/usr/include/spandsp/biquad.h is in libspandsp-dev 0.0.6-2+b2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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* SpanDSP - a series of DSP components for telephony
*
* biquad.h - General telephony bi-quad section routines (currently this just
* handles canonic/type 2 form)
*
* Written by Steve Underwood <steveu@coppice.org>
*
* Copyright (C) 2001 Steve Underwood
*
* All rights reserved.
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU Lesser General Public License version 2.1,
* as published by the Free Software Foundation.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this program; if not, write to the Free Software
* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*/
/*! \page biquad_page Bi-quadratic filter sections
\section biquad_page_sec_1 What does it do?
???.
\section biquad_page_sec_2 How does it work?
???.
*/
#if !defined(_SPANDSP_BIQUAD_H_)
#define _SPANDSP_BIQUAD_H_
typedef struct
{
int32_t gain;
int32_t a1;
int32_t a2;
int32_t b1;
int32_t b2;
int32_t z1;
int32_t z2;
#if FIRST_ORDER_NOISE_SHAPING
int32_t residue;
#elif SECOND_ORDER_NOISE_SHAPING
int32_t residue1;
int32_t residue2;
#endif
} biquad2_state_t;
#if defined(__cplusplus)
extern "C"
{
#endif
static __inline__ void biquad2_init(biquad2_state_t *bq,
int32_t gain,
int32_t a1,
int32_t a2,
int32_t b1,
int32_t b2)
{
bq->gain = gain;
bq->a1 = a1;
bq->a2 = a2;
bq->b1 = b1;
bq->b2 = b2;
bq->z1 = 0;
bq->z2 = 0;
#if FIRST_ORDER_NOISE_SHAPING
bq->residue = 0;
#elif SECOND_ORDER_NOISE_SHAPING
bq->residue1 = 0;
bq->residue2 = 0;
#endif
}
/*- End of function --------------------------------------------------------*/
static __inline__ int16_t biquad2(biquad2_state_t *bq, int16_t sample)
{
int32_t y;
int32_t z0;
z0 = sample*bq->gain + bq->z1*bq->a1 + bq->z2*bq->a2;
y = z0 + bq->z1*bq->b1 + bq->z2*bq->b2;
bq->z2 = bq->z1;
bq->z1 = z0 >> 15;
#if FIRST_ORDER_NOISE_SHAPING
y += bq->residue;
bq->residue = y & 0x7FFF;
#elif SECOND_ORDER_NOISE_SHAPING
y += (2*bq->residue1 - bq->residue2);
bq->residue2 = bq->residue1;
bq->residue1 = y & 0x7FFF;
#endif
y >>= 15;
return (int16_t) y;
}
/*- End of function --------------------------------------------------------*/
#if defined(__cplusplus)
}
#endif
#endif
/*- End of file ------------------------------------------------------------*/
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