Beaver Installations: Learn "How Digital Televisions Works" | Formats, Resolution, Digital, Aspect ratio ...

How Digital Televisions Works

TV Goes Digital

The term "digital TV" is used in many different ways right now, depending on who you are talking to. There is also the term "HDTV" which is the most advanced form of digital TV in use in the Canada. The reason it gets confusing is because digital TV in Canada. combines three different ideas. This section examines those three ideas.

Digital Signal

The first idea that is new to digital TV is the digital signal.

Analog TV started as a broadcast medium. TV stations set up antennas and broadcast radio signals to individual communities. You can put a pair of rabbit ears on your TV and pick up channels 2 through 83 for free. What you receive, as described earlier, is a single, analog composite video signal and a separate sound signal.

Digital TV has started as a free broadcast medium as well. For example, in Toronto, Canada, you can tune in to about a dozen different commercial digital TV stations if you have a digital TV receiver and an antenna. The FCC gave television broadcasters a new frequency to use for their digital broadcasts, so right now each broadcaster has an analog TV channel and a digital TV channel. The digital channel carries a 19.39-megabit-per-second stream of digital data that your digital TV receives and decodes.

Each broadcaster has one digital TV channel, but one channel can carry multiple sub-channels if the broadcaster chooses that option. Here's how it works:

On its digital channel, each broadcaster sends a 19.39-megabit-per-second (Mbps) stream of digital data. Broadcasters have the ability to use this stream in several different ways. For example:

A broadcaster can send a single program at 19.39 Mbps.

A broadcaster can divide the channel into several different streams (perhaps four streams of 4.85 Mbps each). These streams are called sub-channels. For example, if the digital TV channel is channel 53, then 53.1, 53.2 and 53.3 could be three sub-channels on that channel. Each sub-channel can carry a different program.

Formats

The reason that broadcasters can create sub-channels is because digital TV standards allow several different formats. Broadcasters can choose between three formats:

480i - The picture is 704x480 pixels, sent at 60 interlaced frames per second (30 complete frames per second).
480p - The picture is 704x480 pixels, sent at 60 complete frames per second.
720p - The picture is 1280x720 pixels, sent at 60 complete frames per second.
1080i - The picture is 1920x1080 pixels, sent at 60 interlaced frames per second (30 complete frames per second).
1080p - The picture is 1920x1080 pixels, sent at 60 complete frames per second.

(The "p" and "i" designations stand for "progressive" and "interlaced." In a progressive format, the full picture updates every sixtieth of a second. In an interlaced format, half of the picture updates every sixtieth of a second.)

The 480p and 480i formats are called the SD (standard definition) formats, and 480i is roughly equivalent to a normal analog TV picture. When analog TV shows are upconverted and broadcast on digital TV stations, they are broadcast in 480p or 480i.

The 720p, 1080i and 1080p formats are HD (high definition) formats. When you hear about "HDTV," this is what is being discussed -- a digital signal in the 720p, 1080i or 1080p format.

Aspect Ratio

Finally, the HD formats of digital TV have a different aspect ratio than analog TVs. An analog TV has a 4:3 aspect ratio, meaning that the screen is 4 units wide and 3 units high. For example, a "25-inch diagonal" analog TV is 15 inches high and 20 inches wide. The HD format for digital TV has a 16:9 aspect ratio, as shown below:

The type of signal, format and aspect ratio have all changed in the process of converting from analog TV to digital TV in the Canada.

Digital Compression

The idea of sending multiple programs within the 19.39-Mbps stream is unique to digital TV and is made possible by the digital compression system being used. To compress the image for transmission, broadcasters use MPEG-2 compression, and MPEG-2 allows you to pick both the screen size and bit rate when encoding the show. A broadcaster can choose a variety of bit rates within any of the three resolutions.

You see MPEG-2 all the time on the Web, on Web sites that offer streaming video. For example, you will find that you can view streaming video at 56 kilobits-per-second (Kbps), 200 Kbps or 500 Kbps. MPEG-2 allows a technician to pick any bit rate and resolution when encoding a file.

There are many variables that determine how the picture will look at a given bit rate. For example:

If a station wants to broadcast a sporting event (where there is lots of movement in the scene) at 1080i, the entire 19.39 megabits per second is needed to get a high-quality image.

On the other hand, a newscast showing a newscaster's head can use a much lower bit rate. A broadcaster might transmit the newscast at 480p resolution and a 3-Mbps bit rate, leaving 16.39 Mbps of space for other sub-channels.

It is very likely that broadcasters will send three or four sub-channels during the day and then switch to a single high-quality show that consumes the entire 19.39 Mbps at night. Some broadcasters are also experimenting with 1- or 2-Mbps data channels that send information and Web pages along with a show to provide additional information.

Buying a Digital TV Set

If you go to an electronics store today to buy a new TV set, there are four types of sets that you will see on the shelf:

Analog TV sets

Digital-ready sets - They should be called SDTV sets. These TVs are normally 480p displays with an analog tuner (for the normal channels 2 through 83) built in. The problem with these sets is that their maximum resolution is the low 480p SD resolution, which eliminates the HD resolutions and makes the TV essentially useless in the future if you plan to watch HDTV programs.

HDTV-ready sets - These sets are essentially computer monitors able to display 1080i/p resolution in the 16:9 aspect ratio. They may or may not have analog tuners built in.

Integrated HDTV sets - These sets have a digital tuner for broadcast DTV signals integrated into an HDTV display. With the standards changing so much, you may end up paying for an integrated tuner that becomes obsolete.

The preferred way to handle HDTV is to purchase the components separately:

A 16:9 HDTV display capable of 720p and 1080i/p resolution

A digital receiver

An antenna

Since the HDTV display will be the most expensive piece and will likely last 10 years or more, buying the components in this way allows you to change the receiver if you need to.