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