Math: Add functions for mu-law encode and decode

This patch adds to math library functions sofm_mu_law_encode()
and sofm_mu_law_decode(). The main usage is support for VoIP
and 8-bit mu-law encoded wav files support.

Signed-off-by: Seppo Ingalsuo <seppo.ingalsuo@linux.intel.com>

Author: singalsu

src/include/sof/math/mu_law.h

@@ -0,0 +1,36 @@
+/* SPDX-License-Identifier: BSD-3-Clause
+ *
+ * Copyright(c) 2025 Intel Corporation.
+ */
+
+#ifndef __SOF_MATH_MU_LAW_H__
+#define __SOF_MATH_MU_LAW_H__
+
+#include <stdint.h>
+
+/**
+ * sofm_mu_law_encode() - Encode sample with mu-law coding
+ * @param sample: A s16 sample value
+ *
+ * The mu-law codec is defined in ITU-T G.711 standard and has been used
+ * in telecommunications in USA and Japan. The mu-law coding compresses
+ * 14 bit samples to 8 bit data stream. In SOF the high 14 bits of s16
+ * format are used for compatibility with normal audios.
+ *
+ * @return: Compressed 8 bit code value
+ */
+uint8_t sofm_mu_law_encode(int16_t sample);
+
+/**
+ * sofm_mu_law_decode() - Decode mu-law encoded code word
+ * @param byte: Encoded code word
+ *
+ * The mu-law decoder expands a 8 bit code word into a 14 bit sample value.
+ * In the SOF the high 14 bits are aligned to the most significant bits
+ * to be compatible with normal s16 Q1.15 samples.
+ *
+ * @return: Sample value in s16 format
+ */
+int16_t sofm_mu_law_decode(int8_t byte);
+
+#endif /* __SOF_MATH_MU_LAW_H__ */

src/math/CMakeLists.txt

@@ -87,6 +87,10 @@ if(CONFIG_MATH_A_LAW_CODEC)
   list(APPEND base_files a_law.c)
 endif()
 
+if(CONFIG_MATH_MU_LAW_CODEC)
+  list(APPEND base_files mu_law.c)
+endif()
+
 is_zephyr(it_is)
 if(it_is) ###  Zephyr ###
 

src/math/Kconfig

@@ -279,4 +279,14 @@ config MATH_A_LAW_CODEC
 	  to 8 bit coded data. A-law codec can be used in VoIP and
 	  8-bit wav formats.
 
+config MATH_MU_LAW_CODEC
+	bool "mu-law encoder and decoder"
+	help
+	  This option enables functions sofm_mu_law_encode() and
+	  sofm_mu_law_decode(). The mu-law codec is defined in
+	  ITU-T G.711 standard and has been used in telecommunications
+	  in USA and Japan. The mu-law coding compresses 14 bit samples
+	  to 8 bit coded data. Mu-law codec can be used in VoIP and
+	  8-bit wav formats.
+
 endmenu

