Two questions on electricity/electronics/telephony

Two questions just for my electricity, electronics and telephony knowledge:

One: About the typical adaptor transformer rectifier, (rigged up with four diodes for rectifying purposes in order to reduce ‘noises’) for changing household A.C. current to, say, 1 ampere 12 volts D.C. current, here’s the question: Will increasing the power of the output capacitor, to double or greater, make for a more smooth D.C. output?

Two: A.C. electricity is delivered to each home like from a main river with each home receiving the supply by way of a rivulet branching from the river to the home. Now, the question: What about the telephone line supplying the homes in the same street? Is the main line similar to the A.C. electricity supply line feeding the rivulets going into each home? Obviously not, I can see that. O.K., please, the electricity and telephone technicians here, tell me how’s it’s done – in simple language, if I may impose on you good guys. Or are there inside the main telephone trunk(?) line feeding every household smaller cables, as many as there are subscribing homes; and these smaller cables come all the way from an areal depot(?).

Please pardon the lack of technological finesse in the concepts and terms in this post. But I guess electrical and electronic and telephony experts understand my queries.

Thanks and God bless you good guys.

Susma Rio Sep

There is a single physical line for every telephone line, but I’m not sure what you mean by “areal depot.” Each physical phone line gets bundled together into a big cable, which may later get bundled with other ones into even bigger cables, until they get to the central telephone office (CO) for your community. At the CO, each line is plugged into switching equipment which connects the CO to the COs of other nearby towns and to long distance carriers.

There are more modern technologies, such as ATM and VoIP, which use packet-switched networks like Ethernet or the Internet to send voice data, meaning several streams could be sharing the same physical line and bandwidth can be allocated dynamically. But these technologies haven’t made it to the Plain Old Telephone System yet.

One: A larger (not more powerful) capacitor will, indeed, diminish the output ripple BUT the diodes have to be sized accordingly because peak I grows so the diodes could be damaged if they cannot handle the peak load.

TWO: electric power and telephone cables are very different things. Electric power receivers are just all wired in parallel off the mains while each subscriber has a dedicated pair of phone wires which go all the way to the central office.

Telephone systems are kinda neat when you really look at the signals involved. They figured out how to send a ring signal down to the phone, voice signals in both directions, and a way to dial out, all using just two wires, and this was long before the days of the transistor let alone digital circuits. Pretty impressive if you ask me.

Phone systems vary greatly. Generally speaking though, the two wires that are in your house go all the way to a local switching station. From there, signals are going to be combined so that multiple voice signals can be present on a single wire. This can be done with analog signal modulators. This is similar to what is done in your television, where video and audio signals are modulated up to higher channels, then demodulated by your tv tuner so that you get the original tv and audio signals back. At some point the signals will likely change from copper to fiber optic cable, since fiber optic cable has more bandwidth and less signal loss. More bandwidth means that you can stick more voice channels on the same cable. There’s a good chance at some point that your voice gets digitized and the digital packets get routed through a very complex network. Depending on where your phone call is going to, the voice signals may even get beamed up to a satellite via radio waves and beamed back down to another location.

engineer_comp_geek is pretty much on the money – however, there is a special case that bears mentioning, at least for those interested in broadband.

In some phone systems (the former GTE in southern California was notorious for this), the combining of multiple voice signals onto a single wire pair happens well before the switching station (which is usually, but not always, the CO mentioned by freido). In such systems, the multiplexers are much closer to the individual line drops – they can be centralized in a vault in the neighborhood, or even in pole-mounted boxes.

This is usually referred to as PairGain technology, and tends to make it impossible for you to get DSL, because PairGain doesn’t allow enough of a frequency range to carry a DSL signal – sometimes it even screws up 56k dial-up modems.