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Community Newsletter: Q&A forum: Help me untangle the HDTV technical gobbledygook

by: Lee Koo (ADMIN) July 12, 2007 4:45 PM PDT

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Help me untangle the HDTV technical gobbledygook

by Lee Koo (ADMIN) ModeratorCNET staff - 7/12/07 4:45 PM


I keep getting bombarded with ads about HDTV. When I go to a store that carries them (for example, Best Buy or Costco) I find that the clerks really don't know much about them. I came across one that looks interesting, but the description is nothing if not confusing. Perhaps someone can help me untangle the technical gobbledygook. For example: the set I was looking at is a 42-inch HD LCD set with a resolution of 1,920x1,080. It is compatible with 480i, 480p, 720p, 1080i, and 1080p. I think i means interlaced (like my analog TV set) and p means progressive. What is the significance of these designations, as a practical matter? Is i or p defined by the TV station or is that a choice of the receiver? It is said to be compatible to NTSC (I recognize that), ATSC, and QAM (what are these?), and are there other systems out there that need to be covered? What is Media Connectivity? Someone needs to publish a document that ordinary mortals can read and understand. Any suggestions?

Submitted by: Nat C.

This answer was voted most helpful by our community members


De-Mystifying HDTV

Wow! A Lot of questions! Let's start by looking at those confusing HDTV definitions and then we'll look at how the signals are received from your DVD player or Cable box.

Basic, old fashioned TV (the kind you might have bunny ear antennas behind) are 480i. The 480 means that there are 480 lines of "light" hitting the screen of your TV from behind (counting from bottom to top). Now let's call the very bottom-most line, line 1, the one above line 2 and so on, okay?

The i indictor, you are quite right, means "interlaced". The problem with old analogue TV signals is that they can't carry very much data at one time (they have a small "bandwidth"), which makes it difficult to reliably get 480 lines of data to your TV at once. Instead, they actually only broadcast half of the signal, (lines 1, 3, 5, etc) and then right after that the other half (line 2, 4, 6 etc) in a separate transmission. If your TV keeps alternating the picture between odd and even lines fast enough, you don't see much of a difference. It is therefore interlacing the two separate pictures of 240 line each.

naturally this means that the other type of indicator, p (which stands for progressive), simply means that the device is showing you all of the lines all of the time. That is to say instead of updating lines 1, 3, 5 and then 2, 4, 6, it updates 1, 2, 3, 4, 5, 6, which makes for a much smoother looking picture, especially when your are watching something with a lot of fast movement like an action movie.

The screen resolution will also tell you the number of lines on the TV screen (in your example of 1920 x 1080 this would be a resolution of 1080 lines).

There are three "defintions" for TV types, Standard Definition (SD), Enhanced Definition (ED) and High Definition (HD). SD simply means 480i (480 lines, not all shown at once). ED means 480p (480 lines, all seen at once). You will very rarely see ED as a TV feature anymore, and I think most people on this board would strongly recommend against even looking at an ED set. So, anything that's left (anything with more than 480 lines) is considered HD.

Therefore because progressive is the best way to deliver a signal and 1080 lines is the highest number of lines in use today, a 1080p TV will future proof you and provide the highest quality picture options. These are getting so cheap now that unless you are looking for a real budget unit I wouldn't buy anything else.

For a long time LCD and Plasma HDTVs only came in 720p (unless you had LOTS of money to burn), but more and more 1080p sets are now out there, and at very reasonable prices, almost all projection TVs produced new now are 1080p.

Now let's move onto how we get that HD picture to your TV. Just like music, your picture quality will only be as good as your weakest component. If you are listening to an old audio cassette, it doesn't make much difference how expensive your sound setup is, you are not going to get great quality music. The same is true of TV.

People generally get their TV one of 3 ways, Cable, Satellite or the Free over-the-air kind.

All of these ways of receiving TV offer HD content (they broadcast a digital signal that can carry HD information, separate to the analogue signal that older TVs pick up). Most cable and sat providers can rent you an "HD Box" that will allow you access their HD content.

They may broadcast some shows in 720 lines, but most now come in 1080i. You should beware that in order to "save space" both cable and satellite providers compress their HD signals. Decompressing these signals for you to see is what their "HD Box" is doing (just like ZIPing a computer file). You will inevitably lose some picture quality due to this compression process but for most people the difference is minimal. Just a side note, an HD satellite signal is typically less heavily compressed than its cable cousin.

You can watch a 1080 signal on a 480 digital set if you want but you will of course lose some of the detail. Likewise, you can watch a 480 broadcast on a 1080 set. In this case your TV actually has a small "brain" inside it, which creates new lines to make a full 1080 image (it looks at the colors above and below the line it is creating and guesses what should go in the middle). This process is called "up-scaling". If you are going to be watching a lot of regular DVDs (which are in 480p as long as you have a "Progressive Scan" DVD player) then how well the TV up-converts should be a key question you want answered before you buy.

You dont need to worry at all about NTSC versus ATSC versus QAM. NTSC is the name given to the way US broadcasts manage color in the picture. ATSC is simply the name of the council of people who set the rules for HDTV (so that you can buy any brand HDTV and it will work with any HD signal) and QAM is simply a way to modulate the signal to fit more data into the same signal. All HDTVs will use QAM and comply with ATSC.

Finally your question on "Media Connectivity". This simply means it has a lot of plugs in the back and front... that's it. It means you can connect it to a standard digital connection with a Digital Coax Cable, or through an HDMI cable or to a computer with DVI etc. Often you can plug a USB thumb drive or your digital cameras memory card right into the TV to view pictures on them as well....

Although 1080p sets can play up to 1080p signals, there are no broadcasts out there in any definition better than 1080i right now (and due to bandwidth restrictions, there wont be for some time yet). However your 1080p ability comes into play with the new High Definition DVD formats (HD DVD and Blu Ray DVD). These both send out a 1080p signal, for the best picture quality possible.

In short, when you are in the store, look at any 1080p TV. If you are looking at it in a store try to see if they can show you a standard 480 signal on it as well as HD to see how it handles both types of signal.

I hope this helps!

Submitted by: gingaskunk

If you have additional advice for Nat, let's hear them! Click on the "Reply" link to post. Please be detailed as possible in your answer. Thanks!

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