一、三者优缺点

1. PCM

2. PCMA

3. PCMU

讯享网

二、相互转换

获取原始录音字节流数据并转换成相应音频格式编码的字节数据

PCM转PCMA

static byte linear2alaw(short pcm_val){ 
    short mask; short seg; char aval; if(pcm_val >= 0){ 
    mask = 0xD5; }else{ 
    mask = 0x55; pcm_val = (short) (-pcm_val - 1); if(pcm_val < 0){ 
    pcm_val = 32767; } } /* Convert the scaled magnitude to segment number. */ seg = search(pcm_val, seg_end, (short) 8); /* Combine the sign, segment, and quantization bits. */ if (seg >= 8) /* out of range, return maximum value. */ return (byte) (0x7F ^ mask); else { 
    aval = (char) (seg << SEG_SHIFT); if (seg < 2) aval |= (pcm_val >> 4) & QUANT_MASK; else aval |= (pcm_val >> (seg + 3)) & QUANT_MASK; return (byte) (aval ^ mask); } } 

PCM转PCMU

讯享网 static byte linear2ulaw(short pcm_val) /* 2's complement (16-bit range) */ { 
    int mask; int seg; char uval; /* Get the sign and the magnitude of the value. */ // pcm_val = (short) (pcm_val >> 2); if (pcm_val < 0) { 
    pcm_val = (short) -pcm_val; mask = 0x7F; } else { 
    mask = 0xFF; } if (pcm_val > CLIP ) pcm_val = CLIP; /* clip the magnitude 削波*/ pcm_val += (BIAS >> 2); /* Convert the scaled magnitude to segment number. */ seg = search(pcm_val, seg_end, (short) 8); /* * Combine the sign, segment, quantization bits; * and complement the code word. */ if (seg >= 8) /* out of range, return maximum value. */ return (byte) (0x7F ^ mask); else { 
    uval = (char) ((seg << 4) | ((pcm_val >> (seg + 3)) & 0xF)); return (byte) (uval ^ mask); } } 

工具类如下：

public class G711Code { 
    private final static int SIGN_BIT = 0x80; private final static int QUANT_MASK = 0xf; private final static int SEG_SHIFT = 4; private final static int SEG_MASK = 0x70; private final static int BIAS = 0x84; private final static int CLIP = 8159; static short[] seg_end = { 
   0xFF, 0x1FF, 0x3FF, 0x7FF,0xFFF, 0x1FFF, 0x3FFF, 0x7FFF}; static short search(short val,short[] table,short size){ 
    for (short i = 0 ; i < size; i++) { 
    if(val <= table[i]){ 
    return i; } } return size; } static byte linear2alaw(short pcm_val){ 
    short mask; short seg; char aval; if(pcm_val >= 0){ 
    mask = 0xD5; }else{ 
    mask = 0x55; pcm_val = (short) (-pcm_val - 1); if(pcm_val < 0){ 
    pcm_val = 32767; } } /* Convert the scaled magnitude to segment number. */ seg = search(pcm_val, seg_end, (short) 8); /* Combine the sign, segment, and quantization bits. */ if (seg >= 8) /* out of range, return maximum value. */ return (byte) (0x7F ^ mask); else { 
    aval = (char) (seg << SEG_SHIFT); if (seg < 2) aval |= (pcm_val >> 4) & QUANT_MASK; else aval |= (pcm_val >> (seg + 3)) & QUANT_MASK; return (byte) (aval ^ mask); } } static byte linear2ulaw(short pcm_val) /* 2's complement (16-bit range) */ { 
    int mask; int seg; char uval; /* Get the sign and the magnitude of the value. */ // pcm_val = (short) (pcm_val >> 2); if (pcm_val < 0) { 
    pcm_val = (short) -pcm_val; mask = 0x7F; } else { 
    mask = 0xFF; } if (pcm_val > CLIP ) pcm_val = CLIP; /* clip the magnitude 削波*/ pcm_val += (BIAS >> 2); /* Convert the scaled magnitude to segment number. */ seg = search(pcm_val, seg_end, (short) 8); /* * Combine the sign, segment, quantization bits; * and complement the code word. */ if (seg >= 8) /* out of range, return maximum value. */ return (byte) (0x7F ^ mask); else { 
    uval = (char) ((seg << 4) | ((pcm_val >> (seg + 3)) & 0xF)); return (byte) (uval ^ mask); } } static short alaw2linear(byte a_val){ 
    short t; short seg; a_val ^= 0x55; t = (short) ((a_val & QUANT_MASK) << 4); seg = (short) ((a_val & SEG_MASK) >> SEG_SHIFT); switch (seg) { 
    case 0: t += 8; break; case 1: t += 0x108; break; default: t += 0x108; t <<= seg - 1; } return (a_val & SIGN_BIT) != 0 ? t : (short) -t; } / * pcm 转 G711 a率 * @param pcm * @param code * @param size */ public static void G711aEncoder(short[] pcm,byte[] code,int size){ 
    for(int i=0;i<size;i++){ 
    code[i]=linear2alaw(pcm[i]); //Log.e("-------------","数据编码"); } } / * G.711 转 PCM * @param pcm * @param code * @param size */ public static void G711aDecoder(short[] pcm,byte[] code,int size) { 
    for(int i=0;i<size;i++){ 
    pcm[i]=alaw2linear(code[i]); } } / * pcm 转 G711 u率 * @param pcm * @param code * @param size */ public static void G711uEncoder(short[] pcm,byte[] code,int size){ 
    for(int i=0;i<size;i++){ 
    code[i]=linear2ulaw(pcm[i]); //Log.e("-------------","数据编码"); } } }