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HDTV Information

Most digital TV (DTV & HDTV) on the air today comes in three formats;
480i, 720p, and 1080i. Here is an explanation:

The transmitted image is 720x480 which is reduced to 704x480 due to 16 lines of horizontal sync/blanking.
This is further reduced to 640x480 (in other words 640 pixels wide by 480 pixels high) because the displayed aspect ratio is 4:3
The screen is 704x480 or 528x480 (in other words 704 or 528 pixels wide by 480 pixels high).
This is the lowest resolution available with digital TV. 480i is "DTV" not "HDTV" which means all basic DTV televisions (and all HDTV sets) are able to display it.

480i is designed as the digital replacement for standard analog TV. This is because analog TV has approximately 480 visible lines out of the 525 lines ransmitted (the non-visible lines carry close-captioning, etc).

480i looks ok on a very small screen but becomes noticeably "pixelly" (grainy) on larger screens. One advantage of 480i is it uses minimal bandwidth compared to other DTV/HDTV modes, which means that a single TV transmitter can fit five 480i streams channels) inside its signal.

The "i" stands for interlaced. This means the odd-numbered lines are sent as a "frame" followed by the even-numbered lines. The reason for this is to save bandwidth because only 240 lines have to be sent at a time. Interlacing works because the human eye usually doesn't notice it, well unless there are a lot of fast-moving objects on the screen which will cause some noticeable image "artifacting".


A 720p screen is 1280x720 (1280 pixels wide by 720 pixels high).
720p is considered "HDTV" which means a basic DTV set will not display it.
The "p" stands for progressive scan. This means the entire screen is sent (redrawn) for every frame. Basically the opposite of "interlaced" described above.

A 1080i screen is 1920x1088 (1920 pixels wide by 1088 pixels high).
Eight pixels are blanked out which is why its called 1080i instead of 1088i.
This is the theater-style HDTV that looks great !

Now someone might ask why TV stations use 1080i instead of 1080p. There are two answers I can offer for this.
First, 1080p would take twice the bandwidth of 1080i. This would mean all of the TV stations bandwidth would be consumed with a single 1080p
By using 1080i the station will have enough remaining bandwidth to transmit additional video streams, typically one or two 480i channels.
Second, it doesn't offer much performance over 1080i other than minimization of image artifacting.

Here is a chart which shows the difference in screen resolutions:

There are a few tradeoffs to be made when sending DTV/HDTV signals because there is a finite amount of available bandwidth.

Here is a chart which shows an example of bandwidth usage:

Channel 7 is a standard analog TV channel. The little numbers "174" and "180" represent the frequency boundaries of the channel (174-180 mHz).
You'll see there are three main components to an analog TV signal; a large AM video carrier, a small colorburst subcarrier, and an FM stereo sound carrier.
Notice how the AM video carrier appears lopsided ? This is a due to a bandwidth-saving method called Vestigal Sideband or VSB.
A normal AM signal is DSB (double sideband) where the lower and the upper half of the signal are identical and symmetrical.
With VSB we suppress most of the lower half to conserve bandwidth since the information carried there is redundant.
With analog note that we can only fit one TV signal within the 6mhz channel space between 174-180.

Channel 8 is an HDTV channel. There are two HDTV streams available on channel 8; channel "8.1" which is 1080i and channel "8.2" which is 720p.

Channel 9 is a DTV channel. There are five 480i DTV streams available to watch. Notice how the DTV channels are numbered "7.1" thru "7.5" ? With DTV/HDTV the broadcaster can assign channel numbers other than the actual RF channel. For example a TV station could have their main analog signal on Channel 2 and put their digital channels on Channel 2.1 & Channel 2.2, however their signal that carries Channel 2.1 & 2.2 might be RF channel 18.

There are a zillion ways to mix-n-match digital TV. Some broadcasters run a 1080i HDTV picture along with two 480i DTV pictures, while others might run 720p + 480i or perhaps multiple 480i pictures like in the above example. One thing I didn't touch on is compression, where the broadcaster can increase the digital compression to decrease bandwidth usage. The tradeoff here is that picture quality will suffer if too much digital compression is used.

The above diagrams are illustrative. In reality the DTV/HDTV signals are multiple MPEG-2 datastreams which are evenly distributed across the channel bandwidth due to Trellis coding and randomization.

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