So in Australia we’ve all had to change to Digital TVs or Digital Set Top Boxes. The upgrade has provided a range of benefits but at a cost. In this article I will try to explain in simple terms just what the change is all about and how it works.
Analogue TV
The traditional type of TV broadcasting we’ve had in Australia was analogue TV. Analogue TV is based on a radio signal that traces the picture out line by line, with the signal varying by colour and brightness of colour. The result is a picture on your TV screen. Because the radio signal directly controls what’s on the screen with very little processing in between, any interference affecting the radio signal directly affects the picture. This can be evidenced by lines or speckles in the picture. A weak signal is more susceptible to interference and will therefore gradually become snowier and snowier as the signal gets weaker.
Digital TV
Digital TV differs from analogue TV considerably. Instead of sending a signal that directly controls the picture on the screen, Digital TV is sent as a radio signal that carries a stream of binary numbers, 1s and 0s. These 1s and 0s digitally describe the image and sound as well as other information about the TV stations. The type of encoding is basically the same as that which is used on the internet, except it’s one way.
A signal sent in this way requires computer processing to convert into the pictures that are drawn on the screen and the sound you hear. The transmission protocol also contains error correction information called Forward Error Correction. In Forward Error Correction, multiple copies of the picture and sound are sent at different times. The whole picture is effectively sent twice, a fraction of a second apart. So if part of the information required to make the picture is lost due to radio noise, the TV can recover that information when it’s sent a second time a fraction of a second later. The TV handles this automatically so unless the radio signals are really bad you never notice the problem.
Consequently in a location where the signal is really bad on analogue TV, the picture can be perfect on Digital TV. In fact if you have enough signal quality for Digital TV to work, your picture will be perfect. Digital TV either works perfectly or it doesn’t work at all. This phenomena is know as the “Digital Cliff”(see diagram). When you are close to the Digital Cliff a slight disturbance to the signal such as radio noise, lightning or electrical appliances turning on will drop you over the cliff and the picture will “break up”. If you signal quality is past the cliff your picture will regularly break up or be entirely unwatchable.
This may seem like a bad thing, but in practice in locations just above the Digital Cliff, analogue TV would be almost unwatchable. So in almost all circumstances Digital TV’s perfect picture will far surpass analogue TV. The key to good digital reception is to just ensure that there are no faults with your TV aerial, cabling and fly leads because they will intermittently drop you below the cliff.
In another article I will discuss the various Digital TV standards and what they mean.