328 lines
10 KiB
C
328 lines
10 KiB
C
/*
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* SpanDSP - a series of DSP components for telephony
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*
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* g711.h - In line A-law and u-law conversion routines
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*
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* Written by Steve Underwood <steveu@coppice.org>
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*
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* Copyright (C) 2001 Steve Underwood
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*
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* All rights reserved.
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU Lesser General Public License version 2.1,
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* as published by the Free Software Foundation.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU Lesser General Public License for more details.
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*
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* You should have received a copy of the GNU Lesser General Public
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* License along with this program; if not, write to the Free Software
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* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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*
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* $Id: g711.h,v 1.19 2009/04/12 09:12:10 steveu Exp $
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*/
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/*! \file */
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/*! \page g711_page A-law and mu-law handling
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Lookup tables for A-law and u-law look attractive, until you consider the impact
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on the CPU cache. If it causes a substantial area of your processor cache to get
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hit too often, cache sloshing will severely slow things down. The main reason
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these routines are slow in C, is the lack of direct access to the CPU's "find
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the first 1" instruction. A little in-line assembler fixes that, and the
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conversion routines can be faster than lookup tables, in most real world usage.
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A "find the first 1" instruction is available on most modern CPUs, and is a
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much underused feature.
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If an assembly language method of bit searching is not available, these routines
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revert to a method that can be a little slow, so the cache thrashing might not
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seem so bad :(
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Feel free to submit patches to add fast "find the first 1" support for your own
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favourite processor.
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Look up tables are used for transcoding between A-law and u-law, since it is
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difficult to achieve the precise transcoding procedure laid down in the G.711
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specification by other means.
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*/
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#if !defined(_SPANDSP_G711_H_)
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#define _SPANDSP_G711_H_
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/* The usual values to use on idle channels, to emulate silence */
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/*! Idle value for A-law channels */
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#define G711_ALAW_IDLE_OCTET 0x5D
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/*! Idle value for u-law channels */
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#define G711_ULAW_IDLE_OCTET 0xFF
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enum
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{
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G711_ALAW = 0,
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G711_ULAW
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};
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/*!
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G.711 state
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*/
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typedef struct g711_state_s g711_state_t;
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#if defined(__cplusplus)
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extern "C"
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{
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#endif
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/* N.B. It is tempting to use look-up tables for A-law and u-law conversion.
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* However, you should consider the cache footprint.
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*
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* A 64K byte table for linear to x-law and a 512 byte table for x-law to
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* linear sound like peanuts these days, and shouldn't an array lookup be
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* real fast? No! When the cache sloshes as badly as this one will, a tight
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* calculation may be better. The messiest part is normally finding the
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* segment, but a little inline assembly can fix that on an i386, x86_64 and
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* many other modern processors.
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*/
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/*
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* Mu-law is basically as follows:
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*
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* Biased Linear Input Code Compressed Code
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* ------------------------ ---------------
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* 00000001wxyza 000wxyz
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* 0000001wxyzab 001wxyz
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* 000001wxyzabc 010wxyz
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* 00001wxyzabcd 011wxyz
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* 0001wxyzabcde 100wxyz
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* 001wxyzabcdef 101wxyz
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* 01wxyzabcdefg 110wxyz
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* 1wxyzabcdefgh 111wxyz
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*
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* Each biased linear code has a leading 1 which identifies the segment
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* number. The value of the segment number is equal to 7 minus the number
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* of leading 0's. The quantization interval is directly available as the
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* four bits wxyz. * The trailing bits (a - h) are ignored.
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*
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* Ordinarily the complement of the resulting code word is used for
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* transmission, and so the code word is complemented before it is returned.
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*
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* For further information see John C. Bellamy's Digital Telephony, 1982,
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* John Wiley & Sons, pps 98-111 and 472-476.
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*/
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/* Enable the trap as per the MIL-STD */
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//#define ULAW_ZEROTRAP
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/*! Bias for u-law encoding from linear. */
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#define ULAW_BIAS 0x84
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/*! \brief Encode a linear sample to u-law
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\param linear The sample to encode.
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\return The u-law value.
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*/
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static __inline__ uint8_t linear_to_ulaw(int linear)
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{
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uint8_t u_val;
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int mask;
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int seg;
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/* Get the sign and the magnitude of the value. */
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if (linear >= 0)
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{
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linear = ULAW_BIAS + linear;
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mask = 0xFF;
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}
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else
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{
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linear = ULAW_BIAS - linear;
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mask = 0x7F;
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}
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seg = top_bit(linear | 0xFF) - 7;
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/*
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* Combine the sign, segment, quantization bits,
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* and complement the code word.
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*/
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if (seg >= 8)
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u_val = (uint8_t) (0x7F ^ mask);
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else
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u_val = (uint8_t) (((seg << 4) | ((linear >> (seg + 3)) & 0xF)) ^ mask);
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#ifdef ULAW_ZEROTRAP
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/* Optional ITU trap */
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if (u_val == 0)
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u_val = 0x02;
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#endif
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return u_val;
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}
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/*- End of function --------------------------------------------------------*/
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/*! \brief Decode an u-law sample to a linear value.
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\param ulaw The u-law sample to decode.
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\return The linear value.
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*/
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static __inline__ int16_t ulaw_to_linear(uint8_t ulaw)
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{
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int t;
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/* Complement to obtain normal u-law value. */
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ulaw = ~ulaw;
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/*
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* Extract and bias the quantization bits. Then
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* shift up by the segment number and subtract out the bias.
