telecommunication

[Deck]

How does the humble telephone transmit the human voice almost instaneously even to the other side of the world so that you can have a conversation without any time lag or delay. Conversely, why is it that the same cannot be said for a "live" satellite link up discussion?

[incitatus]

The delays in satellite link transmission are more to do with the operators at each end than with the transmission itself. In fact international phone calls are routinely sent by satellite using uplink stations (such as goonhilly in cornwall) so your delay free phonecall is likely to be a satellite link! The transmissions appear instantaneous as they are travelling at near light speed.

[lorven]

The answer lies in the fact of how many times the signal has to be diverted. A telephone call to the USA or Australia may take a fibre-optic cable route or a satalite connection, depending on what the telecom system finds open at that precise time. The signal takes the most direct path available. Fibre-optic cable takes the least distance, as it travels under the sea. That is why there is not usually an 'echo'. If a signal has to bounce up to a satellite first, it travels 76 000 kilometres before it reaches the earth station again. That causes a lapse of up to a second. In a "live' satellite linkup discussion, the signal may have to be 'bounced' back and forth by up to three satellites, depending where the two parties talking to each other are. So a delay of up to 3 seconds or more develops from the person talking, to when it is received at the studio. The interviewer has to take this into account. That is why the person being interviewed will usually signal the anchorman that his sentence is finished by calling their name. That way the interviewer knows he can ask his next question. Of course, after the signal comes into the studio, it has to be sent out again, so the words you actually hear at home may have been spoken up to 5 seconds before.
One may also experience a delay or an echo when speaking on a mobile phone. Your signal is first relayed to the service provider's main distibution centre. Then it is sent back to the phone you are speaking to. If you are in a remote part of the country from the main distribution centre then it may take a second before your signal comes back. This can make your voice echo in your own ear.