1 /*
2 * Copyright (C) 2004-2010 NXP Software
3 * Copyright (C) 2010 The Android Open Source Project
4 *
5 * Licensed under the Apache License, Version 2.0 (the "License");
6 * you may not use this file except in compliance with the License.
7 * You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17
18 #include "LVM_Types.h"
19 #include "LVM_Macros.h"
20 #include "ScalarArithmetic.h"
21 #include "BIQUAD.h"
22 #include "Filter.h"
23
24 /*-------------------------------------------------------------------------*/
25 /* FUNCTION: */
26 /* void LVM_FO_LPF( LVM_INT32 w , */
27 /* FO_C32_Coefs_t *pCoeffs); */
28 /* */
29 /* */
30 /* DESCRIPTION: */
31 /* This function calculates the coefficient of first order low pass */
32 /* filter. It uses the equations: */
33 /* */
34 /* B1 = (tan(w/2) - 1 ) / (tan(w/2) + 1 ) */
35 /* A0 = (1 - B1) / 2 */
36 /* A1 = A0 */
37 /* */
38 /* The value of B1 is then calculated directly from the value w by a */
39 /* polynomial expansion using a 9th order polynomial. It uses the */
40 /* following table of 32-bit integer polynomial coefficients: */
41 /* */
42 /* Coefficient Value */
43 /* A0 -8388571 */
44 /* A1 33547744 */
45 /* A2 -66816791 */
46 /* A3 173375308 */
47 /* A4 -388437573 */
48 /* A5 752975383 */
49 /* A6 -1103016663 */
50 /* A7 1121848567 */
51 /* A8 -688078159 */
52 /* A9 194669577 */
53 /* A10 8 */
54 /* */
55 /* Y = (A0 + A1*X + A2*X2 + A3*X3 + �.. + AN*xN) << AN+1 */
56 /* */
57 /* */
58 /* PARAMETERS: */
59 /* */
60 /* w Sample rate in radians, where: */
61 /* w = 2 * Pi * Fc / Fs */
62 /* Fc is the corner frequency in Hz */
63 /* Fs is the sample rate in Hz */
64 /* w is in Q2.29 format and data range is [0 Pi] */
65 /* pCoeffs Points to the filter coefficients calculated here */
66 /* in Q1.30 format */
67 /* RETURNS: */
68 /* */
69 /*-------------------------------------------------------------------------*/
LVM_FO_HPF(LVM_FLOAT w,FO_FLOAT_Coefs_t * pCoeffs)70 LVM_FLOAT LVM_FO_HPF(LVM_FLOAT w, FO_FLOAT_Coefs_t* pCoeffs) {
71 LVM_FLOAT Y, Coefficients[13] = {-0.999996f, 0.999801f, -0.497824f, 0.322937f, -0.180880f,
72 0.087658f, -0.032102f, 0.008163f, -0.001252f, 0.000089f,
73 0, 0, 0};
74 Y = LVM_Polynomial((LVM_UINT16)9, Coefficients, w);
75
76 pCoeffs->B1 = -Y; /* Store -B1 in filter structure instead of B1!*/
77 /* A0=(1-B1)/2= B1/2 - 0.5*/
78 Y = Y / 2.0f; /* A0=Y=B1/2*/
79 Y = Y - 0.5f; /* A0=Y=(B1/2 - 0.5)*/
80
81 pCoeffs->A0 = Y * FILTER_LOSS_FLOAT; /* Apply loss to avoid overflow*/
82 pCoeffs->A1 = -pCoeffs->A0; /* Store A1=-A0*/
83
84 return 1;
85 }
86