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*/
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t = (((ulaw & 0x0F) << 3) + ULAW_BIAS) << (((int) ulaw & 0x70) >> 4);
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return (int16_t) ((ulaw & 0x80) ? (ULAW_BIAS - t) : (t - ULAW_BIAS));
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}
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/*- End of function --------------------------------------------------------*/
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/*
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* A-law is basically as follows:
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*
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* Linear Input Code Compressed Code
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* ----------------- ---------------
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* 0000000wxyza 000wxyz
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* 0000001wxyza 001wxyz
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* 000001wxyzab 010wxyz
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* 00001wxyzabc 011wxyz
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* 0001wxyzabcd 100wxyz
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* 001wxyzabcde 101wxyz
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* 01wxyzabcdef 110wxyz
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* 1wxyzabcdefg 111wxyz
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*
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* For further information see John C. Bellamy's Digital Telephony, 1982,
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* John Wiley & Sons, pps 98-111 and 472-476.
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*/
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/*! The A-law alternate mark inversion mask */
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#define ALAW_AMI_MASK 0x55
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/*! \brief Encode a linear sample to A-law
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\param linear The sample to encode.
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\return The A-law value.
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*/
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static __inline__ uint8_t linear_to_alaw(int linear)
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{
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int mask;
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int seg;
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if (linear >= 0)
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{
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/* Sign (bit 7) bit = 1 */
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mask = ALAW_AMI_MASK | 0x80;
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}
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else
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{
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/* Sign (bit 7) bit = 0 */
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mask = ALAW_AMI_MASK;
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linear = -linear - 1;
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}
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/* Convert the scaled magnitude to segment number. */
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seg = top_bit(linear | 0xFF) - 7;
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if (seg >= 8)
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{
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if (linear >= 0)
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{
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/* Out of range. Return maximum value. */
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return (uint8_t) (0x7F ^ mask);
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}
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/* We must be just a tiny step below zero */
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return (uint8_t) (0x00 ^ mask);
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}
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/* Combine the sign, segment, and quantization bits. */
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return (uint8_t) (((seg << 4) | ((linear >> ((seg) ? (seg + 3) : 4)) & 0x0F)) ^ mask);
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}
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/*- End of function --------------------------------------------------------*/
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/*! \brief Decode an A-law sample to a linear value.
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\param alaw The A-law sample to decode.
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\return The linear value.
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*/
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static __inline__ int16_t alaw_to_linear(uint8_t alaw)
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{
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int i;
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int seg;
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alaw ^= ALAW_AMI_MASK;
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i = ((alaw & 0x0F) << 4);
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seg = (((int) alaw & 0x70) >> 4);
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if (seg)
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i = (i + 0x108) << (seg - 1);
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else
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i += 8;
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return (int16_t) ((alaw & 0x80) ? i : -i);
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}
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/*- End of function --------------------------------------------------------*/
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/*! \brief Transcode from A-law to u-law, using the procedure defined in G.711.
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\param alaw The A-law sample to transcode.
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\return The best matching u-law value.
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*/
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SPAN_DECLARE(uint8_t) alaw_to_ulaw(uint8_t alaw);
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/*! \brief Transcode from u-law to A-law, using the procedure defined in G.711.
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\param ulaw The u-law sample to transcode.
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\return The best matching A-law value.
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*/
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SPAN_DECLARE(uint8_t) ulaw_to_alaw(uint8_t ulaw);
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/*! \brief Decode from u-law or A-law to linear.
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\param s The G.711 context.
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\param amp The linear audio buffer.
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\param g711_data The G.711 data.
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\param g711_bytes The number of G.711 samples to decode.
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\return The number of samples of linear audio produced.
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*/
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SPAN_DECLARE(int) g711_decode(g711_state_t *s,
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int16_t amp[],
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const uint8_t g711_data[],
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int g711_bytes);
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/*! \brief Encode from linear to u-law or A-law.
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\param s The G.711 context.
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\param g711_data The G.711 data.
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\param amp The linear audio buffer.
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\param len The number of samples to encode.
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\return The number of G.711 samples produced.
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*/
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SPAN_DECLARE(int) g711_encode(g711_state_t *s,
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uint8_t g711_data[],
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const int16_t amp[],
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int len);
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/*! \brief Transcode between u-law and A-law.
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\param s The G.711 context.
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\param g711_out The resulting G.711 data.
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\param g711_in The original G.711 data.
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\param g711_bytes The number of G.711 samples to transcode.
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\return The number of G.711 samples produced.
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*/
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SPAN_DECLARE(int) g711_transcode(g711_state_t *s,
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uint8_t g711_out[],
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const uint8_t g711_in[],
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int g711_bytes);
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/*! Initialise a G.711 encode or decode context.
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\param s The G.711 context.
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\param mode The G.711 mode.
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\return A pointer to the G.711 context, or NULL for error. */
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SPAN_DECLARE(g711_state_t *) g711_init(g711_state_t *s, int mode);
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/*! Release a G.711 encode or decode context.
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\param s The G.711 context.
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\return 0 for OK. */
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SPAN_DECLARE(int) g711_release(g711_state_t *s);
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/*! Free a G.711 encode or decode context.
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\param s The G.711 context.
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\return 0 for OK. */
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SPAN_DECLARE(int) g711_free(g711_state_t *s);
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#if defined(__cplusplus)
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}
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#endif
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#endif
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/*- End of file ------------------------------------------------------------*/
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