It wowed audiences in 1964 at the New York World’s fair. It’s been “just around the corner” for decades. And now, at last, it’s here: the videophone has arrived.
AT&T and MCI will both be selling these picture-sending telephones soon, for anywhere from $750 to $1,500 U.S., as the perfect way to show off new grandchildren. I, on the other hand, see them as the perfect reason to write about telephones.
Despite digital readouts, computer memories and annoying technological rudenesses like call-waiting, today’s telephones are very similar to the device Alexander Graham Bell patented in 1876 and 1877. They still turn sound waves into electrical oscillations, transmit those oscillations to a receiver, and turn them back into sound waves.
Air molecules set vibrating by the speaker’s voice generate matching vibrations in a thin aluminum diaphragm inside the mouthpiece, which are passed on to a metallic box filled with small granules of carbon. These vibrations alternately force the granules closer together, so they conduct electricity more easily, and loosen them up, so electricity has a harder time getting through. The electrical current flowing through the granules therefore varies in strength as it passes into the telephone line.
At some point in the transmission process the signal may be turned into pulses of light and sent through a fibre-optic cable, or turned into radio waves and bounced off a satellite, but eventually it gets turned back into a varying electrical current which controls the strength with which an electromagnet attracts a second metallic membrane in the receiving telephone’s earpiece. As the magnetic attraction varies, the movements of this membrane match the vibrations of the mouthpiece membrane in the transmitting telephone, setting up vibrations in the air that recreate the speaker’s voice.
The telephone has to be part of a complete electrical circuit for all this to work, so it needs a power source, a conductor and a switch. The power source is a centralized battery in the telephone system. The telephone wires are the conductors, and the telephone switch is also the circuit’s switch: lifting the handset completes the circuit, causing the dial tone; hanging up breaks the circuit.
When you dial (or punch in) a number, you send an electrical code to an automatic station telling it which switches to throw to connect you to a particular telephone among the millions in the system. The first automatic switching systems used mechanical swtiches, which is why dial telephones make you wait between numbers: mechanical switches need time to move. Today’s switches are electronic, so no delay is required.
Despite such improvements, telephone systems can’t take full advantage of advances in technology because of the need to insure that a pre-war telephone in Istanbul can still connect you to a computerized telephone in Regina, should you run into a vacation emergency. Getting this inherently limited system to accept the new videophone hasn’t been easy, because it’s a lot harder to transmit pictures than voices.
A typical telephone line can handle about 19,000 bits of information per second (a bit is an on/off signal, the basic element of computer communication). Video images require 92 million bits per second. Obviously, video had to go on a drastic diet.
First step: reduce the number of images sent per second from the 30 used by standard TV to just 10, saving 60 million bits and still giving an image as good as most camcorders provide. Second step: use a small TV screen, only 83 millimetres on a side, so you don’t need as much information to fill it.
Third step: a device called a “codec” (for “coding” and “decoding”) which divides the image into 224 blocks, analyzes each, and transmits only those blocks (or parts of blocks) that have changed from one frame to the next. The background probably doesn’t change at all, so it might only need to be sent once. A hand wave might require sending two or three blocks, but moving lips probably only require slight changes in one block.
The final result is a video signal compressed by 99 percent, from 92 million bits per second to just 11,200, leaving plenty of room for the audio signal. The two can be sent together as a varying electrical current and reconstructed by the receiving videophone.
Do people really want to be able to see each other when talking on the telephone? Some doubt it — myself among them.
But then, I still don’t understand why people want to talk on the phone while driving.