What is High Definition TV, HDTV, DIGITAL TV, SDTV, EDTV & Their Relationships ?

To properly explain traditional analog TV, and EDTV vs HDTV, we need to step back a bit. Traditional TV has 525 lines of video, which are interlaced. Interlacing just means that every frame of video is split into two fields and each field is shown alternately. Although there are 525 lines in the signal, only 480 of those lines contain video, so it is sometimes referred to as 480i (480 lines interlaced). This is known as Standard Definition TV, or SDTV. While this has worked well enough for over fifty years, as TV sets have gotten bigger and projectors more popular, the quality deficiencies of this format have become increasingly apparent.

The ultimate answer to improving the quality of the image displayed on your TV is HDTV, however, as a step along the road towards HDTV, the industry came up with Enhanced Definition TV, or EDTV. This system contains the same 480 lines as SDTV, but they are instead a progressive scan. Hence EDTV is also known as 480p. Progressive scanning just means that instead of splitting the signal into two fields and showing half the lines at a time, all 480 lines of video are shown simultaneously. This results in a noticeable improvement in the quality of the video displayed.

The bad news in relation to EDTV vs HDTV is that you need a completely new TV or projector to display both EDTV and HDTV. Most DVD players on the market today are progressive scan, meaning that they output EDTV signals, however, in order to view the progressive scan picture in all its glory, you need a TV or projector capable of displaying it and that, unfortunately, means getting a new TV.

In terms of content, there is very limited TV content currently available in HDTV format, and there is still some wrangling about the standards for HD DVDs. However, every DVD disc on the market can be displayed in EDTV right now. As consumer acceptance of HDTV has increased so too the programming availability has been increasing and the prices for HDTV sets have been declining.

What is the bottom line result? Almost every HDTV projector or TV on the market today will display EDTV as well. So while there may not seem like a great deal of point in getting a new display for the limited HDTV content that is available today, it is worthwhile to watch your DVD programming collection in EDTV, the difference between SDTV and EDTV really is rather dramatic.

In the past this sort of resolution was considered to be remarkable even amazing, but today it is rather lackluster. The lowest computer monitor resolution in common usage today is 640x480 pixels, and most new computers offer 800x600 or even 1024x768 as a standard. We have come to expect that degree of high resolution, sharpness and brightness as a standard for our computer displays and ANALOG TV can not even come close in emulating that performance.

Today, much of our TV and Radio programming is relayed around the world by satellite linkages or it is made portable and transferable by recording the images and sound on DVD's. What these systems have in common is the reliance on DIGITAL encoding schemes for recording as well as, transmission. Often this digital information/material/programming must be converted to analog format in order to display it on the traditional analog television sets. The transmitted and converted images look great when compared to say images fro a VHS tape, but they would be dramatically better if the conversion to analog format had never taken place. There has been a move underway for a number of years to convert all TV sets, programming and transmission to DIGITAL format.

When people discuss or refer to Digital Television or DTV what they are talking about is the transmission of pure digital television signals, along with the reception and display of those signals on a truly digital TV set that totally avoids the need for any digital/analog conversion. The digital signals might be broadcast over the air, or be transmitted by a cable or satellite feed. When those signals arrive at your home or office, a decoding device such as a set top box, receives the signal and uses it in digital format to directly drive your digital TV set. Alternatively, the digital signal received by the set top box can be converted to an analog signal and be displayed through a conventional analog television set.

HDTV requires new production and transmission equipment at the HDTV and the capital costs for such conversions have led to some foot dragging in terms of making such conversions. As well, new equipment for reception must be purchased by the consumers or. The higher resolution picture, the rectangular movie screen picture format and Dolby Digital Surround Sound are the main features or selling points for HDTV. Imagine 720 or 1080 lines of resolution as compared to the 525 lines in North America and the 625 lines currently in use in Europe. This makes for a rather dramatic difference in picture quality.

Of the 18 DTV formats that are currently under consideration, six are HDTV formats, five of which are based upon progressive scanning and one on interlaced scanning. Of the remaining formats, eight are SDTV high-aspect ratio or wide movie screen formats (16:9), and four conventional formats with 4:3 aspect ratios, and the remaining four are Video Graphic Arrays (VGA) formats. Stations are free to choose whichever formats they will elect to broadcast.

There is one class of digital television that is defined by a set of industry standards that is called high-definition-television or HDTV. HDTV is a high resolution, digital television format (DTV) that is combined with Dolby Digital Surround Sound (AC-3) into an industry prescribed format. HDTV represents the highest DTV resolution in all the new set of standards and it uses a high aspect ratio for its display.

• 720p - 1280x720 pixels progressive
• 1080i - 1920x1080 pixels interlaced
• 1080p -1920x1080 pixels progressive

The terms "Interlaced" and "Progressive" refer to the methods of scanning or scanning system. In an Interlaced format, the screen displays every odd line at one scan of the screen, then follows that up with the even lines in a second scan. Since there are 30 frames shown per second, the screen shows or displays one half of the frame every sixtieth of a second. For smaller screens, this is less noticeable. But, as screens get larger, the problems with Interlacing manifests as "flicker".

By contrast, Progressive scanning displays the whole picture, every line in a single showing, every sixtieth of a second. This delivers a much smoother non-flickering picture but it uses significantly more bandwidth. Broadcasters are having to squeeze the increased picture detail and higher quality surround sound into the same 6MHz bandwidth that was historically used for traditional analog television signals. Compression software, very similar to that used on personal computers allows this to be accomplished.

Digital TV relies on a compressions and encoding scheme known as MPEG-2 to fit its stunning images and surround sound into a reasonable amount of bandwidth For each image, the MPEG-2 software records or processes only a portion of the total image, but sufficient that the human eye does not detect that some detail is missing. In subsequent frames, the software only detects and records the changes to the original image and it leaves the rest of the image unchanged from the previous frame. This digital compression and encoding reduces the amount of data to be displayed by a ratio of 55:1.

MPEG-2 is already the industry standard for video on DVD's and for some satellite TV broadcasts. Compression reduces image quality somewhat below the quality that would be observed with say a digital camera in a studio setting. But, MPEG-2 is great at selectively discarding image details that are most often ignored by the human eye anyway. The quality of the remaining simplified image is still very good and is significantly better than that for the traditional analog TV systems.