - m)^2 / (2 * s^2), in Q10. Replacing
// division by two with one shift.
// Q-domain: (Q11 * Q7) >> 8 = Q10.
tmp32 = (*delta * tmp16) >> 9;
// If the exponent is small enough to give a non-zero probability we calculate
// `exp_value` ~= exp(-(x - m)^2 / (2 * s^2))
// ~= exp2(-log2(exp(1)) * `tmp32`).
if (tmp32 < kCompVar) {
// Calculate `tmp16` = log2(exp(1)) * `tmp32`, in Q10.
// Q-domain: (Q12 * Q10) >> 12 = Q10.
tmp16 = (int16_t)((kLog2Exp * tmp32) >> 12);
tmp16 = -tmp16;
exp_value = (0x0400 | (tmp16 & 0x03FF));
tmp16 ^= 0xFFFF;
tmp16 >>= 10;
tmp16 += 1;
// Get `exp_value` = exp(-`tmp32`) in Q10.
exp_value >>= tmp16;
}
// Calculate and return (1 / s) * exp(-(x - m)^2 / (2 * s^2)), in Q20.
// Q-domain: Q10 * Q10 = Q20.
return inv_std * exp_value;
}
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