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Layer1的编码更简单，解码端的代码也就比Layer2还简单不少。网络上还有部分老歌是采用Layer2压缩的，但Layer1编码方式的就很难找到了，手头的编码器没有Layer1编码方式，所以这些代码没有经过测试，不详细讲解解码过程了。作为一个完整的MPEG Audio解码器，我还是加入了这部分代码。解码Layer1的源码：

/*
* Layer1.java -- MPEG 1.0 Audio Layer I Decoder
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program.  If not, see <http://www.gnu.org/licenses/>.
*
* If you would like to negotiate alternate licensing terms, you may do
* so by contacting the author: <http://jmp123.sourceforge.net/>
*/
package decoder;public final class Layer1 implements ILayer123 {Header header;BitStream bs;Synthesis filter;float[] factor;byte[][] allocation; //[2][32]byte[][] scalefactor;  //[2][32]float[][] syin;            //[2][32]public Layer1(BitStream bitstream,Header h, Synthesis filter, int wch) {header = h;bs = bitstream;this.filter = filter;allocation = new byte[2][32];scalefactor = new byte[2][32];syin = new float[2][32];factor = Layer2.factor;       // ISO/IEC 11172-3 Table 3-B.1.}/** 逆量化公式:* s'' = (2^nb / (2^nb - 1)) * (s''' + 2^(-nb + 1))* s' = factor * s''*/float requantization(int ch, int sb, int nb) {int samplecode = bs.getBits17(nb);int nlevels = (1 << nb);float requ = 2.0f * samplecode / nlevels - 1.0f;  //s'''requ += (float)Math.pow(2, 1-nb);requ *= nlevels / (nlevels - 1);       //s''requ *= factor[scalefactor[ch][sb]];    //s'return requ;}public void decodeFrame() throws Exception {int sb, gr, ch, nb;int nch = header.getChannels();int bound = (header.getMode() == 1) ? ((header.getModeExtension() + 1) * 4) : 32;int slots = header.getMainDataSlots();bs.append(slots);int maindata_begin = bs.getBytePos();//1. Bit allocation decodingfor (sb = 0; sb < bound; sb++)for (ch = 0; ch < nch; ++ch) {nb = bs.getBits9(4);if (nb == 15)throw new Exception("decodeFrame()->nb=15");allocation[ch][sb] = (byte)((nb != 0) ? (nb + 1) : 0);}for (sb = bound; sb < 32; sb++) {nb = bs.getBits9(4);if (nb == 15)throw new Exception("decodeFrame()->nb=15");allocation[0][sb] = (byte)((nb != 0) ? (nb + 1) : 0);}//2. Scalefactor decodingfor (sb = 0; sb < 32; sb++)for (ch = 0; ch < nch; ch++)if (allocation[ch][sb] != 0)scalefactor[ch][sb] = (byte)bs.getBits9(6);for (gr = 0; gr < 12; gr++) {//3. Requantization of subband samplesfor (sb = 0; sb < bound; sb++)for (ch = 0; ch < nch; ch++){nb = allocation[ch][sb];if(nb == 0)syin[ch][sb] = 0;elsesyin[ch][sb] = requantization(ch, sb, nb);}//mode=1(Joint Stereo)for (sb = bound; sb < 32; ++sb)if ((nb = allocation[0][sb]) != 0)for (ch = 0; ch < nch; ++ch)syin[ch][sb] = requantization(ch, sb, nb);elsefor (ch = 0; ch < nch; ++ch)syin[ch][sb] = 0;//4. Synthesis subband filterfor (ch = 0; ch < nch; ch++)filter.synthesisSubBand(syin[ch], ch);}//5. Ancillary bitsint discard = slots + maindata_begin - bs.getBytePos();bs.skipBytes(discard);}
}

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