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#ifndef PA_DITHER_H
#define PA_DITHER_H
/*
* $Id: pa_dither.h,v 1.1.2.2 2002/06/05 22:37:03 rossb 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.
*
* 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.
*
* 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.
*/
#ifdef __cplusplus
extern "C"
{
#endif /* __cplusplus */
typedef struct PaUtilTriangularDitherGenerator{
unsigned long previous;
unsigned long randSeed1;
unsigned long randSeed2;
} PaUtilTriangularDitherGenerator;
/**< State needed to generate a dither signal */
void PaUtil_InitializeTriangularDitherState( PaUtilTriangularDitherGenerator *ditherState );
/**< Initialize dither state */
signed long PaUtil_Generate16BitTriangularDither( PaUtilTriangularDitherGenerator *ditherState );
/**<
Calculate 2 LSB dither signal with a triangular distribution.
Ranged for adding to a 1 bit right-shifted 32 bit integer
prior to >>15. eg:
<pre>
signed long in = *
signed long dither = PaUtil_Generate16BitTriangularDither( ditherState );
signed short out = (signed short)(((in>>1) + dither) >> 15);
</pre>
@return
A signed long with a range of +32767 to -32768
*/
float PaUtil_GenerateFloatTriangularDither( PaUtilTriangularDitherGenerator *ditherState );
/**<
Calculate 2 LSB dither signal with a triangular distribution.
Ranged for adding to a pre-scaled float.
<pre>
float in = *
float dither = PaUtil_GenerateFloatTriangularDither( ditherState );
// use smaller scaler to prevent overflow when we add the dither
signed short out = (signed short)(in*(32766.0f) + dither );
</pre>
@return
A float with a range of -2.0 to +1.99999.
*/
/*
The following alternate dither algorithms are known...
*/
/*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;
}
*/
#ifdef __cplusplus
}
#endif /* __cplusplus */
#endif /* PA_DITHER_H */
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