What is HDTV?

High Definition Television (HDTV) does not have any enforced set of standards, so the term itself is rather generic.  It is often used as a marketing buzz-word and does not guarantee any particular set of features or level of performance.  However, in most cases, when you hear the term High Definition (or simply HD), it means the product or service is able to do "better than traditional TV resolution".

Our traditional TV system, sometimes called Standard Definition TV (SD or SDTV), has kept essentially the same image structure ever since it was introduced in the 1940s.  The resolution of SD television is said to be approximately 704 x 480 pixels.  Although analog signals do not technically have a fixed "resolution", this value can be estimated based on the information carrying capacity of the video signal.  So anything capable of resolutions significantly higher than 704 x 480 pixels is usually classified as HD.  There are many variations of this, but the most common ones you'll see are:

Resolution	Classified as	Other names or equivalents
704 x 480	SD	DVD, NTSC, 480i, 480p
1280 x 720	HD	720p
1920 x 1080	HD	1080i, 1080p

The term HD can refer to both the display (e.g., a TV or monitor) as well as the media sources (e.g., satellite, cable, broadcast, video discs, game consoles, etc.).  It is important to remember that YOU NEED BOTH a high definition source and a high definition display to actually see anything in high definition.

 

Is Digital the same as HD?

No.  High Definition describes the resolution of an image, while Digital describes how the information is being processed or transported.

For example, a normal movie DVD is a digital media source, but it is not High Definition.  The video data is stored on the disc in a digital format, but since the viewing resolution of the movie is normally 704x480 pixels, this does not count as HD.  Even if the video is "upscaled" to higher resolutions, it still doesn't really count as HD because the image detail is limited by the resolution of the source material.

Some digital media is capable of both SD and HD content.

Broadcast digital TV, also known as ATSC, is capable of sending both SD and HD content.  Most of the major networks now broadcast a good amount of HD content, especially for popular shows, news programs, and sporting events.  However, there are still a lot of shows (mostly older ones) that were not produced in HD format and therefore come in SD format when aired.

Almost all HD sources are digital these days, but not all digital sources are HD.  The old NTSC standard has not been updated to support HD resolutions, so chances are that if you're getting HD content, it's coming from some kind of digital source (like ATSC, Blu-ray, digital cable, satellite, etc.).

 

Are there differences in HD Quality?

Yes, there can be.  Not all HD sources are created equal.  HD generally only refers to the resolution (number of pixels) used to display a picture.  However, there are many other factors that determine the overall picture quality that you see.  It's possible for a picture to degrade very badly even when it is being displayed at high resolution.

Some of the more common visible differences include things like "motion artifacts", "macro-blocking" (aka., "pixellation"), or "washed-out colors".  These are mostly caused by the video compression schemes used to send the HD content.  In order to reduce the data size of a video stream, the video compressors are told to discard details below a certain threshold.  If those settings are tuned very agressively, you may end up with visible degradations in the picture, even when displayed at HD resolutions.

Each method of HD content delivery has to make their video streams fit within their available transport channel.  For example, ATSC television signals are limited to the capacity of their broadcast channel (~19.39 Mbps).  Satellite and cable operators are generally even more constrained because they need to stuff lots of channels (from a few dozen to a few hundred) in their fast, but limited delivery pipes.

There are also situations where video streams get "re-compressed" for delivery.  For example, some cable or satellite operators offer "local" channels by taking the already compressed over-the-air broadcasts from local stations and re-compressing them into a format that's compatible with their delivery system.  When this happens, the resulting image will have the degradations caused by both levels of video compressors.

As a general rule of thumb, the best quality HD content will come from the least compressed sources.  Blu-rays (or HD-DVDs) will usually have the best picture because they have the fewest bandwidth constraints and can preserve the maximum amount of information.  Next, original content from over-the-air network TV broadcasts (news, sports, and shows they produce themselves) usually ranks second.  Some dedicated HD channels from cable and satellite (e.g., HBO, ESPN, Discovery, etc.) might tie for second.  Re-compressed local TV stations or other "lower priority" HD channels on cable or satellite often end up ranking third in quality due to visible over-compression.

Quality itself is a very subjective matter, so what really matters is how good the picture looks to you.  HD almost always looks better than its SD equivalent (except in rare cases when the compression artifacts really get out of control), but if you're not completely wow'd by HD, you might want to check the source.  The term HD spans everything from "slightly better than SD" to "jaw-dropping" picture detail and richness if you've got the right source.