For heavy duty applications such as MARS, high power RTTY and SSB. Price $89500
W. Edmund Hood W2FEZ 223 Pullman Ave. Rochester NY 14615
As long as there have been radio amateurs, hams have been interested in the telephone system and sought ways to integrate it into ham communications. The worth of this idea needs no explaining. With their great tradition of public service and emergency work, ham operators have time and again proven their skill and efficiency in time of need. Nonetheless, in order to be a truly effective service, ham radio should be ready, whenever necessary, to utilize any and all media for communications. In spite of the fact that the two services grew up together, comparatively little data of any value is available for our use. Therefore the ham is often inclined to experiment on his own. This clandestine activity can have tragic results in the form of angry telephone officials, and, in some cases, can produce real damage to the telephone system, to say nothing of the expensive ham equipment.
In all fairness to the telephone people, it should be pointed out that a service so widespread as theirs can become night-marishly complex. Take the telephone found in the average home, it's a lot more intricate than the cheap field phones they used to sell in the Radio Shack.
The diagram is the result of the dissection of a surplus telephone, rather than any revelation by Mr. Bell. With the receiver hung up, S4 and S5 are held open. A strong ac signal can pass through C2 and ring the bell, but the rest of the instrument is disconnected. When you pick up the phone, S4 and S5 close. As you start to dial, SI
opens disconnecting the earphone, and S2 closes. As the dial returns, S3 opens and closes pulsing the line. You can follow the circuit from the red lead through S2, which remains closed until the dial reaches its rest position, then through the pulser, S3, through S5 closed because the receiver is up) and out via the green lead. A dc voltage is present on the line, and the closing of S3 shorts it out, producing pulses which are sensed by the stepping relays at the central office.
After you have finished dialing, SI and S3 are closed, and S2 is open. The dc voltage now reaches the mike from the green lead, through S5 and S3, through the mike, then into the autotransformer, L, where it is stepped up and sent out over the line. The
capacitor, CI blocks dc from the autotransformer, he dc voltage on the line is in the order of 48 volts when it leaves the exchange, but can be considerably reduced by the time it reaches the subscriber. Sufficient tolerance is built into the system to allow a wide variation. The signal which rings the bell is a 20 hz. The voltage of the ac is 48 volts, but since it is superimposed on the dc, it can sum up to a total of 96 v. The dial tone varies from exchange to exchange. Anything that will make a sound will do.
A line from the exchange to the subscriber is called a loop. The overall impedance can range from 900 ohms to about 1-5 k., although they like to shoot for a happy medium of 1.2 k. Since the subscriber's telephone is midway around the loop, it looks into an impedance of around 600 ohms, and this value is the standard value used by radio stations in a remote broadcast loop. The frequency bandpass for conversational subscribers is 300 to 3khz, This has proven most effective for transmission of speech with the highest intelligibility. Broadcast loops come in several classes. One has a passband essentially the same as a standard telephone loop, his class is used laost often for sports and newscasts. The class of loop used for disc jocky type programs, where some transmission of music is required, passes up to 8 khz., and finally, FM stations occasionally use the most expensive class with a full audio passband. (With very few exceptions, AM broadcast stations have an audio bandwidth limiting at 6 khz.)
Audio levels for the telephone system are centered around a reference (O db) of 1 milliwatt across a 600 ohm load. Broadcast loops generally hold their audio levels at 0 to +6 db. Home subscribers have no engineer to watch a level meter, and so it is difficult to pin down the audio level. Bell system engineers have no way of knowing whether the user will be a love-sick bobby-soxer or an eight-year-old telling grandpa about the fish he just caught. These represent two extremes for audio level. For design purposes, they try to consider the average of all voltage levels over a three-second period at -12 db. Using this figure as a design center, they are at least able to minimize crosstalk between lines.
