by January 2, 2003 0 comments

Very few people question how a TV set shows their favorite TV programs–they are simply content with watching these programs. PCQuest readers don’t generally fall in this category. Most of you would know how a TV works. So, we have something new: how HDTV (high-definition television) works. 

Before we talk more of HDTV, let’s see how DTV (digital TV) works. Digital signals are transmitted over air or through cable or satellite, and on your end, a decoder (receiver) receives and deciphers the signals and, of course, the digital TV displays the signals. 

Because the transmission is digital, the strength of the signal, terrain, weather conditions, etc that can effect the transmission for regular TV are not of concern here. So, you don’t have snowy or distorted pictures that you sometimes get on traditional TVs that most of us have grown up watching. Moreover, in traditional TV, as you move away from the transmission center, your picture quality slowly deteriorates. This is not the case with DTV. Here, you get perfect picture until the signals get too weak, at which point the picture completely disappears. So, there’s no scope for snowy pictures. 

1/3rd times
wider than

Now, let’s see where HDTV fits in the realm of DTV. There are various types of DTVs (there are 18 ATSC–Advanced Television Standards Committee–approved formats for DTV). HDTV is one of them. It is a high-resolution, wide-screen TV based on digital technology and digital surround sound, making the experience of watching HDTV comparable to watching a movie in a cinema hall. 

Superior resolution
The sharpness of an HDTV image comes from its resolution, which is twice that offered by NTSC (used in the US) or PAL (used in India and Europe) formats, making the picture twice as sharp. HDTV has 720 or 1080 lines of resolution compared to 525 lines for the NTSC format and 625 lines for PAL. 

What are 720 or 1080 lines? The image you see on a regular TV is actually a series of horizontal lines, which, after they are caused to glow by energy beams, create the images that you see. These lines can be displayed either as interlaced or progressive scans. An interlaced scan shows every odd line on the screen at 1/60th of a second, and every even line in the next 1/60th of a second. The problem here is that as the screen size increases, the images begin to flicker. A progressive scan, on the other hand, shows all the lines at one time, giving a much smoother picture. The problem here is that this requires more bandwidth. Now, HDTV can use either of the two formats in the order of 1080 interlaced or 720 progressive lines. It is up to the broadcaster to decide which format he wants to follow. For example, ABC uses 720 while CBS uses 1080. At your end, you must ensure that your HDTV set is capable of viewing both signals. Also, an HDTV image contains six times more pixels than a regular TV image, providing a picture that is much more crisp and detailed. How does it accommodate so many more pixels? The pixels in an NTSC TV are rectangular, while those on an HDTV are squares, like in computer monitors. These squares are also much smaller and placed closer together, such that there are 4.5 HDTV pixels in the same space of one
NTSC pixel.

Wider picture
Ever wondered why a Cinemascope or VistaVision movie makes for a richer viewing experience? Because it is wide format–since our field of vision is more rectangular than square, wide-screen images fill more of our field of vision. Well, the same concept applies to HDTV. 

An HDTV screen’s aspect ratio (the ratio between the width and the height of the screen) is 16:9. Compare this with NTSC’s 4:3. DTVs can, however, have as aspect ratio of either 16:9 or 4:3. (But, remember that aspect ratio is not the same as screen size, which is the screen’s diagonal measurement.)

Here’s an interesting fact: the 4:3 aspect ratio was first developed in 1889 by WKL Dickson while working in Thomas Edison’s lab. It is been used ever since. 

CD-quality sound
Now listen to what sound HDTV has to offer. Dolby AC-3 encoded digital sound with 5.1 channels of discrete digital sound–front (left, center, right), back (left, right) and a subwoofer (this is the .1 channel). These channels are also discrete, meaning they are completely separate from one another. Each channel plays across the audible spectrum from 20 Hz to 20,000 Hz. Compare this with your regular TV’s low-range analog sound, and your
sensibilities swerve towards HDTV’s movie-theater like sound experience. 

MPEG-2 and bandwidth 
Better video and sound require more bandwidth. In the US, analog TV transmission is over 6 MHz bandwidth. It is here that HDTV broadcasters face a problem–they have to squeeze their content into the existing 6 MHz. So, what do they do? They use MPEG-2 to compress the video, and at your end, your HDTV decodes the signal and displays the picture. 

MPEG-2 is not entirely perfect; what you see on your HDTV is not exactly what the digital camera would have captured, though it is many times better than an image on a regular TV–MPEG-2 can reduce the amount of bits by about 55 to 1. In Japan, however, HDTV transmission is over 20 MHz of bandwidth. 

Juhi Bhambal

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