1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
|
/*
* $Id: pa_dither.c,v 1.3 2007-12-28 03:43:01 millerpuckette Exp $
* Portable Audio I/O Library triangular dither generator
*
* Based on the Open Source API proposed by Ross Bencina
* Copyright (c) 1999-2002 Phil Burk, Ross Bencina
*
* Permission is hereby granted, free of charge, to any person obtaining
* a copy of this software and associated documentation files
* (the "Software"), to deal in the Software without restriction,
* including without limitation the rights to use, copy, modify, merge,
* publish, distribute, sublicense, and/or sell copies of the Software,
* and to permit persons to whom the Software is furnished to do so,
* subject to the following conditions:
*
* The above copyright notice and this permission notice shall be
* included in all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
* IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR
* ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF
* CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
* WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/
/*
* The text above constitutes the entire PortAudio license; however,
* the PortAudio community also makes the following non-binding requests:
*
* Any person wishing to distribute modifications to the Software is
* requested to send the modifications to the original developer so that
* they can be incorporated into the canonical version. It is also
* requested that these non-binding requests be included along with the
* license above.
*/
/** @file
@ingroup common_src
@brief Functions for generating dither noise
*/
#include "pa_dither.h"
#include "pa_types.h"
#define PA_DITHER_BITS_ (15)
void PaUtil_InitializeTriangularDitherState( PaUtilTriangularDitherGenerator *state )
{
state->previous = 0;
state->randSeed1 = 22222;
state->randSeed2 = 5555555;
}
signed long PaUtil_Generate16BitTriangularDither( PaUtilTriangularDitherGenerator *state )
{
signed long current, highPass;
/* Generate two random numbers. */
state->randSeed1 = (state->randSeed1 * 196314165) + 907633515;
state->randSeed2 = (state->randSeed2 * 196314165) + 907633515;
/* Generate triangular distribution about 0.
* Shift before adding to prevent overflow which would skew the distribution.
* Also shift an extra bit for the high pass filter.
*/
#define DITHER_SHIFT_ ((SIZEOF_LONG*8 - PA_DITHER_BITS_) + 1)
current = (((signed long)state->randSeed1)>>DITHER_SHIFT_) +
(((signed long)state->randSeed2)>>DITHER_SHIFT_);
/* High pass filter to reduce audibility. */
highPass = current - state->previous;
state->previous = current;
return highPass;
}
/* Multiply by PA_FLOAT_DITHER_SCALE_ to get a float between -2.0 and +1.99999 */
#define PA_FLOAT_DITHER_SCALE_ (1.0f / ((1<<PA_DITHER_BITS_)-1))
static const float const_float_dither_scale_ = PA_FLOAT_DITHER_SCALE_;
float PaUtil_GenerateFloatTriangularDither( PaUtilTriangularDitherGenerator *state )
{
signed long current, highPass;
/* Generate two random numbers. */
state->randSeed1 = (state->randSeed1 * 196314165) + 907633515;
state->randSeed2 = (state->randSeed2 * 196314165) + 907633515;
/* Generate triangular distribution about 0.
* Shift before adding to prevent overflow which would skew the distribution.
* Also shift an extra bit for the high pass filter.
*/
#define DITHER_SHIFT_ ((SIZEOF_LONG*8 - PA_DITHER_BITS_) + 1)
current = (((signed long)state->randSeed1)>>DITHER_SHIFT_) +
(((signed long)state->randSeed2)>>DITHER_SHIFT_);
/* High pass filter to reduce audibility. */
highPass = current - state->previous;
state->previous = current;
return ((float)highPass) * const_float_dither_scale_;
}
/*
The following alternate dither algorithms (from musicdsp.org) could be
considered
*/
/*Noise shaped dither (March 2000)
-------------------
This is a simple implementation of highpass triangular-PDF dither with
2nd-order noise shaping, for use when truncating floating point audio
data to fixed point.
The noise shaping lowers the noise floor by 11dB below 5kHz (@ 44100Hz
sample rate) compared to triangular-PDF dither. The code below assumes
input data is in the range +1 to -1 and doesn't check for overloads!
To save time when generating dither for multiple channels you can do
things like this: r3=(r1 & 0x7F)<<8; instead of calling rand() again.
int r1, r2; //rectangular-PDF random numbers
float s1, s2; //error feedback buffers
float s = 0.5f; //set to 0.0f for no noise shaping
float w = pow(2.0,bits-1); //word length (usually bits=16)
float wi= 1.0f/w;
float d = wi / RAND_MAX; //dither amplitude (2 lsb)
float o = wi * 0.5f; //remove dc offset
float in, tmp;
int out;
//for each sample...
r2=r1; //can make HP-TRI dither by
r1=rand(); //subtracting previous rand()
in += s * (s1 + s1 - s2); //error feedback
tmp = in + o + d * (float)(r1 - r2); //dc offset and dither
out = (int)(w * tmp); //truncate downwards
if(tmp<0.0f) out--; //this is faster than floor()
s2 = s1;
s1 = in - wi * (float)out; //error
--
paul.kellett@maxim.abel.co.uk
http://www.maxim.abel.co.uk
*/
/*
16-to-8-bit first-order dither
Type : First order error feedforward dithering code
References : Posted by Jon Watte
Notes :
This is about as simple a dithering algorithm as you can implement, but it's
likely to sound better than just truncating to N bits.
Note that you might not want to carry forward the full difference for infinity.
It's probably likely that the worst performance hit comes from the saturation
conditionals, which can be avoided with appropriate instructions on many DSPs
and integer SIMD type instructions, or CMOV.
Last, if sound quality is paramount (such as when going from > 16 bits to 16
bits) you probably want to use a higher-order dither function found elsewhere
on this site.
Code :
// This code will down-convert and dither a 16-bit signed short
// mono signal into an 8-bit unsigned char signal, using a first
// order forward-feeding error term dither.
#define uchar unsigned char
void dither_one_channel_16_to_8( short * input, uchar * output, int count, int * memory )
{
int m = *memory;
while( count-- > 0 ) {
int i = *input++;
i += m;
int j = i + 32768 - 128;
uchar o;
if( j < 0 ) {
o = 0;
}
else if( j > 65535 ) {
o = 255;
}
else {
o = (uchar)((j>>8)&0xff);
}
m = ((j-32768+128)-i);
*output++ = o;
}
*memory = m;
}
*/
|
