In an effort to make movie soundtracks more dramatic and engaging, Dolby Labs created a signal encoding which encodes more than just two channels of audio onto the stereo signal.
Many popular receivers and home-theater systems include the
required circuitry to decode these signals. These components
are referred to as Pro Logic, Dolby Pro Logic, or Surround
Sound components. Very few audio recordings contain this
encoding, but it is very common with movie soundtracks and
some network TV programs.
Best Surround Sound reproduction requires five separate
speaker systems, but some improvement is claimed from a
surround sound receiver and three speakers over two speakers.
In its best implementation, surround sound will give a fuller
sense of being in the middle of the action. The quality of the
image is a function of the recording, the broadcast quality,
and the choice of reproduction components.