alaw.c File Reference

A-law algorithm for encoding and decoding (16bit pcm <=> a-law). This is the implementation of G.711 in C. More...

#include <assert.h>
#include <inttypes.h>
#include <stdio.h>

Include dependency graph for alaw.c:

Macros
#define	LEN   ((size_t)8)
	Linear input code | Compressed code | Linear output code ---------------—+--------------—+----------------— s0000000abcdx | s000abcd | s0000000abcd1 s0000001abcdx | s001abcd | s0000001abcd1 s000001abcdxx | s010abcd | s000001abcd10 s00001abcdxxx | s011abcd | s00001abcd100 s0001abcdxxxx | s100abcd | s0001abcd1000 s001abcdxxxxx | s101abcd | s001abcd10000 s01abcdxxxxxx | s110abcd | s01abcd100000 s1abcdxxxxxxx | s111abcd | s1abcd1000000.

Functions
void	encode (uint8_t *out, int16_t *in, size_t len)
	16bit pcm to 8bit alaw
void	decode (int16_t *out, uint8_t *in, size_t len)
	8bit alaw to 16bit pcm
static void	test (int16_t *pcm, uint8_t *coded, int16_t *decoded, size_t len)
	Self-test implementations.
int	main (int argc, char *argv[])
	Main function.

Variables
int16_t	pcm [LEN] = {1000, -1000, 1234, 3200, -1314, 0, 32767, -32768}
uint8_t	r_coded [LEN] = {250, 122, 230, 156, 97, 213, 170, 42}
int16_t	r_decoded [LEN] = {1008, -1008, 1248, 3264, -1312, 8, 32256, -32256}

Detailed Description

A-law algorithm for encoding and decoding (16bit pcm <=> a-law). This is the implementation of G.711 in C.

Author

sunzhenliang

Macro Definition Documentation

LEN

#define LEN   ((size_t)8)

Linear input code | Compressed code | Linear output code ---------------—+--------------—+----------------— s0000000abcdx | s000abcd | s0000000abcd1 s0000001abcdx | s001abcd | s0000001abcd1 s000001abcdxx | s010abcd | s000001abcd10 s00001abcdxxx | s011abcd | s00001abcd100 s0001abcdxxxx | s100abcd | s0001abcd1000 s001abcdxxxxx | s101abcd | s001abcd10000 s01abcdxxxxxx | s110abcd | s01abcd100000 s1abcdxxxxxxx | s111abcd | s1abcd1000000.

Compressed code: (s | eee | abcd) for assert for appropriate size int types for IO operations

Function Documentation

decode()

void decode	(	int16_t *	out,
		uint8_t *	in,
		size_t	len
	)

8bit alaw to 16bit pcm

Parameters

out	signed 16bit pcm array
in	unsigned 8bit alaw array
len	length of alaw array

Returns

void

{
    uint8_t alaw = 0;
    int32_t pcm = 0;
    int32_t sign = 0;
    int32_t eee = 0;
    for (size_t i = 0; i < len; i++)
    {
        alaw = *in++;
 
        /* Re-toggle toggled bits */
        alaw ^= 0xD5;
 
        /* Get sign bit */
        sign = alaw & 0x80;
 
        /* Get eee bits */
        eee = (alaw & 0x70) >> 4;
 
        /* Get abcd bits and add 1/2 quantization step */
        pcm = (alaw & 0x0f) << 4 | 8;
 
        /* If quantization level > 0, there need `1` bit before abcd bits */
        pcm += eee ? 0x100 : 0x0;
 
        /* Left shift according quantization level */
        pcm <<= eee > 1 ? (eee - 1) : 0;
 
        /* Use the right sign */
        *out++ = sign ? -pcm : pcm;
    }
}

encode()

void encode	(	uint8_t *	out,
		int16_t *	in,
		size_t	len
	)

