274 lines
7.0 KiB
Dart
274 lines
7.0 KiB
Dart
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/**
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* G711Tool - G.711音频编解码工具类
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*
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* G.711是一种用于音频压缩的ITU-T标准，主要用于电话系统。
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* 它有两种主要变体：A-law（主要用于欧洲和国际电话系统）和μ-law（主要用于北美和日本）。
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*
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* 该类提供了PCM线性音频数据与G.711 A-law/μ-law格式之间的转换功能。
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* 编码过程将16位线性PCM样本压缩为8位，解码过程则相反。
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*/
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class G711Tool {
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// 常量定义
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static const int SIGN_BIT = 0x80; // A-law字节的符号位
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static const int QUANT_MASK = 0xf; // 量化字段掩码
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static const int NSEGS = 8; // A-law段数
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static const int SEG_SHIFT = 4; // 段号左移位数
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static const int SEG_MASK = 0x70; // 段字段掩码
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static const int BIAS = 0x84; // 线性编码的偏置值
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// 查找表
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/**
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* μ-law到A-law的转换表
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* 用于在两种编码格式之间进行转换而不需要先解码为线性PCM
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*/
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static final List<int> _u2a = [
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// u- to A-law conversions
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1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7, 8, 8,
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9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
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25, 27, 29, 31, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44,
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46, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62,
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64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79,
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81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96,
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97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112,
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113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127,
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128
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];
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/**
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* A-law到μ-law的转换表
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* 用于在两种编码格式之间进行转换而不需要先解码为线性PCM
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*/
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static final List<int> _a2u = [
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// A- to u-law conversions
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1, 3, 5, 7, 9, 11, 13, 15, 16, 17, 18, 19, 20, 21, 22, 23,
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24, 25, 26, 27, 28, 29, 30, 31, 32, 32, 33, 33, 34, 34, 35, 35,
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36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 48, 49, 49,
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50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 64,
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65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 79,
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80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95,
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96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111,
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112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126,
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127
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];
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/**
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* 段结束值表
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* 用于确定PCM值应该映射到哪个段
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* 每个段代表不同的量化步长
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*/
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static final List<int> _segEnd = [
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0xFF,
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0x1FF,
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0x3FF,
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0x7FF,
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0xFFF,
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0x1FFF,
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0x3FFF,
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0x7FFF
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];
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/**
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* 在表中搜索值所属的段
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*
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* @param val 要搜索的值
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* @param table 段结束值表
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* @param size 表大小
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* @return 段索引
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*/
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static int _search(int val, List<int> table, int size) {
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for (int i = 0; i < size; i++) {
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if (val <= table[i]) return i;
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}
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return size;
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}
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/**
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* 将线性PCM值转换为A-law编码值
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*
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* 转换步骤:
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* 1. 确定符号位和掩码
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* 2. 对负值进行特殊处理
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* 3. 确定段号
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* 4. 根据段号计算A-law值
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*
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* @param pcmVal 16位线性PCM值
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* @return 8位A-law编码值
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*/
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static int _linear2alaw(int pcmVal) {
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int mask;
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int seg;
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int aval;
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if (pcmVal >= 0) {
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mask = 0xD5; // sign (7th) bit = 1
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} else {
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mask = 0x55; // sign bit = 0
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pcmVal = -pcmVal - 1;
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if (pcmVal < 0) pcmVal = 32767;
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}
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seg = _search(pcmVal, _segEnd, 8);
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if (seg >= 8) return 0x7F ^ mask;
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aval = seg << SEG_SHIFT;
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if (seg < 2) {
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aval |= (pcmVal >> 4) & QUANT_MASK;
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} else {
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aval |= (pcmVal >> (seg + 3)) & QUANT_MASK;
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}
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return aval ^ mask;
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}
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/**
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* 将A-law编码值转换为线性PCM值
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*
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* 转换步骤:
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* 1. 异或操作恢复原始位模式
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* 2. 提取量化值和段号
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* 3. 根据段号计算线性值
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* 4. 应用符号位
