記錄一個很簡單的壓縮編碼--ADPCM

田帅康学习笔记發表於2024-12-10

此篇文章在2022年9月22日被記錄

ADPCM是一種很簡單實現的音訊編碼方式，真正的PCM相當佔用記憶體，這對網路和記憶體的壓力是相當大的，因此通常需要壓縮編碼，ADPCM是一種可以執行在微控制器上的編碼方式，原理如下：

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由於聲音訊號具有波形上的連續性，因此相鄰兩個取樣值大小也非常接近，記錄單個取樣值通常需要 16bit，而記錄前後兩個取樣點的差值（差分法），往往只需要 4bit，這便是 ADPCM 壓縮編碼的基本原理，因此透過 ADPCM 編碼的音訊檔案，其大小隻有 PCM 格式的四分之一。

不僅如此，ADPCM 的智慧之處在於，對於變化劇烈的波形，演算法透過自適應機制，能自動改變差分值的度量粒度，即使是抖動較大的訊號，也可以保證前後取樣差值總能用固定的 4bit 表示。在 PCM 編碼的基礎上增加 「差分」和「自適應」的特性，便是 ADPCM（Adaptive Differential Pulse Code Modulation 自適應差分脈衝編碼調製） 名稱的由來。

當然，ADPCM 演算法實現簡單、壓縮率高的同時，必然要付出音質損失的代價 —— ADPCM 格式檔案的聲音聽起來會略為粗糙，被同樣是有失真壓縮的 MP3 編碼吊打，不過用於提示音、人聲講話等場合還是綽綽有餘。

實際上在STM32L476@80Mhz微控制器上測試，編碼320個16bit資料需要時間在1ms內，解碼幾乎不佔用時間，這意味著在微控制器上具有較強的實時性。

如何實現：
在網上嫖到的adpcm.c、adpcm.h

-------------------------------原始檔-----------------------------------

#include "adpcm.h"
#include <stdio.h> /*DBG*/

#ifndef __STDC__
#define signed
#endif

/* Intel ADPCM step variation table */
static int indexTable[16] = {
    -1, -1, -1, -1, 2, 4, 6, 8,
    -1, -1, -1, -1, 2, 4, 6, 8,
};

static int stepsizeTable[89] = {
    7, 8, 9, 10, 11, 12, 13, 14, 16, 17,
    19, 21, 23, 25, 28, 31, 34, 37, 41, 45,
    50, 55, 60, 66, 73, 80, 88, 97, 107, 118,
    130, 143, 157, 173, 190, 209, 230, 253, 279, 307,
    337, 371, 408, 449, 494, 544, 598, 658, 724, 796,
    876, 963, 1060, 1166, 1282, 1411, 1552, 1707, 1878, 2066,
    2272, 2499, 2749, 3024, 3327, 3660, 4026, 4428, 4871, 5358,
    5894, 6484, 7132, 7845, 8630, 9493, 10442, 11487, 12635, 13899,
    15289, 16818, 18500, 20350, 22385, 24623, 27086, 29794, 32767
};

void adpcm_coder(short *indata, signed char *outdata, int len, struct adpcm_state *state)
{
    short *inp;			/* Input buffer pointer */
    signed char *outp;		/* output buffer pointer */
    int val;			/* Current input sample value */
    int sign;			/* Current adpcm sign bit */
    int delta;			/* Current adpcm output value */
    int diff;			/* Difference between val and valprev */
    int step;			/* Stepsize */
    int valpred;		/* Predicted output value */
    int vpdiff;			/* Current change to valpred */
    int index;			/* Current step change index */
    int outputbuffer;		/* place to keep previous 4-bit value */
    int bufferstep;		/* toggle between outputbuffer/output */

    outp = (signed char *)outdata;
    inp = indata;

    valpred = state->valprev;
    index = state->index;
    step = stepsizeTable[index];

    bufferstep = 1;

    for ( ; len > 0 ; len-- ) {
	val = *inp++;

	/* Step 1 - compute difference with previous value */
	diff = val - valpred;
	sign = (diff < 0) ? 8 : 0;
	if ( sign ) diff = (-diff);

	/* Step 2 - Divide and clamp */
	/* Note:
	** This code *approximately* computes:
	**    delta = diff*4/step;
	**    vpdiff = (delta+0.5)*step/4;
	** but in shift step bits are dropped. The net result of this is
	** that even if you have fast mul/div hardware you cannot put it to
	** good use since the fixup would be too expensive.
	*/
	delta = 0;
	vpdiff = (step >> 3);

	if ( diff >= step ) {
	    delta = 4;
	    diff -= step;
	    vpdiff += step;
	}
	step >>= 1;
	if ( diff >= step  ) {
	    delta |= 2;
	    diff -= step;
	    vpdiff += step;
	}
	step >>= 1;
	if ( diff >= step ) {
	    delta |= 1;
	    vpdiff += step;
	}