16bit pcm to 8bit alaw

Parameters

out	unsigned 8bit alaw array
in	signed 16bit pcm array
len	length of pcm array

Returns

void

{
    uint8_t alaw = 0;
    int16_t pcm = 0;
    int32_t sign = 0;
    int32_t abcd = 0;
    int32_t eee = 0;
    int32_t mask = 0;
    for (size_t i = 0; i < len; i++)
    {
        pcm = *in++;
        /* 0-7 kinds of quantization level from the table above */
        eee = 7;
        mask = 0x4000; /* 0x4000: '0b0100 0000 0000 0000' */
 
        /* Get sign bit */
        sign = (pcm & 0x8000) >> 8;
 
        /* Turn negative pcm to positive */
        /* The absolute value of a negative number may be larger than the size
         * of the corresponding positive number, so here needs `-pcm -1` after
         * taking the opposite number. */
        pcm = sign ? (-pcm - 1) : pcm;
 
        /* Get eee and abcd bit */
        /* Use mask to locate the first `1` bit and quantization level at the
         * same time */
        while ((pcm & mask) == 0 && eee > 0)
        {
            eee--;
            mask >>= 1;
        }
 
        /* The location of abcd bits is related with quantization level. Check
         * the table above to determine how many bits to `>>` to get abcd */
        abcd = (pcm >> (eee ? (eee + 3) : 4)) & 0x0f;
 
        /* Put the quantization level number at right bit location to get eee
         * bits */
        eee <<= 4;
 
        /* Splice results */
        alaw = (sign | eee | abcd);
 
        /* The standard specifies that all resulting even bits (LSB
         * is even) are inverted before the octet is transmitted. This is to
         * provide plenty of 0/1 transitions to facilitate the clock recovery
         * process in the PCM receivers. Thus, a silent A-law encoded PCM
         * channel has the 8 bit samples coded 0xD5 instead of 0x80 in the
         * octets. (Reference from wiki above) */
        *out++ = alaw ^ 0xD5;
    }
}

main()

int main	(	int	argc,
		char *	argv[]
	)

Main function.

Parameters

argc	commandline argument count (ignored)
argv	commandline array of arguments (ignored)

Returns

0 on exit

{
    /* output alaw encoded by encode() */
    uint8_t coded[LEN];
 
    /* output pcm decoded by decode() from coded[LEN] */
    int16_t decoded[LEN];
 
    test(pcm, coded, decoded, LEN);  // run self-test implementations
 
    /* print test pcm inputs */
    printf("inputs: ");
    for (size_t i = 0; i < LEN; i++)
    {
        printf("%d ", pcm[i]);
    }
    printf("\n");
 
    /* print encoded alaw */
    printf("encode: ");
    for (size_t i = 0; i < LEN; i++)
    {
        printf("%u ", coded[i]);
    }
    printf("\n");
 
    /* print decoded pcm */
    printf("decode: ");
    for (size_t i = 0; i < LEN; i++)
    {
        printf("%d ", decoded[i]);
    }
    printf("\n");
 
    /* It can be seen that the encoded alaw is smaller than the input PCM, so
     * the purpose of compression is achieved. And the decoded PCM is almost the
     * same as the original input PCM, which verifies the correctness of the
     * decoding. The reason why it is not exactly the same is that there is
     * precision loss during encode / decode.  */
 
    return 0;
}

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test()

static void test ( int16_t *  pcm, uint8_t *  coded, int16_t *  decoded, size_t  len  )

static

Self-test implementations.

Parameters

pcm	signed 16bit pcm array
coded	unsigned 8bit alaw array
decoded	signed 16bit pcm array
len	length of test array

Returns

void

{
    /* run encode */
    encode(coded, pcm, len);
 
    /* check encode result */
    for (size_t i = 0; i < len; i++)
    {
        assert(coded[i] == r_coded[i]);
    }
 
    /* run decode */
    decode(decoded, coded, len);
 
    /* check decode result */
    for (size_t i = 0; i < len; i++)
    {
        assert(decoded[i] == r_decoded[i]);
    }
}

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