Television is ancient technology trying to stay compatible on a modern world.
The first television processes date back to the early 1900s and were perfected by the 1940s use tube technologies.
A tube is like a light bulb. Most people still use “tubes” for their TV set and computer. That big boxy screen you watch that comes packed in a big box. That’s called a Cathode Ray Tube or CRT. It was invented around 1915.
Once upon a time that who box had lots of tubes inside of it, but with the transistor revolution of the 1960s and later the computer “chip” revolution of the 1980s these smaller, cooler devices replaced tubes inside of TV sets.
The CRT works by moving a beam of electrical charges across the face of the screen to illuminate a chemical. You can draw shapes directly with this beam, sort of like an Etch-A-Sketch (called vector graphics) or you can design a system to scan the entire front surface on a regular basis to created full glow across the entire chemical screen. We call this glow a “raster” and when you make images this way it is called raster graphics.
How you scan the image is up to you and around the world everyone scanned it differently. By the year 1950 the U.S. adopted a fixed standard to keep each network from have their own process, which meant buying a TV set for each network, a very expensive proposition.
The U.S. decided to scan the TV signal in two passes, known as interlaced scanning. The picture would be made up of 525 lines or row from top to bottom, of which only 480 actually showed picture image. One pass of the interlaced scan (known as a field) would do all the even numbered lines and the other pass would do all the odd numbered lines.
To keep the picture synchronized the U.S. decided to use household electricity standards of 60 cycle (Hertz) current for each scan. Thus there would be 60 fields (scan of half images) per second that would generate 30 full images per second. We call this 30 frames per second (FPS).
A few years later Europe adopted its own standards based on their 50 cycle AC current, thus they did 50 fields per second to generated 25 frames per second. Several systems evolved of which the British PAL (Phase Alteration Line) was the most popular. France and Russia used another system called SECAM which was similar but had a different color process.
The European TV sets scanned 625 lines for 580 lines of picture, thus they had finer detail than American TV programs, but they also had less channels as there is only so much spectrum to broadcast a complex color TV signal.
TV signals were broadcast in raw form. Every scan line was transmitted, along with sound. This is a big, complex signal.
All of this went along quite nicely until the 1990s with the advent of the digital revolution. With digital technology you don’t have to broadcast a complex color and sound wave in its entirety. You simply need to turn this into number data, remove the repetitive information (such as 5 minutes of blue skies in a video scene) and broadcast this data on a different spectrum.
In order to appreciate digital mediums we must remember the audio cassette tape, vinyl phonograph records, VHS video tape and the digital counterparts of the audio CD and video DVD.
CDs have no hiss, they don’t deteriorate with age, they don’t go snap, crackle and pop!
DVDs have twice the clarity of video tape.
So the advantages of digital are obvious and now we can make TV this way. The only drawback is everyone needs to buy a new TV set or get a tuner to decode these new signals.
That is where we now are. Starting next year all TV sets over 13” must be “ready” to show digital images as well as traditional TV stations. Traditional TV stations are due to stop operating on the current “channels” by the year 2015 and then all there will be is digital TV (or DTV).
There are two basic types of digital TV: One that shows an image of the same size and shape as your current TV set, only made digitally instead of from a raw signal and we call this SDTV or Standard Definition Television.
Anyone can buy a tuner and hook it up to a TV set made in 1955 to see SDTV digital images.
The other process has more scan lines and higher resolution and it is called HDTV or High Definition Television.
Now, your current TV image has a sharpness rating of 330 in the U.S. and 400 in Europe, although most modern TV sets can easily display sharpness of 500 or better. DVD disks have a sharpness of 480. VHS video tape has a sharpness of 240. Digital video camcorders have a sharpness of 530. SDTV has a sharpness potential of over 500 and in some instances it can put out close to 700. HDTV has a sharpness rating of 700 to 1,200. That is close to three times sharper than your current TV picture when viewing traditional TV shows on traditional TV stations.
Traditional TV set used 1 million dots in three colors to make an image. Since those three colors are required to make full color this means a TV image has 333,333 picture area or elements (pixels).
HDTV at best uses 6 million dots in three colors or 2 million pixels to make an image.
There are, however, two different HDTV processes and one of them uses less dots for a smaller screen size. The bigger HDTV has 1920 columns across the front of the set and 1280 rows down. The smaller HDTV system uses 1280 columns across and 720 rows down. Both sets can view both HDTV formats, as well as SDTV and traditional TV.
The bigger HDTV process scans the same way traditional TV does, in two passes of odd and even lines. The smaller HDTV process uses a progressive line by line scanning in a single pass.
HDTV has been broadcast in the US since the year 2000 and this year almost every major show is in HDTV.
SDTV has only been in limited operation for the last two years. One by one TV stations are converting their equipment over from traditional TV to digital SDTV, which, by the way, allows them to continue sending out traditional TV signals at the same times (bi-packed signals). In fact the new systems allow them to provide the same show in HDTV, SDTV and traditional TV all at the same time.
As with traditional TV and U.S. and Europe have adopted different systems, which we explore elsewhere in this issue with Digital Television World Wide.