LVM_FO_LPF.c revision 2c8e5cab3faa6d360e222b7a6c40a80083d021ac 
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/************************************************************************/
19/*                                                                      */
20/*     Project::                                                        */
21/*     $Author: nxp27078 $*/
22/*     $Revision: 762 $*/
23/*     $Date: 2010-06-11 14:50:33 +0200 (vr, 11 jun 2010) $*/
24/*                                                                      */
25/************************************************************************/
26
27#include "LVM_Types.h"
28#include "LVM_Macros.h"
29#include "ScalarArithmetic.h"
30#include "BIQUAD.h"
31#include "Filter.h"
32
33
34/*-------------------------------------------------------------------------*/
35/* FUNCTION:                                                               */
36/*   void LVM_FO_LPF(   LVM_INT32       w ,                                */
37/*                      FO_C32_Coefs_t  *pCoeffs);                         */
38/*                                                                         */
39/*                                                                         */
40/* DESCRIPTION:                                                            */
41/*    This function calculates the coefficient of first order low pass     */
42/*    filter. It uses the equations:                                       */
43/*                                                                         */
44/*    B1    = (tan(w/2) - 1 )  /  (tan(w/2) + 1 )                          */
45/*    A0    = (1 + B1) / 2                                                 */
46/*    A1    = A0                                                           */
47/*                                                                         */
48/*    The value of B1 is then calculated directly from the value w by a    */
49/*    polynomial expansion using a 9th order polynomial. It uses the       */
50/*    following table of 32-bit integer polynomial coefficients:           */
51/*                                                                         */
52/*   Coefficient    Value                                                  */
53/*   A0             -8388571                                               */
54/*   A1             33547744                                               */
55/*   A2             -66816791                                              */
56/*   A3             173375308                                              */
57/*   A4             -388437573                                             */
58/*   A5             752975383                                              */
59/*   A6             -1103016663                                            */
60/*   A7             1121848567                                             */
61/*   A8             -688078159                                             */
62/*   A9             194669577                                              */
63/*   A10            8                                                      */
64/*                                                                         */
65/*  Y = (A0 + A1*X + A2*X2 + A3*X3 + �.. + AN*xN) << AN+1                  */
66/*                                                                         */
67/*                                                                         */
68/* PARAMETERS:                                                             */
69/*                                                                         */
70/*  w               Sample rate in radians,  where:                        */
71/*                  w = 2 * Pi * Fc / Fs                                   */
72/*                  Fc   is the corner frequency in Hz                     */
73/*                  Fs   is the sample rate in Hz                          */
74/*                  w is in Q2.29 format and data range is [0 Pi]          */
75/*  pCoeffs         Points to the filter coefficients calculated here      */
76/*                  in Q1.30 format                                        */
77/* RETURNS:                                                                */
78/*                                                                         */
79/*-------------------------------------------------------------------------*/
80
81LVM_INT32 LVM_FO_LPF(   LVM_INT32       w,
82                        FO_C32_Coefs_t  *pCoeffs)
83{
84    LVM_INT32 Y,Coefficients[13]={  -8388571,
85                                    33547744,
86                                    -66816791,
87                                    173375308,
88                                    -388437573,
89                                    752975383,
90                                    -1103016663,
91                                    1121848567,
92                                    -688078159,
93                                    194669577,
94                                    8};
95    Y=LVM_Polynomial(           (LVM_UINT16)9,
96                                 Coefficients,
97                                 w);
98    pCoeffs->B1=-Y;     // Store -B1 in filter structure instead of B1!
99                        // A0=(1+B1)/2= B1/2 + 0.5
100    Y=Y>>1;             // A0=Y=B1/2
101    Y=Y+0x40000000;     // A0=Y=(B1/2 + 0.5)
102    MUL32x16INTO32(Y, FILTER_LOSS ,pCoeffs->A0 ,15)    // Apply loss to avoid overflow
103    pCoeffs->A1=pCoeffs->A0;
104    return 1;
105}
106
107