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 "Mixer.h"
21 #include "LVM_Mixer_FilterCoeffs.h"
22
23
24 /************************************************************************/
25 /* FUNCTION: */
26 /* LVM_Mix_GetTimeConstant */
27 /* */
28 /* DESCRIPTION: */
29 /* This function calculates the filter coefficient using the following */
30 /* equation: */
31 /* Alpha = exp(ln(0.1)/ (tc * Update + 1.0)) */
32 /* */
33 /* This is to be used with the follow first order filter, called at a */
34 /* rate of Update times a second. tc is the required time constant in */
35 /* units of 100us. */
36 /* */
37 /* Output(n) = Alpha * Output(n-1) + (1 - Alpha) * Target(n) */
38 /* */
39 /* The function assumes the block size is large, i.e. the update rate */
40 /* is approximately a fixed, and correct factor of the value of Fs */
41 /* given in the call. This is especially not true when the block size */
42 /* is very small, see the mixer documentation for further details. */
43 /* */
44 /* The function does not support all possible combinations of input */
45 /* values: */
46 /* */
47 /* 1. NumChannels is limited to the values 1 (Mono) and 2 (Stereo) */
48 /* 2. The product tc * Fs is limited approximately to the range */
49 /* 8 < (tc * Fs) < 2^35 */
50 /* */
51 /* PARAMETERS: */
52 /* tc - the time constant in 100us steps, i.e. 10 = 1ms */
53 /* Fs - the filter update rate in Hz */
54 /* NumChannels - Number of channels 1=Mono, 2=Stereo */
55 /* */
56 /* RETURNS: */
57 /* Alpha - the filter coefficient Q31 format */
58 /* */
59 /************************************************************************/
60
LVM_Mixer_TimeConstant(LVM_UINT32 tc,LVM_UINT16 Fs,LVM_UINT16 NumChannels)61 LVM_UINT32 LVM_Mixer_TimeConstant(LVM_UINT32 tc,
62 LVM_UINT16 Fs,
63 LVM_UINT16 NumChannels)
64 {
65
66 LVM_UINT32 Product;
67 LVM_INT16 Interpolate;
68 LVM_UINT16 Shift;
69 LVM_INT32 Diff;
70 LVM_UINT32 Table[] = {ALPHA_0, /* Log spaced look-up table */
71 ALPHA_1,
72 ALPHA_2,
73 ALPHA_3,
74 ALPHA_4,
75 ALPHA_5,
76 ALPHA_6,
77 ALPHA_7,
78 ALPHA_8,
79 ALPHA_9,
80 ALPHA_10,
81 ALPHA_11,
82 ALPHA_12,
83 ALPHA_13,
84 ALPHA_14,
85 ALPHA_15,
86 ALPHA_16,
87 ALPHA_17,
88 ALPHA_18,
89 ALPHA_19,
90 ALPHA_20,
91 ALPHA_21,
92 ALPHA_22,
93 ALPHA_23,
94 ALPHA_24,
95 ALPHA_25,
96 ALPHA_26,
97 ALPHA_27,
98 ALPHA_28,
99 ALPHA_29,
100 ALPHA_30,
101 ALPHA_31,
102 ALPHA_32,
103 ALPHA_33,
104 ALPHA_34,
105 ALPHA_35,
106 ALPHA_36,
107 ALPHA_37,
108 ALPHA_38,
109 ALPHA_39,
110 ALPHA_40,
111 ALPHA_41,
112 ALPHA_42,
113 ALPHA_43,
114 ALPHA_44,
115 ALPHA_45,
116 ALPHA_46,
117 ALPHA_47,
118 ALPHA_48,
119 ALPHA_49,
120 ALPHA_50};
121
122
123 /* Calculate the product of the time constant and the sample rate */
124 Product = ((tc >> 16) * (LVM_UINT32)Fs) << 13; /* Stereo value */
125 Product = Product + (((tc & 0x0000FFFF) * (LVM_UINT32)Fs) >> 3);
126
127 if (NumChannels == 1)
128 {
129 Product = Product >> 1; /* Mono value */
130 }
131
132 /* Normalize to get the table index and interpolation factor */
133 for (Shift=0; Shift<((Alpha_TableSize-1)/2); Shift++)
134 {
135 if ((Product & 0x80000000)!=0)
136 {
137 break;
138 }
139
140 Product = Product << 1;
141 }
142 Shift = (LVM_UINT16)((Shift << 1));
143
144 if ((Product & 0x40000000)==0)
145 {
146 Shift++;
147 }
148
149 Interpolate = (LVM_INT16)((Product >> 15) & 0x00007FFF);
150
151 Diff = (LVM_INT32)(Table[Shift] - Table[Shift+1]);
152 MUL32x16INTO32(Diff,Interpolate,Diff,15)
153 Product = Table[Shift+1] + (LVM_UINT32)Diff;
154
155 return Product;
156 }
157