It can be, there's provision in MPGEG2, and of course it is on sat, and some (usually non UK) digital terrestrial boxes have a digital link, but it isn't currently in UK due to bandwidth limitations (it has to be separately transmitted, in addition to the existing stereo sound, which would mean dropping some other stuff, possibly a tv channel!).
However, many films made since 1973 (and most now) have Dolby surround encoded onto the analogue soundtrack, actually this info can't be removed! You can get a slight out-of-phase effect just on stereo speakers. Some decoders will correctly produce surround from this 2-channel signal. To experiment, you can cross-wire - reverse phase - 2 smaller rear speakers across the front 2, the "poor man's surround sound"! Not brilliant, but better than nothing.
I am currently feeding analogue stereo into an AV amp, so I am decoding Dolby Pro Logic. My question was whether digital audio offered Dolby Digtial or DTS as DVD. Anyway, I got an answer from the beeb, as follows for your info:-
The BBC, in common with most other European digital broadcasters, has no plans at present to support any Digital Surround Sound system. The digital multi-channel sound systems are very hungry of the 'bandwidth' available so it was decided not to include one when the European specifications for digital terrestrial TV were being drawn up. However, the stereo sound system chosen for Digital broadcasts (called MPEG-2) is capable of supporting the phase-encoded surround sound systems (e.g. Dolby ProLogic) in the same way that NICAM does on analogue TV. In the UK, only pre-recorded media such as DVD, or some commercial satellite channels have sufficient bandwidth to accommodate full multi-channel Digital Surround Sound.
Most films made since the early 1990s are therefore broadcast in surround sound although at the present time, few ordinary programmes are.
Yes! Currently, The Simpsons says "surround", and some other progs also. Better point out that I don't decode it myself, not something I particularly desire, so can't tell you directly!
The original Dolby surround is on some films from 1973 onwards (first - I think - was Ken Russell's "The Music Lovers" - I might be wrong!). But is standard now, films without it are rare.
Films from 1990s on are often in "Dolby spectral", still analogue, but with improved noise suppression, more closely approaching digital quality.
Note that, ther's often extra audio processing added by the tv companies, after the film analogue soundtrack and before transmission. Frequently a "noise gate", to suppress film hiss, but this can create strange "pumping", clip words off, generally sound odd, and completely muck up surround info if the 2 audio channels aren't "ganged together".
Sometimes, Dolby noise reduction decoding isn't done properly - or not at all - it can happen it's not switched in, or the decoder hasn't been lined up on Dolby Tone, etc, Then, you usually get muffled sound, although the surround info is still valid.
Dolby surround sound, as on The Simpsons and many other programs, isn't necessarily a digital sound process. The extra information is buried in the standard stereo soundtrack and can be extracted using circuitry developed by Dolby labs.
The first circuit dsigned by Dolby was Dolby 3 channel which could extract an extra rear channel. Dolby Pro-logic circuits could extract an extra 2 channels - a centre channel for speach and another for very low frequencies (the sub-woofer) In both these circuits the rear channel is the same on the right and left sides.
In Dolby digital the audio signals are encoded into a digital stream. There are 6 six separate tacks (front left, front right, centre, rear left, rear right and the subwoofer) although, because the subwoofer has a very limited requency range its known as .1 of a channel - hence Dolbt 5.1.
Dolby labs have produced many extremely clever circuits over the years such as the various methods of reducing hiss on audio cassettes and latterly some audio processing for films that can only be used in cinemas.
Note that MPEG2 has a provision for Dolby 5.1 - regardless of whether on DTT or satellite - so it could be transmitted! The point is, 6 (or 7!) discrete sound channels - even when compression is used - are very "bandwidth hungry".
(As so often, Wicky has a very good brief article, with links etc).
It's often said Dolby A nr was first used in films preparing "A Clockwork Orange" (actual location sound then being good enough for film soundtrack - later "lipsyncing" not required - so obviously cost effective and attractive!). Wicky says film soundtrack Dolby introduced 1975, though I thought The music Lovers had it (I might be wrong!).