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Math: Add functions for A-law encode and decode
This patch adds to math library functions sofm_a_law_encode() and sofm_a_law_decode(). The main usage is support for 8-bit A-law encoded wav files support. Signed-off-by: Seppo Ingalsuo <seppo.ingalsuo@linux.intel.com>
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‎src/include/sof/math/a_law.h‎

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/* SPDX-License-Identifier: BSD-3-Clause
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*
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* Copyright(c) 2025 Intel Corporation.
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*/
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#ifndef __SOF_MATH_A_LAW_H__
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#define __SOF_MATH_A_LAW_H__
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#include <stdint.h>
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/**
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* sofm_a_law_encode() - Encode sample with A-law coding
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* @param sample: A s16 sample value
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*
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* The A-law codec is defined in ITU-T G.701 standard and has been used
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* in telecommunications in e.g. Europe. In SOF the main purpose is
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* support for wav format. The A-law coding compresses 13 bit samples
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* to 8 bit data stream. In SOF the high 13 bits of s16 format are used
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* for compatibility with normal audios.
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*
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* @return: Compressed 8 bit code value
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*/
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uint8_t sofm_a_law_encode(int16_t sample);
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/**
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* sofm_a_law_decode() - Decode A-law encoded code word
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* @param byte: Encoded code word
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*
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* The A-law decoder expands a 8 bit code word into a 13 bit sample value.
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* In the SOF the high 13 bits are aligned to the most significant bits
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* to be compatible with normal s16 Q1.15 samples.
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*
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* @return: Sampple value in s16 format
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*/
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int16_t sofm_a_law_decode(int8_t byte);
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#endif /* __SOF_MATH_A_LAW_H__ */

‎src/math/CMakeLists.txt‎

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@@ -83,6 +83,10 @@ if(CONFIG_MATH_DCT)
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list(APPEND base_files dct.c)
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endif()
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if(CONFIG_MATH_A_LAW_CODEC)
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list(APPEND base_files a_law.c)
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endif()
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is_zephyr(it_is)
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if(it_is) ### Zephyr ###
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‎src/math/Kconfig‎

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transform for data is done as matrix multiply with the
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returned DCT matrix.
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config MATH_A_LAW_CODEC
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bool "A-law encoder and decoder"
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default n
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help
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This option enables functions sofm_a_law_encode() and
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sofm_a_law_decode(). The A-law codec is defined in
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ITU-T G.701 standard and has been used in telecommunications
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in e.g. Europe. In SOF the main purpose is support for wav
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format. The A-law coding compresses 13 bit samples to 8 bit
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coded data.
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endmenu

