G.711音頻編解碼演算法

一、概述

G.711是國際電信聯盟（ITU）為語音通信和錄音等應用而制定的音頻編解碼標準。該標準主要用於傳輸純語音信息，包括電話、對講機等，不支持音頻信號的壓縮。G.711採用時域抽樣，將模擬音頻採樣率從8kHz上升至16kHz，保證傳輸音頻質量。

二、G.711編解碼原理

G.711有兩種編解碼方式，分別為u-law（μ-law）和A-law。這兩種編碼方式是根據編碼器的壓縮特性進行選擇的。

1. u-law編碼

short linear2ulaw(int pcm_val)
{
    int     mask;
    int     seg;
    unsigned char   uval;

    pcm_val = pcm_val >> 2;

    if (pcm_val  0x3FFF) {
        pcm_val = 0x3FFF;
    }

    pcm_val += 0x20;
    seg = compand[pcm_val>>6];
    uval = (unsigned char)(seg | ((pcm_val >>(seg + 1)) & 0x0F));
    return (short)(uval ^ mask);
}

2. A-law編碼

short linear2alaw(int pcm_val)
{
    int     mask;
    int     seg;
    unsigned char   aval;

    if (pcm_val >= 0) {
        mask = 0xD5;
    } else {
        mask = 0x55;
        pcm_val = -pcm_val - 8;
    }

    if (pcm_val  0x7FF) {
        pcm_val = 0x7FF;
    }

    pcm_val >>= 4;
    seg = search(pcm_val, seg_end, 8);
    if (seg >= 8) {
        return (short)(0x7F ^ mask);
    } else {
        aval = (unsigned char)(seg <> (seg + 1) & 0x0F));
        return (short)(aval ^ mask);
    }
}

三、G.711編碼器的實現

以下代碼展示了一個簡單的G.711編碼器實現：

public class G711Encoder {
    static final int ZEROTRAP = 1;
    static final int BIAS = 0x84;
    static final int CLIP = 8159;

    private static byte[] _law2linear;

    /**
     * 靜態塊，初始化_law2linear表
     */
    static {
        _law2linear = new byte[1024];
        for (int i = 0; i < 256; i++) {
            int x = i ^ 0x55;
            x <<= 7;
            short t = (short) x;
            if ((x & 0x8000) != 0) {
                t = (short) (0x7fff - (x & 0x7fff));
            }

            t -= BIAS <>> 8);
            _law2linear[i + 256] = (byte) (t & 0xff);
            x = (i & 0x7f) << 8;
            t = (short) x;
            if ((x & 0x8000) != 0) {
                t = (short) (0x7fff - (x & 0x7fff));
            }

            t -= (BIAS <>> 8);
            _law2linear[i + 768] = (byte) (t & 0xff);
        }
    }

    /**
     * 編碼PCM到G.711
     *
     * @param pcm 輸入PCM數據
     * @return 返回G.711編碼後的數據
     */
    public byte[] encode(byte[] pcm) {
        byte[] g711 = new byte[pcm.length / 2];
        int idx, x;
        for (int i = 0, s = 0; i < pcm.length; i += 2) {
            idx = ((pcm[i + 1] & 0xff) << 8) | (pcm[i] & 0xff);
            if (idx  32767) {
                idx = 32767;
            }

            x = _law2linear[idx];
            if (pcm[i + 1] < 0) {
                x = -x;
            }

            x += 0x80;
            if (x  0xFF) {
                x = 0xFF;
            }

            g711[s++] = (byte) x;
        }
        return g711;
    }

}

四、G.711解碼器的實現

以下代碼展示了一個簡單的G.711解碼器實現：

public class G711Decoder {
    static final int ZEROTRAP = 1;
    static final int BIAS = 0x84;
    static final int CLIP = 8159;

    private static short[] _linear2law;

    /**
     * 靜態塊，初始化_linear2law表
     */
    static {
        int[] seg_end = {0xFF, 0x1FF, 0x3FF, 0x7FF, 0xFFF, 0x1FFF, 0x3FFF, 0x7FFF};
        _linear2law = new short[65536];
        for (int i = 0; i < 65536; i++) {
            short sign = 0;
            if (i < 0) {
                i = -i;
                sign = 0x80;
            }
            int exponent = search(i, seg_end, 8) <> (exponent + 3)) & 0x0F;
            _linear2law[i] = (short) (sign | exponent | mantissa);
        }
    }

    /**
     * 解碼G.711到PCM
     *
     * @param g711 輸入G.711數據
     * @return 返回PCM解碼後的數據
     */
    public byte[] decode(byte[] g711) {
        byte[] pcm = new byte[g711.length * 2];
        int idx, sign, x;
        for (int i = 0, s = 0; i < g711.length; i++, s += 2) {
            sign = (g711[i] & 0x80) != 0 ? (byte) 0x00FF : 0;
            idx = ((g711[i] ^ sign) & 0xff);
            x = _linear2law[idx] + BIAS <> 8);
        }
        return pcm;
    }
}

五、應用舉例

G.711主要在電話語音通信、對講機等領域中得到廣泛使用。以下是一個Java語言中使用G.711編解碼的例子：

public class G711Example {
    public static void main(String[] args) {
        short[] pcmData = new short[]{0, 100, 200, 300, 400, 500};
        G711Encoder encoder = new G711Encoder();
        G711Decoder decoder = new G711Decoder();
        byte[] g711Data = encoder.encode(ArrayUtils.toBytes(pcmData)); // PCM -> G.711
        short[] pcmData2 = ArrayUtils.toShorts(decoder.decode(g711Data)); // G.711 -> PCM
        System.out.println(Arrays.toString(pcmData));
        System.out.println(Bytes.toHexString(g711Data, "", " ", ""));
        System.out.println(Arrays.toString(pcmData2));
    }
}