SDDS stands for Sony Dynamic Digital Sound, which is a cinema sound system developed by Sony. Digital sound information is recorded on both outer edges of the 35mm release print. 7 main channels (5 screen + 2 surround) plus a sub-bass channel are available, usually only 3 of the available 5 screen channels are used as most cinemas do not have the extra speakers installed. The digital track includes 4 fall-back tracks for occasions when the full signal is unrecoverable due to dirt or damage on the film.

Sony Official SDDS Website: