dolby ac - 3 and mpeg - 2 audio permit the transmission of audio signals in packet form , via more than two independent audio channels . if a reproduction device has only a two channel audio decoder an external multi channel audio decoder may be used for multi channel audio reproduction . in this instance , packetized signal is received by a first decoder and is decoded for a first mode of reproduction and applied to a reproduction device . the received signal is unpacked in the first decoder and stored in memory . the stored signal is reformatted for application to a second decoder for decoding and reproduction according to a second mode of reproduction .