Hello DTV pals!
All audio delivered in a digital television transmission, whether via terrestrial, cable or satellite, is encoded into a Dolby Digital data stream and decoded through your set-top box or external decoder or home theatre integrated amplifier. Just remember that Dolby Digital and "5.1" are not the same thing. Dolby Digital is scalable and can carry a single mono program as well as a 5.1 presentation, and anywhere in between.
In the case of a full blown home theatre 5.1 presentation, the vast majority of providers are performing a "pass through" of a network feed, since local affiliates have limited ability to create their own 5.1 audio content. While the Dolby Digital professional encoders and reference decoders are not that expensive for a local affiliate to buy, other equipment like HD recorders, file servers, master control switchers, etc. are very pricey and out of the question for most local broadcasters at this point. That's a big reason for the lack of HD programming. Rest assured however that if you are receiving a 5.1 Dolby Digital broadcast either through a cable, satellite or terrestrial transmission, the broadcaster is using a Dolby Digital professional encoder (Model #DP569) and reference decoder (Model #DP562).
In nearly all cases when receiving an upconverted broadcast, the audio will be a stereo (2/0) Dolby Digital data stream. Dolby Digital also carries what's called "metadata" that allows the broadcaster to flag or identify this stereo broadcast as a Pro Logic encoded stereo presentation. When this flag is received by your decoder at home, it automatically performs a Pro Logic decode on the audio after the Dolby Digital data stream has been decoded to baseband digital audio. In this way you will receive and enjoy a surround broadcast, even though it may not be full 5.1 theatrical presentation.
There are a host of other parameters that encompass the library of flags and controls that are collectively called "metadata". You can find more comprehensive explanations of metadata on our web site at
www.dolby.com.
Best regards,
Mike Babbitt
Dolby Laboratories