Understanding High Definition TV

High definition TV, commonly known as HDTV, is the highest state in television picture quality evolution. Actually, revolution and not evolution is the right word to use because since the standardization of high definition TV, television picture format remained virtually unchanged for nearly 50 years.

There are six different high definition TV picture formats.

High definition TV (HDTV) is digital, but it is not to be confused with DTV, which stands for digital television. The Advanced Television Systems Committee (ATSC) has standardized 18 formats for digital television, 6 of which are high definition TV formats. The high definition formats are of the highest resolution among the 18, and therefore support the largest number of pixels per picture. Of the remaining 12 formats, 8 are SDTV (standard definition TV), which are digital formats whose resolution is comparable to regular (non-digital analog) television, and 4 are VGA (video graphics array) formats, which are compatible with computer video.

The following table summarizes the 18 DTV formats:

The Format Type column indicates which of the three DTV types the format falls under, i.e. HDTV, SDTV, or VGA.

Resolution indicates the dimensions (width x height) of the picture, not as physical size but rather as number of pixels (the smallest color elements in a TV picture). If you multiply these numbers you get the total number of pixels on a screen. For the best high definition TV picture, the number of pixels is 1920x1080 = 2,073,600. For standard definition (both analog and digital), the number of pixels is 704x480 = 337,920. Therefore the high definition TV picture is about six times more detailed than standard TV.

The Aspect Ratio column shows the relative physical dimensions of the picture. Until DTV was introduced, a 4:3 ratio was the only option. The 16:9 ratio is the wide screen format and is the only one available for high definition TV. SDTV supports both the 4:3 and 16:9 aspect ratios. The wide screen 16:9 ratio is closer to how movies are shown in the theaters. The advantage of wide screen is that it better represents how humans perceive the world. Our field of vision is wider than it is tall. Because of that, a 4:3 television picture tends to look framed and confined much more so that a 16:9 picture.  Note that the SDTV picture dimensions require the pixels to be rectangular and not square, and that's because 704/480 does not equal 4/3 nor 16/9.

The Scan column indicates the scan rates and interlacing options available. A scan rate is the number of pictures, also called frames, shown per second. The interlacing option can either be i (interlaces) or p (progressive). In interlaced scanning, any picture is not shown all at once. Instead, every odd scan line (a row of pixels) is shown first, and then every even scan line is shown, in an alternating fashion. The rapid switch between the displayed lines appears to the eye as a whole picture. In progressive scanning every picture is simply displayed all at once. The interlacing technique came about as an effort to reduce picture flicker using slower and cheaper hardware, especially in older TV display technologies. In fact, the analog NTSC TV standard is an interlaced standard.

The numerous DTV formats offer different bandwidth requirements and compatibility with various television technologies. Digital TV stations are free to choose which formats to broadcast in. The higher resolution and faster frame rate formats require more bandwidth, i.e. more data bits to be transmitted per second.  Therefore the lower and slower formats leave room for broadcasters to transmit other information within the limited bandwidth allocated to them. This information may be whole other TV channels. A station might choose to broadcast multiple low quality channels during the day, and then unify its program for a single high quality broadcast during prime time hours.

Any given television set or HDTV receiver box might also support all or some of these formats. Furthermore, a TV that's capable of receiving a high definition TV signal might not necessarily be capable of displaying it as a high definition picture. That is, it will show a picture, but transform it to a lower resolution it is capable of displaying. These are important factors you should be aware of while purchasing a high definition TV set. Most importantly, if you want true high definition TV, be sure that the set you're considering has a physical display resolution that is at least as great as the HDTV standards.