【Android】【NDK】解决Lame库编码MP3声音速度变快的问题

这个问题可能是由于把单声道PCM作为双声道PCM来编码造成的
这样就有一半的数据从左声道被编到了右声道，于是播放速度快了一倍，总时长短了一半
我看网上大多Lame库的Demo都默认是双声道，没有考虑到单声道的问题

解决方法：单声道和双声道分开进行编码

	//转码单声道PCM
	void encodeSingleChannelPcm() {
	    int bufferSize = 1024 * 256;
	    short *buffer = new short[bufferSize / 2];
	    unsigned char *mp3Buffer = new unsigned char[bufferSize];
	    size_t readBufferSize = 0;
	    while ((readBufferSize = fread(buffer, 2, bufferSize / 2, pcmFile)) > 0) {
	        size_t wroteSize = lame_encode_buffer(lame, buffer, nullptr, readBufferSize, mp3Buffer, bufferSize);
	        fwrite(mp3Buffer, 1, wroteSize, mp3File);
	    }
	    delete[] buffer;
	    delete[] mp3Buffer;
	}
	
	//转码双声道PCM
	void encodeDoubleChannelPcm() {
	    int bufferSize = 1024 * 256;
	    short *buffer = new short[bufferSize / 2];
	    short *leftBuffer = new short[bufferSize / 4];
	    short *rightBuffer = new short[bufferSize / 4];
	    unsigned char *mp3Buffer = new unsigned char[bufferSize];
	    size_t readBufferSize = 0;
	    while ((readBufferSize = fread(buffer, 2, bufferSize / 2, pcmFile)) > 0) {
	        for (int i = 0; i < readBufferSize; i++) {
	            if (i % 2 == 0)
	                leftBuffer[i / 2] = buffer[i];
	            else
	                rightBuffer[i / 2] = buffer[i];
	        }
	        size_t wroteSize = lame_encode_buffer(lame, leftBuffer, rightBuffer, readBufferSize / 2, mp3Buffer, bufferSize);
	        fwrite(mp3Buffer, 1, wroteSize, mp3File);
	    }
	    delete[] buffer;
	    delete[] leftBuffer;
	    delete[] rightBuffer;
	    delete[] mp3Buffer;
	}
	
	//转码
	extern "C" void encode() {
	    if (singlechannel)
	        encodeSingleChannelPcm();
	    else
	        encodeDoubleChannelPcm();
	}