The Digithetre is a standalone Dolby Digital decoder, amp and speakers.
It is suitable for smaller home set-ups and PC's, it also decodes Dolby Pro Logic for existing videos and alike. The quality is not as good as that of a dedicated system however it is not to bad for approximately £200 all in including an active sub and makes an ideal entry into the world of Dolby Digital.

There is also a DTS model avalable offering better speakers and sound.
There is a review of the DTS model on http://www.dvd.reviewer.co.uk/