	/* Step 3 - Update previous value */
	if ( sign )
	  valpred -= vpdiff;
	else
	  valpred += vpdiff;

	/* Step 4 - Clamp previous value to 16 bits */
	if ( valpred > 32767 )
	  valpred = 32767;
	else if ( valpred < -32768 )
	  valpred = -32768;

	/* Step 5 - Assemble value, update index and step values */
	delta |= sign;

	index += indexTable[delta];
	if ( index < 0 ) index = 0;
	if ( index > 88 ) index = 88;
	step = stepsizeTable[index];

	/* Step 6 - Output value
	if ( bufferstep ) {
	    outputbuffer = (delta << 4) & 0xf0;
	} else {
	    *outp++ = (delta & 0x0f) | outputbuffer;
	}*/
    if ( bufferstep ) {
	    outputbuffer = delta & 0x0f;
	} else {
	    *outp++ = ((delta << 4) & 0xf0) | outputbuffer;
	}
	bufferstep = !bufferstep;
    }

    /* Output last step, if needed */
    if ( !bufferstep )
      *outp++ = outputbuffer;

    state->valprev = valpred;
    state->index = index;
}

void adpcm_decoder(signed char *indata, short *outdata, int len, struct adpcm_state *state)
{
    signed char *inp;		/* Input buffer pointer */
    short *outp;		/* output buffer pointer */
    int sign;			/* Current adpcm sign bit */
    int delta;			/* Current adpcm output value */
    int step;			/* Stepsize */
    int valpred;		/* Predicted value */
    int vpdiff;			/* Current change to valpred */
    int index;			/* Current step change index */
    int inputbuffer;		/* place to keep next 4-bit value */
    int bufferstep;		/* toggle between inputbuffer/input */

    outp = outdata;
    inp = (signed char *)indata;

    valpred = state->valprev;
    index = state->index;
    step = stepsizeTable[index];

    bufferstep = 0;

    for ( ; len > 0 ; len-- ) {

	/* Step 1 - get the delta value */
	if ( !bufferstep ) {
	    inputbuffer = *inp++;
	    delta = inputbuffer & 0xf;
	} else {
	    delta = (inputbuffer >> 4) & 0xf;
	}
	bufferstep = !bufferstep;

	/* Step 2 - Find new index value (for later) */
	index += indexTable[delta];
	if ( index < 0 ) index = 0;
	if ( index > 88 ) index = 88;

	/* Step 3 - Separate sign and magnitude */
	sign = delta & 8;
	delta = delta & 7;

	/* Step 4 - Compute difference and new predicted value */
	/*
	** Computes 'vpdiff = (delta+0.5)*step/4', but see comment
	** in adpcm_coder.
	*/
	vpdiff = step >> 3;
	if ( delta & 4 ) vpdiff += step;
	if ( delta & 2 ) vpdiff += step>>1;
	if ( delta & 1 ) vpdiff += step>>2;

	if ( sign )
	  valpred -= vpdiff;
	else
	  valpred += vpdiff;

	/* Step 5 - clamp output value */
	if ( valpred > 32767 )
	  valpred = 32767;
	else if ( valpred < -32768 )
	  valpred = -32768;

	/* Step 6 - Update step value */
	step = stepsizeTable[index];

	/* Step 7 - Output value */
	*outp++ = valpred;
    }

    state->valprev = valpred;
    state->index = index;
}

------------------------------------------------標頭檔案---------------------------

#ifndef ADPCM_H
#define ADPCM_H
#include <stdint.h>

struct adpcm_state
{
    int valprev;
    int index;
};

extern void adpcm_coder(short *indata, signed char *outdata, int len, struct adpcm_state *state);
extern void adpcm_decoder(signed char *indata, short *outdata, int len, struct adpcm_state *state);

#endif /*ADPCM_H*/

HOW TO USE ?

ADPCM不用設定那麼多引數，直接來解碼編碼：
建立兩個編解碼引數，主要是用來儲存上次的資料

struct adpcm_state myENadpcm,myDEadpcm;
myENadpcm.index=0;
myENadpcm.valprev=0;

myDEadpcm.index=0;
myDEadpcm.valprev=0;

編解碼函式呼叫：

+ adpcm_coder(原始資料陣列, 編碼後的資料陣列, 編碼前的資料長度, &myENadpcm);
+ adpcm_decoder(編碼後的資料, 解碼後的資料, 編碼後的資料長度, &myDEadpcm);

實際測試上，在微控制器上執行，相比於原聲具有較強的電子音，就像是牙籤的babiQ~，相對來說，OPUS的效果更好，但是資源佔用也更高。

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