Plain Old Analog TV
In the previous section, a CRT screen displayed information from a computer video adapter that started as a set of digital points. During the sweep of the beams across the screen, each digital value is converted to a voltage level for the period of time associated with one dot in the current resolution. Something of the same process occurs when a TV picture is generated from a digital source (DVD, digital cable, or PC TV Out).
However, conventional TV (broadcast to an antenna, VHS tape, or the cable signal on Channels 2-99) is an "analog" signal. A TV only has 240 lines. The analog broadcast signal is marked with a clear beginning and end to each line. However, within the line the signal is simply a wave form with components for intensity and color. This signal is decoded into a voltage level for the red, green, and blue electrodes. The signal could change continuously as the beam plays across the screen. When the beam happens to hit a hole in the grid and a phosphor on the screen, the current level of the signal contributes to the brightness of that dot of color on the screen.
The TV signal begins when a camera focuses the image on a sensor grid. A conventional TV camera records 480 lines (twice as many as the TV can display) and repeats the process 30 times a second. Today, a TV camera is digital, so it has a specific number of sensor points per line but back in the 1950's, the TV signal was scanned as a continuous wave in the camera. The camera transmitted this wave as an electrical signal across wires to the transmitter. The transmitter converted the electric wave to an electromagnetic wave broadcast through the air. The antenna at a house converted the electromagnetic wave back to an electric signal that the TV set amplified and sent to the electrodes at the back of the picture tube. Throughout the entire process there was no digital technology and it would have been misleading to say that the signal intended the TV set to have any particular resolution in terms of the number of dots per line. That noted, any individual TV set actually had some specific number of holes in the mesh and phosphor dots on each line.
The TV set had only 240 lines on the screen, and rewrites every one of them 60 times a second. The camera recorded 480 lines of picture 30 times a second. To get the appearance of more vertical resolution, the TV signal is "interlaced". Every other line is transmitted (240 lines of picture) in 1/60th of a second. Then a second pass transmits the 240 intermediate lines that we skipped in the previous pass. Every 1/30th of a second a complete frame of 480 lines is transmitted, but they are displayed on the screen as alternate sets of 240 lines.
A VHS VCR receives this analog broadcast signal and records it to tape. When you play the tape, you get back the same analog signal (plus any noise introduced by imperfections in the recording process).
The standards for broadcast analog TV (and therefore the conventional analog output from anything that feeds the TV: cable TV box, DVD player, VHS tape player, or PC) have not changed since the '50s. All the equipment has certainly changed. All modern TV cameras are digital. In the studio they record digital signals on digital tape. The information can be copied onto a hard disk, edited by computer software, and written back to tape. Old analog tape could not be edited as easily, and there was a loss of signal quality whenever you made a copy. Digital information is easily edited and every copy is perfect. The networks feed their programming to individual stations through big satellite dishes or fiber optic cable. This network feed is a digital signal in a form of MPEG 2 compression. In the end, however, the local TV station converts the digital signal back to a 1950 era analog signal to broadcast it to your TV set.
The analog signal transmits 240 lines. The start and end of each line is obvious from the signal. As each line is drawn, the analog signal continuously varies the amount of Red, Green, and Blue content to "draw" the line of picture on the screen. This signal can be digitized by a circuit that samples the amount of Red, Green, and Blue signal generated for a period of time that corresponds to one "dot", then stores the measurement as three numbers and continues on to measure the next dot. Every analog TV PC receiver card (and all Digital Video Recorder devices like Tivo) turn the analog signal into a stream of numbers. Typically these numbers are immediately compressed to a smaller, reasonable amount of data.
Any consumer device (TiVo, Replay) expects to get 240 lines of TV signal refreshed 60 times a second. Yes, this is an interlaced signal, but consumer devices do not expect to convert the recorded information to any other format and they do not expect to display the picture on anything except a TV. So they can simply record it as 240 lines 60 times a second.
However, a computer screen is not interlaced. To display the TV picture on a computer, the software has to "de-interlace" the original signal and create a picture with the original 480 lines refreshed 30 times a second. The digital file formats support a wide range of number of lines, number of dots per line, either interlaced or non-interlaced (also called "progressive" scan).
If a conventional TV set quotes a resolution, it reflects the number of horizontal phosphor dots per line. The vertical number of lines is fixed at 240. Horizontal TV "resolution" is different from computer display resolution. A computer display adapter is digital and the screen image is generated as dots or "pixels" of color in the computer memory. These dots are then transmitted to the display. A TV signal is a continuous analog signal, essentially a "wave" of colors painted right to left across the screen. There are no discrete dots in the TV signal, but the colors change continuously across the screen. Every time the electron beam sweeps over a set of phosphors on the screen, it generates a dot of picture. More horizontal dots will display more of the fine details in the original wave, provided that those details are present in the analog signal. A higher resolution conventional TV might display a better picture from a DVD player.
The US standard for TV signals is called "NTSC". The US power grid generates alternating current at 60 cycles per second, and back when TV's were invented there was no better timing reference. NTSC represents a combination of lines and refresh rates that made sense given the vacuum tube electronics of the 40's if you had to do your operations at a rate of 60 per second.
Each TV channel has a fixed amount of bandwidth. In other countries the electric power cycles 50 times a second and their TV pictures change at that rate. Given a fixed bandwidth, if you transmit fewer screen images per second you can add a few extra lines to each image. So traditional European analog TV standards feature more vertical lines in each picture, but change the screen image less frequently.
Obviously we can do much better in the modern world of computers, where internal timers measure speed in billionths of a second and microchips provide enormous processing power. The real problem is that this technology is continuing to improve. Any standard that is comfortable today will be obsolete in a few years. The new digital TV standards take this into consideration by selecting values that are just a little beyond the reach of today's mass market devices. The industry can spend the next decade letting the mass market catch up and completing the conversion.
To position it against new digital TV standard, the Plain Old Analog TV standard is referred to as "480i". It has 480 lines of image, but they are interlaced so that only 240 lines show on the screen at a time. By flashing alternate views of every pair of lines, the TV presents what appears to be a sharper picture than 240 lines, but not as clear a picture as if the set had 480 lines of picture to display all at once.