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*
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* @param aVal 8位A-law编码值
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* @return 16位线性PCM值
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*/
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static int _alaw2linear(int aVal) {
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int t;
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int seg;
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aVal ^= 0x55;
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t = (aVal & QUANT_MASK) << 4;
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seg = (aVal & SEG_MASK) >> SEG_SHIFT;
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switch (seg) {
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case 0:
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t += 8;
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break;
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case 1:
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t += 0x108;
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break;
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default:
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t += 0x108;
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t <<= seg - 1;
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}
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return (aVal & SIGN_BIT) != 0 ? t : -t;
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}
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/**
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* 将线性PCM值转换为μ-law编码值
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*
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* 转换步骤与A-law类似，但使用不同的偏置和掩码
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*
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* @param pcmVal 16位线性PCM值
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* @return 8位μ-law编码值
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*/
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static int _linear2ulaw(int pcmVal) {
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int mask;
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int seg;
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int uval;
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if (pcmVal < 0) {
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pcmVal = BIAS - pcmVal;
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mask = 0x7F;
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} else {
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pcmVal += BIAS;
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mask = 0xFF;
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}
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seg = _search(pcmVal, _segEnd, 8);
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if (seg >= 8) return 0x7F ^ mask;
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uval = (seg << 4) | ((pcmVal >> (seg + 3)) & 0xF);
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return uval ^ mask;
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}
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/**
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* 将μ-law编码值转换为线性PCM值
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*
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* @param uVal 8位μ-law编码值
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* @return 16位线性PCM值
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*/
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static int _ulaw2linear(int uVal) {
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uVal = ~uVal;
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int t = ((uVal & QUANT_MASK) << 3) + BIAS;
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t <<= (uVal & SEG_MASK) >> SEG_SHIFT;
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return (uVal & SIGN_BIT) != 0 ? (BIAS - t) : (t - BIAS);
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}
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/**
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* 将A-law编码值转换为μ-law编码值
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*
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* 使用查找表直接转换，避免解码再编码的性能损失
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*
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* @param aval 8位A-law编码值
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* @return 8位μ-law编码值
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*/
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static int alaw2ulaw(int aval) {
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aval &= 0xff;
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return (aval & 0x80) != 0
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? (0xFF ^ _a2u[aval ^ 0xD5])
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: (0x7F ^ _a2u[aval ^ 0x55]);
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}
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/**
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* 将μ-law编码值转换为A-law编码值
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*
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* 使用查找表直接转换，避免解码再编码的性能损失
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*
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* @param uval 8位μ-law编码值
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* @return 8位A-law编码值
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*/
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static int ulaw2alaw(int uval) {
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uval &= 0xff;
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return (uval & 0x80) != 0
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? (0xD5 ^ (_u2a[0xFF ^ uval] - 1))
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: (0x55 ^ (_u2a[0x7F ^ uval] - 1));
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}
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/**
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* 将PCM数据编码为G.711格式
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*
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* @param pcm 输入PCM数据
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* @param lawFlag 编码类型标志: 0表示A-law, 1表示μ-law
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* @return 编码后的G.711数据
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*/
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static List<int> encode(List<int> pcm, int lawFlag) {
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List<int> code = List<int>.filled(pcm.length, 0);
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if (lawFlag == 0) {
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for (int i = 0; i < pcm.length; i++) {
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code[i] = _linear2alaw(pcm[i]);
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}
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} else {
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for (int i = 0; i < pcm.length; i++) {
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code[i] = _linear2ulaw(pcm[i]);
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}
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}
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return code;
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}
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/**
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* 将G.711数据解码为PCM格式
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*
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* @param code 输入G.711数据
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* @param lawFlag 编码类型标志: 0表示A-law, 1表示μ-law
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* @return 解码后的PCM数据
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*/
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static List<int> decode(List<int> code, int lawFlag) {
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List<int> pcm = List<int>.filled(code.length, 0);
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if (lawFlag == 0) {
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for (int i = 0; i < code.length; i++) {
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pcm[i] = _alaw2linear(code[i]);
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}
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} else {
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for (int i = 0; i < code.length; i++) {
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pcm[i] = _ulaw2linear(code[i]);
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}
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}
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return pcm;
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}
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}
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