‎src/math/a_law.c‎

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// SPDX-License-Identifier: BSD-3-Clause
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//
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// Copyright(c) 2025 Intel Corporation.
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//
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// Author: Seppo Ingalsuo <seppo.ingalsuo@linux.intel.com>
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#include <sof/audio/format.h>
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#include <sof/math/a_law.h>
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#include <stdint.h>
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#define SOFM_ALAW_SIGN_BIT 0x80
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#define SOFM_ALAW_MAX 4095
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#define SOFM_ALAW_TOGGLE_EVEN_BITS 0x55
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#define SOFM_ALAW_MANTISSA_MASK 0x0f
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#define SOFM_ALAW_MANTISSA_BITS 4
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#define SOFM_ALAW_SHIFT_MASK 0x07
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#define SOFM_ALAW_DEC_ONES_MASK 0x21 /* 0b100001 for "1abcd1", see below */
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/*
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* A-law encode table (sign bit is b12)
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*
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* Input values 11:0 Output values 6:0
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*
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* 0 0 0 0 0 0 0 a b c d x 0 0 0 a b c d
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* 0 0 0 0 0 0 1 a b c d x 0 0 1 a b c d
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* 0 0 0 0 0 1 a b c d x x 0 1 0 a b c d
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* 0 0 0 0 1 a b c d x x x 0 1 1 a b c d
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* 0 0 0 1 a b c d x x x x 1 0 0 a b c d
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* 0 0 1 a b c d x x x x x 1 0 1 a b c d
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* 0 1 a b c d x x x x x x 1 1 0 a b c d
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* 1 a b c d x x x x x x x 1 1 1 a b c d
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*
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*
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* A-law decode table (sign bit is b7)
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*
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* Input values 6:0 Output values 11:0
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*
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* 0 0 0 a b c d 0 0 0 0 0 0 0 a b c d 1
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* 0 0 1 a b c d 0 0 0 0 0 0 1 a b c d 1
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* 0 1 0 a b c d 0 0 0 0 0 1 a b c d 1 0
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* 0 1 1 a b c d 0 0 0 0 1 a b c d 1 0 0
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* 1 0 0 a b c d 0 0 0 1 a b c d 1 0 0 0
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* 1 0 1 a b c d 0 0 1 a b c d 1 0 0 0 0
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* 1 1 0 a b c d 0 1 a b c d 1 0 0 0 0 0
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* 1 1 1 a b c d 1 a b c d 1 0 0 0 0 0 0
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*
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*/
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/* Shift values lookup table for above table for 7
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* highest sample value bits.
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*/
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static uint8_t alaw_encode_shifts[128] = {
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1, 1, 2, 2, 3, 3, 3, 3,
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4, 4, 4, 4, 4, 4, 4, 4,
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5, 5, 5, 5, 5, 5, 5, 5,
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5, 5, 5, 5, 5, 5, 5, 5,
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6, 6, 6, 6, 6, 6, 6, 6,
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6, 6, 6, 6, 6, 6, 6, 6,
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6, 6, 6, 6, 6, 6, 6, 6,
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6, 6, 6, 6, 6, 6, 6, 6,
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7, 7, 7, 7, 7, 7, 7, 7,
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7, 7, 7, 7, 7, 7, 7, 7,
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7, 7, 7, 7, 7, 7, 7, 7,
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7, 7, 7, 7, 7, 7, 7, 7,
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7, 7, 7, 7, 7, 7, 7, 7,
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7, 7, 7, 7, 7, 7, 7, 7,
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7, 7, 7, 7, 7, 7, 7, 7,
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7, 7, 7, 7, 7, 7, 7, 7,
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};
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/**
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* sofm_a_law_encode() - Encode sample with A-law coding
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* @param sample: A s16 sample value
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*
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* The A-law codec is defined in ITU-T G.701 standard and has been used
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* in telecommunications in e.g. Europe. In SOF the main purpose is
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* support for wav format. The A-law coding compresses 13 bit samples
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* to 8 bit data stream. In SOF the high 13 bits of s16 format are used
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* for compatibility with normal audios.
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*
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* @return: Compressed 8 bit code value
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*/
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uint8_t sofm_a_law_encode(int16_t sample)
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{
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int sign = SOFM_ALAW_SIGN_BIT;
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int shift = 0;
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int low_bits;
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uint8_t byte;
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/* Convert to 13 bits with shift */
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sample = sample >> 3;
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/* Negative samples are 1's complement with zero sign bit */
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if (sample < 0) {
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sign = 0;
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sample = -sample - 1;
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}
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if (sample > SOFM_ALAW_MAX)
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sample = SOFM_ALAW_MAX;
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if (sample > 31) {
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shift = alaw_encode_shifts[sample >> 5];
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low_bits = (sample >> shift) & SOFM_ALAW_MANTISSA_MASK;
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} else {
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low_bits = (sample >> 1) & SOFM_ALAW_MANTISSA_MASK;
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}
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byte = (shift << SOFM_ALAW_MANTISSA_BITS) | low_bits;
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byte = (byte | sign) ^ SOFM_ALAW_TOGGLE_EVEN_BITS;
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return byte;
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}
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/**
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* sofm_a_law_decode() - Decode A-law encoded code word
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* @param byte: Encoded code word
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*
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* The A-law decoder expands a 8 bit code word into a 13 bit sample value.
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* In the SOF the high 13 bits are aligned to the most significant bits
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* to be compatible with normal s16 Q1.15 samples.
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*
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* @return: Sampple value in s16 format
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*/
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int16_t sofm_a_law_decode(int8_t byte)
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{
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int low_bits;
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int shift;
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int sign;
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int16_t value;
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byte ^= SOFM_ALAW_TOGGLE_EVEN_BITS;
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low_bits = byte & SOFM_ALAW_MANTISSA_MASK;
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shift = (byte >> SOFM_ALAW_MANTISSA_BITS) & SOFM_ALAW_SHIFT_MASK;
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sign = byte & SOFM_ALAW_SIGN_BIT;
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if (shift > 0)
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value = (low_bits << shift) | (SOFM_ALAW_DEC_ONES_MASK << (shift - 1));
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else
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value = (low_bits << 1) | 1;
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if (!sign)
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value = -value;
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/* Shift 13 bit Q1.12 to 16 bit Q1.15 */
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return value << 3;
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}

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