You just get one signal per line, the voice signal. There will always be some voltage on the circuit as long as at least one line is established. But if your line is on a trunk (connections between exchanges), you can't do this voltage trick because the trunk has several signals on it, each shifted to a different frequency so as not to overlap ( Frequency-Division Multiplexing). When the phone is on-hook, the DC circuit is blocked by a capacitor and should have no voltage. If the phone circuit has a voltage at all, it's off-hook. The voice signal is the basic perturbations of voltage you'd see on a normal microphone or loudspeaker wire, making a nice waveform. If a phone is directly wired to your exchange, it has an on/off-hook signal in addition to the main voice signal. When you run a telephone exchange / central office, you constantly want to know which of your lines are in use, so you don't put through a call to a line that's on another call, and you know how much to charge based on when the customer hangs up. You can transmit silence perfectly well, but how can the receiver tell the difference between a signal of absolute silence, and no signal being sent at all? Early telephony couldn't, and that was a problem. The problem that 2600Hz signals solve isn't one of transmission, but reception. Sorry, what I said is a bit awkward and misleading. Please also read the site-wide Reddiquette. Please avoid reposting TILs that have already made the front page in the past on YouTube)Īdd or tags to your posts, as necessary. Link to the appropriate start time when referencing videos (e.g. Link to the appropriate heading when referencing an article (particularly on Wikipedia) If you are interested in reading about the TILs on this list use the search box feature and enter the keywords to pull up past TILs.Īvoid mobile versions of websites (e.g. The purpose is to keep content fresh on /r/todayilearned as requested by its members. The titles have been abridged for the sake of brevity, however the context remains the same. If your TIL is found on this list, it will be removed.
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