2 * $Id: pa_dither.c,v 1.1 2006/06/10 21:30:55 dmazzoni Exp $
3 * Portable Audio I/O Library triangular dither generator
5 * Based on the Open Source API proposed by Ross Bencina
6 * Copyright (c) 1999-2002 Phil Burk, Ross Bencina
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33 @brief Functions for generating dither noise
37 #include "pa_dither.h"
40 #define PA_DITHER_BITS_ (15)
43 void PaUtil_InitializeTriangularDitherState( PaUtilTriangularDitherGenerator *state )
46 state->randSeed1 = 22222;
47 state->randSeed2 = 5555555;
51 signed long PaUtil_Generate16BitTriangularDither( PaUtilTriangularDitherGenerator *state )
53 signed long current, highPass;
55 /* Generate two random numbers. */
56 state->randSeed1 = (state->randSeed1 * 196314165) + 907633515;
57 state->randSeed2 = (state->randSeed2 * 196314165) + 907633515;
59 /* Generate triangular distribution about 0.
60 * Shift before adding to prevent overflow which would skew the distribution.
61 * Also shift an extra bit for the high pass filter.
63 #define DITHER_SHIFT_ ((SIZEOF_LONG*8 - PA_DITHER_BITS_) + 1)
64 current = (((signed long)state->randSeed1)>>DITHER_SHIFT_) +
65 (((signed long)state->randSeed2)>>DITHER_SHIFT_);
67 /* High pass filter to reduce audibility. */
68 highPass = current - state->previous;
69 state->previous = current;
74 /* Multiply by PA_FLOAT_DITHER_SCALE_ to get a float between -2.0 and +1.99999 */
75 #define PA_FLOAT_DITHER_SCALE_ (1.0f / ((1<<PA_DITHER_BITS_)-1))
76 static const float const_float_dither_scale_ = PA_FLOAT_DITHER_SCALE_;
78 float PaUtil_GenerateFloatTriangularDither( PaUtilTriangularDitherGenerator *state )
80 signed long current, highPass;
82 /* Generate two random numbers. */
83 state->randSeed1 = (state->randSeed1 * 196314165) + 907633515;
84 state->randSeed2 = (state->randSeed2 * 196314165) + 907633515;
86 /* Generate triangular distribution about 0.
87 * Shift before adding to prevent overflow which would skew the distribution.
88 * Also shift an extra bit for the high pass filter.
90 #define DITHER_SHIFT_ ((SIZEOF_LONG*8 - PA_DITHER_BITS_) + 1)
91 current = (((signed long)state->randSeed1)>>DITHER_SHIFT_) +
92 (((signed long)state->randSeed2)>>DITHER_SHIFT_);
94 /* High pass filter to reduce audibility. */
95 highPass = current - state->previous;
96 state->previous = current;
97 return ((float)highPass) * const_float_dither_scale_;
102 The following alternate dither algorithms (from musicdsp.org) could be
106 /*Noise shaped dither (March 2000)
109 This is a simple implementation of highpass triangular-PDF dither with
110 2nd-order noise shaping, for use when truncating floating point audio
113 The noise shaping lowers the noise floor by 11dB below 5kHz (@ 44100Hz
114 sample rate) compared to triangular-PDF dither. The code below assumes
115 input data is in the range +1 to -1 and doesn't check for overloads!
117 To save time when generating dither for multiple channels you can do
118 things like this: r3=(r1 & 0x7F)<<8; instead of calling rand() again.
122 int r1, r2; //rectangular-PDF random numbers
123 float s1, s2; //error feedback buffers
124 float s = 0.5f; //set to 0.0f for no noise shaping
125 float w = pow(2.0,bits-1); //word length (usually bits=16)
127 float d = wi / RAND_MAX; //dither amplitude (2 lsb)
128 float o = wi * 0.5f; //remove dc offset
135 r2=r1; //can make HP-TRI dither by
136 r1=rand(); //subtracting previous rand()
138 in += s * (s1 + s1 - s2); //error feedback
139 tmp = in + o + d * (float)(r1 - r2); //dc offset and dither
141 out = (int)(w * tmp); //truncate downwards
142 if(tmp<0.0f) out--; //this is faster than floor()
145 s1 = in - wi * (float)out; //error
150 paul.kellett@maxim.abel.co.uk
151 http://www.maxim.abel.co.uk
156 16-to-8-bit first-order dither
158 Type : First order error feedforward dithering code
159 References : Posted by Jon Watte
162 This is about as simple a dithering algorithm as you can implement, but it's
163 likely to sound better than just truncating to N bits.
165 Note that you might not want to carry forward the full difference for infinity.
166 It's probably likely that the worst performance hit comes from the saturation
167 conditionals, which can be avoided with appropriate instructions on many DSPs
168 and integer SIMD type instructions, or CMOV.
170 Last, if sound quality is paramount (such as when going from > 16 bits to 16
171 bits) you probably want to use a higher-order dither function found elsewhere
176 // This code will down-convert and dither a 16-bit signed short
177 // mono signal into an 8-bit unsigned char signal, using a first
178 // order forward-feeding error term dither.
180 #define uchar unsigned char
182 void dither_one_channel_16_to_8( short * input, uchar * output, int count, int * memory )
185 while( count-- > 0 ) {
188 int j = i + 32768 - 128;
193 else if( j > 65535 ) {
197 o = (uchar)((j>>8)&0xff);
199 m = ((j-32768+128)-i);