它是把四声道立体声在录制时通过特定的编码手段合成为两声道，即将原来的左声道（L）、右声道（R）、中置声道（C）、环绕声道（S）的4个信号，经编码后合成为LT、RT复合双声道信号，重放时通过解码器将已编码的双声道复合信号LT和RT还原为编码的左、右、中、环绕四个互不干犹的独立信号，经放大后分别输入左音箱、右音箱、中置音箱和环绕音箱。(It is the four track stereo recording by means of encoding specific synthesis is two channel, to the left and right channel (L) (R), middle channel (C), surround channel (S) of the 4 signals, after encoding for the synthesis of LT, RT double channel signal playback when the decoder will have encoding two channel composite signal LT and the reduction of RT to encoding the left and right, surrounded by four mutually independent signals and stem, enter the left speaker, right speakers, speakers and surround speakers after amplification.)