- The source for television information in Juneau Alaska


What is HDTV?

High Definition Television, known as HDTV for short, is a relatively new television format that brings a wider-screen image with up to 6 times the picture sharpness of SD and other enhancements such as surround-sound audio.

SD (Standard Definition) TV (in the USA) is broadcast using the NTSC standard which has a vertical resolution of 525 "lines" of video. Meaning that the resolution of the final image is about 700 pixels across by 525 pixels high.

HDTV can be either broadcast with a vertical resolution of "720 lines" of video, a resolution of 1280 x 720 pixels. Or with a vertical resolution of "1080 lines" of video, a resolution of 1920 x 1080 pixels.

If viewed side-by-side at the same resolution (with the same "sharpness"), it's clear to see just how much more detail there is in the HD resolutions:
Comparison of SDTV and HDTV resolutions.
Comparison of SDTV and HDTV resolutions (Click for full size image).

If you click on the above image (which is shrunk down to 25% of the original), it will load at "full" resolution. You'll notice how big HDTV images really are, the image probably won't even fit on your computer screen!

Of course HD TV's aren't necessarily physically larger than SD TV's. Viewed on a (HD) screen of the same size, the HD images are sharper and appear to have more detail. Simply put, the HD pictures simply look "better". If you flip back-and-forth between a SD channel and a HD channel on a HDTV, the difference is clear.

You may have heard terms like "720p", "1080i" and "1080p" mentioned. The number of course relates to the number of lines in the signal - "1080" is about twice as sharp as "720". But what about the "i" and the "p"?

Video images can be captured, transmitted and displayed one whole frame at a time, or half an image - odd lines first, then even lines - at a time. Whole frames at once are called "progressive" video. Frames created from two "half frames", called "fields", are called "interlaced" video. This image shows how it works:
Comparison of interlaced & progressive frames.
Comparison of interlaced & progressive frames.

'i' Vs 'p'

NTSC SD video is always transmitted as interlaced. As the "i" is assumed, it's simply referred to as "525" unless otherwise noted. NTSC SD has a frame rate of 30 frames per second (fps), meaning that the image is actually 60 fields (of 262.5 lines) per second. As such the correct full technical designation is "525i60".

"720p" video, as the name implies is a progressive video format. It consists of 60 full frames (twice as many as SD!) every second. So the technical designation is 720p60. Double the frame-rate means twice as smooth video, which is great for faster action like fast sports or computer games.

"1080i" is an interlaced format. Just like NTSC, it's only 30 frames per second, again made of 60 fields. So technically: 1080i60. Though it's got half the frame-rate, it's got twice the resolution of 720p. This makes it better for slower but more detailed content. Most dramatic content - and all movies (shot on film) - are actually shot at 24fps, which being slower than 30fps, fits well into a 1080i60 signal.

1080p60 - the "holey grail" of HD video - has both the sharpness of 1080 lines, and the 60fps frame-rate (of 720p). This allows for maximum detail and the smoothest motion. The problem is that that's a lot of information! Too much for most TV equipment to process, and too much to transmit as a Digital TV signal. Because of this, 1080p60 is only used by some high-end computer game systems and other similar "non-broadcast" applications. Most TV's that can display any 1080 picture do so progressively and as such are sold as "1080p" TV's.

1080p24 is a bit of a strange one. Movies shot on film (and many Dramatic TV shows & commercials) are shot at 24 frames per second. The 1080p24 format allows for these images to be presented at their maximum resolution at the same frame-rate that they where shot at, this means a better quality image as there is no conversion necessary. Most modern 1080p ready TV's can also display the 1080p24 format. BluRay (for example) can carry a movie as a 1080p24 signal without having those extra 6 frames in there to "fill it up" to 1080p30 (1080i60) - this means that you can fit a longer movie onto a single disc! Due to compatibility issues, TV broadcasts don't make use of the 1080p24 format.

What you need to watch HDTV

There are 3 parts to the mix to watch HDTV, just like a chain, if you miss one, you won't be getting HD.

A HDTV is of course the first part. Though the resolution of the screen can vary, any TV sold as a "HDTV" can display all the HDTV signals. Of course the higher the resolution of the screen, the sharper an image it can give. Smaller TV's normally have "720 line" screens, they simply scale down 1080 line images to fit the screen, normally you can't see the missing detail - unless you have really good vision! Larger HDTV's normally have 1080 line displays, making use of all the detail of 1080 HD signals for the larger screen. All TV's can of course also display the regular SD images too by scaling them up to fill the screen - of course many leave black bars on the sides as the aspect ratio (shape) of the SD image isn't as wide as the HD images.

The next part of the puzzle is a HDTV receiver. All HDTV's have a receiver built in that can tune into free over-the-air HDTV signals, so if you get your TV through an antenna (on your roof, or even simple "rabbit ears") you have all you need built into the TV! If you have cable, you'll probably need a HD Cable box provided by the cable company. In Juneau you'll need the silver "HD-DVR" from GCI, contact them for details at 1 (800) 800-4800. If you have DishNet or DirecTV satellite TV, then you'll need a HD Satellite receiver, again call them for details. Or if you have a BluRay (or HD-DVD [a format that competed with and lost to BluRay]) player, then that acts as your "receiver" (player) for BluRay discs. Make sure that you also use the proper cables to hook up your HDTV to the HD receiver or player, the digital "HDMI" cable is the best way to do this, though you can also use "Component" cables to do the job. "Composite" (A yellow RCA connector) won't give you a HD signal - you'll probably see a down-converted SD image (which your TV then has to re-scale back up to HD for it's screen - ugly!.

The last part is actually getting the HD content. If you watch over the air, it's as simple as tuning into a Digital TV station broadcasting a HDTV signal. On cable or satellite, you may need to pay extra to get access to some or all HDTV channels. "Basic Cable" from GCI for example includes HD versions of some channels or networks already provided in SD on their Basic service. Or if you have a BluRay player as your "receiver", then it's time to rent or buy some BluRay movies.


There's a common misconception that all DTV is HDTV, this isn't the case. Analog TV is always the same format (525i60), because of this a analog TV can assume it'll always be getting the same type of signal: SD NTSC. So Analog TV is always SD.

However a digital TV (DTV) signal can include extra information to tell the TV what type of signal it is. Because of this, a DTV signal can be a variety of formats, SD or HD.

So not all DTV is HDTV, but HDTV can only be transmitted digitally. So HDTV is always DTV.