src/math/mu_law.c

@@ -0,0 +1,138 @@
+// SPDX-License-Identifier: BSD-3-Clause
+//
+// Copyright(c) 2025 Intel Corporation.
+//
+// Author: Seppo Ingalsuo <seppo.ingalsuo@linux.intel.com>
+
+#include <sof/audio/format.h>
+#include <sof/math/mu_law.h>
+#include <stdint.h>
+
+#define SOFM_MULAW_BIAS			33
+#define SOFM_MULAW_MAX			8191
+#define SOFM_MULAW_TOGGLE_BITS		0x7f
+#define SOFM_MULAW_SIGN_BIT		0x80
+#define SOFM_MULAW_MANTISSA_MASK	0x0f
+#define SOFM_MULAW_MANTISSA_BITS	4
+#define SOFM_MULAW_SHIFT_MASK		0x07
+#define SOFM_MULAW_DEC_ONES_MASK	0x21  /* 0b100001 for "1abcd1", see below */
+
+/*
+ * mu-law encode table (sign bit is b12)
+ *
+ * Input values 12:0		Output values 6:0
+ *
+ * 0 0 0 0 0 0 0 1 a b c d x      0 0 0 a b c d
+ * 0 0 0 0 0 0 1 a b c d x x      0 0 1 a b c d
+ * 0 0 0 0 0 1 a b c d x x x      0 1 0 a b c d
+ * 0 0 0 0 1 a b c d x x x x      0 1 1 a b c d
+ * 0 0 0 1 a b c d x x x x x      1 0 0 a b c d
+ * 0 0 1 a b c d x x x x x x      1 0 1 a b c d
+ * 0 1 a b c d x x x x x x x      1 1 0 a b c d
+ * 1 a b c d x x x x x x x x      1 1 1 a b c d
+ *
+ * mu-law decode table (sign bit is b7)
+ *
+ * Input values 6:0	Output values 12:0
+ *
+ * 0 0 0 a b c d	0 0 0 0 0 0 0 1 a b c d 1
+ * 0 0 1 a b c d	0 0 0 0 0 0 1 a b c d 1 0
+ * 0 1 0 a b c d	0 0 0 0 0 1 a b c d 1 0 0
+ * 0 1 1 a b c d	0 0 0 0 1 a b c d 1 0 0 0
+ * 1 0 0 a b c d	0 0 0 1 a b c d 1 0 0 0 0
+ * 1 0 1 a b c d	0 0 1 a b c d 1 0 0 0 0 0
+ * 1 1 0 a b c d	0 1 a b c d 1 0 0 0 0 0 0
+ * 1 1 1 a b c d        1 a b c d 1 0 0 0 0 0 0 0
+ */
+
+/* Shift values lookup table for above table for 7
+ * highest sample value bits.
+ */
+static uint8_t mulaw_encode_shifts[128] = {
+	0, 1, 2, 2, 3, 3, 3, 3,
+	4, 4, 4, 4, 4, 4, 4, 4,
+	5, 5, 5, 5, 5, 5, 5, 5,
+	5, 5, 5, 5, 5, 5, 5, 5,
+	6, 6, 6, 6, 6, 6, 6, 6,
+	6, 6, 6, 6, 6, 6, 6, 6,
+	6, 6, 6, 6, 6, 6, 6, 6,
+	6, 6, 6, 6, 6, 6, 6, 6,
+	7, 7, 7, 7, 7, 7, 7, 7,
+	7, 7, 7, 7, 7, 7, 7, 7,
+	7, 7, 7, 7, 7, 7, 7, 7,
+	7, 7, 7, 7, 7, 7, 7, 7,
+	7, 7, 7, 7, 7, 7, 7, 7,
+	7, 7, 7, 7, 7, 7, 7, 7,
+	7, 7, 7, 7, 7, 7, 7, 7,
+	7, 7, 7, 7, 7, 7, 7, 7,
+};
+
+/**
+ * sofm_mu_law_encode() - Encode sample with mu-law coding
+ * @param sample: A s16 sample value
+ *
+ * The mu-law codec is defined in ITU-T G.711 standard and has been used
+ * in telecommunications in USA and Japan. The mu-law coding compresses
+ * 14 bit samples to 8 bit data stream. In SOF the high 14 bits of s16
+ * format are used for compatibility with normal audios.
+ *
+ * @return: Compressed 8 bit code value
+ */
+
+uint8_t sofm_mu_law_encode(int16_t sample)
+{
+	int sign = SOFM_MULAW_SIGN_BIT;
+	int shift = 0;
+	int low_bits;
+	uint8_t byte;
+
+	/* Convert to 14 bits with shift */
+	sample >>= 2;
+
+	/* Negative samples are 1's complement with zero sign bit */
+	if (sample < 0) {
+		sign = 0;
+		sample = -sample - 1;
+	}
+
+	sample += SOFM_MULAW_BIAS;
+	if (sample > SOFM_MULAW_MAX)
+		sample = SOFM_MULAW_MAX;
+
+	shift = mulaw_encode_shifts[sample >> 6];
+	low_bits = (sample >> (shift + 1)) & SOFM_MULAW_MANTISSA_MASK;
+
+	byte = (shift << SOFM_MULAW_MANTISSA_BITS) | low_bits | sign;
+	byte ^= SOFM_MULAW_TOGGLE_BITS;
+	return byte;
+}
+
+/**
+ * sofm_mu_law_decode() - Decode mu-law encoded code word
+ * @param byte: Encoded code word
+ *
+ * The mu-law decoder expands a 8 bit code word into a 14 bit sample value.
+ * In the SOF the high 14 bits are aligned to the most significant bits
+ * to be compatible with normal s16 Q1.15 samples.
+ *
+ * @return: Sampple value in s16 format
+ */
+int16_t sofm_mu_law_decode(int8_t byte)
+{
+	int low_bits;
+	int shift;
+	int sign;
+	int16_t value;
+
+	sign = byte & SOFM_MULAW_SIGN_BIT;
+	byte ^= SOFM_MULAW_TOGGLE_BITS;
+	low_bits = byte & SOFM_MULAW_MANTISSA_MASK;
+	shift = (byte >> SOFM_MULAW_MANTISSA_BITS) & SOFM_MULAW_SHIFT_MASK;
+	value = (low_bits << (shift + 1)) | (SOFM_MULAW_DEC_ONES_MASK << shift);
+	value -= SOFM_MULAW_BIAS;
+	if (!sign)
+		value = -value;
+
+	/* Shift 14 bit Q1.13 to 16 bit Q1.15 */
+	return value << 2;
+}