The biggest difference with digital TV audio is that it can be used to broadcast full-fledged surround sound. Analog broadcasting can only broadcast a stereo signal, in which the sound is carried by two channels. This can be used to carry Dolby Pro Logic signals, in which there is only one rear, or surround, signal, and no specific signal to a subwoofer. A stereo signal can also be turned into Dolby Pro Logic II, which offers full 5.1 channel surround sound, but this is created by the listener's equipment.
Digital TV audio can carry Dolby Digital signals, which offer the full 5.1 channels. This means a center, left and right signals at the front, separate left and right surround channels, and a dedicated subwoofer channel. Stations can then can broadcast the same sound quality which you would get by watching the same programming on a DVD. As with watching a DVD, you will need a surround sound decoder and speakers to enjoy the full surround sound experience. This equipment is often marketed in a package along with a DVD player as a “home cinema system."
To receive Dolby Digital, you will need to connect your cable or digital terrestrial box to your surround sound system through a digital connection. There are two leads available for this. One is known as optical and transmits information as light signals. The other is known as coaxial, or simply digital, and transmits in the same way as most other TV cables.
Which cable is best for your needs is debatable. In many cases you will be limited by which sockets are available on both your surround sound system and your digital TV box. If you have both sockets available, you may find optical works better if you have a lot of audio-video or power cables nearby which could cause interference. However, optical cable is more likely to suffer performance problems when it is bent around corners. Generally you should aim to use the shortest lead which will work without being stretched or twisted.
You may find the only digital TV audio option on your box is through a standard stereo lead socket. This will allow you stereo and Dolby Pro Logic reception, but not full Dolby Digital. You may consider exploring whether you can upgrade to a different box if you want the best possible digital TV audio. Whether this is worth paying extra may depend on what programming you watch most often. Generally movies and some sporting events are most likely to be broadcast in Dolby Digital.