When the receiver is lifted, and switches S4 and S5 close, a dc path is closed for the 48 volts on the line. The resultant current closes a relay in the central exchange which connects you into an available line to dial, and isolates your phone from incoming calls. As you dial, the pulser, S3, produces a series of brief shorts, so far as your telephone is concerned. At the exchange, however, these "shorts'* are seen as a series of current pulses. These pulses go into a device similar to a stepping relay, which counts the pulses and connects your phone to a set of contacts determined by the first digit you dialed. Each set of contacts is connected to a second stepping relay. The relay on the set of contacts corresponding to your first digit is pulsed when you dial the second digit, 11 lis connects you to a third stepping relay, and so on until you have dialed the full number. The final position of the last relay is connected to the telephone you are calling. The presence of your signal trying to reach this number closes another relay which sets the ringing mechanism into motion,
!'he ringing mechanism sends a 48 voit ac pulse into the line you are calling. This pulse passes through the blocking capacitor and rings the bell. When that line is picked up, its current censor closes making the final connection. The ring pulse, by the way, is one of two possible signals, depending on the service you have. Both are based on a 5-second cycle. A "long'1 ring is on for one third of the time and off two thirds. Some phones ring in a sequence of two short rings, each one sixth of the time with one. sixth interval between them, and then pause for one half of the time.
1209 1336 1477 1633
1209 1336 1477 1633
The touch-tone system, now available in some localities, sends a combination of two audio tones down the line which trip a frequency sensitive relay. From there the sequence is similar to the dial system except that frequency sensitive relays are used instead of the stepping relays. The combinations are given in the matrix shown here. The tones for any given digit are determined by the intersection of the tone-lines at which the button is located. For example, the digit, 4, would send out a combination of 770 hz, and 1209 hz, The two figures in the bottom row, # and * are only available on certain military and industrial phones. The 941 hz signal only appears in a standard phone when the zero button is pressed. The fourth vertical row (1633 hz) is not used at present, except in certain data-transmission systems. For the most part, it is reserved for further expansion.
The descriptions given so far are a very simplified version of what goes on when you make a phone call. Multiply this by a theoretical ten million possible number combinations and you can get a slight idea of the nightmarish complexity of the system. One small goof by some character who doesn't quite know what he is doing, reflecting back through the central exchange, can upset the whole applecart. The telephone system, being an emergency service, is therefore protected by federal and state laws against any tampering that might disrupt the service or invade anybody's privacy. As one telephone engineer explained to me, "It's not that we're trying to monopolize the equipment, but unless we know what is hung on that line, we can't do our job. Also, you're paying for the service, not the lines. If you put on an extension phone of your own, you're taking service that you're not paying for."
Over the years, Amateur Radio has proven its worth time and again in emergencies, When disaster strikes, it is good that all services can work together and combine all available facilities for the public good. Now the telephone company cannot and does not object to the infinitesimal amount of competition offered by hams chatting long distance over the air. But when we use their own facilities to phone-patch a pleasure call m competition with them, you've got to admit that's getting a bit dirty. While the telephone company won't go broke from the loss of an occasional service charge, the reputation of Amateur Radio is far too valuable to allow it to be cheapened by so small a thing as that. What 1 have told about the telephone system so far, as well as what I
am about to tell, is not by any means being revealed so that my brother hams can outsmart Mr, Bell, but rather, if the need should ever arise, that they might have-the know-how necessary to make use of every available facility for the good of their fellow-men. In such a case as that, even the strictest of the telephone people would very probably go along with you. And so I clear my conscience with the reminder that you receive with this knowledge a responsibility to use it only to the greater credit of the fraternity. One clown can ruin things for all of us.
When it comes to coupling into a telephone line with equipment of other than Bell System design, we should look first at the broadcast stations. They do this sort of thing on a far larger scale than any other telephone company customer. In the broadcast industry the most common method is transformer coupling. Broadcast loops are generally treated as if they were 600 ohm balanced lines. Therefore, any 600 ohm transformer winding should offer a decent match. It is worth noting that the telephone people sometimes clear their lines with a high-power surge at around 500 volts dc. A blocking capacitor of sufficient capacity with a good high voltage rating can save you a lot of grief. Remember also that the 600 ohm winding only looks like 600 ohms if the other winding is properly matched. If the transformer has, for instance, a 10 K primary and a 600 ohm secondary, it must have a 10 K load across the primary. Also, some means should be provided to insure that the audio level does not exceed about 0 db, even if it means buying a meter. For those who want to stay on the good side of Mr. Bell, the telephone company makes available in many places a coupling transformer together with a telephone fitted to automatically remove it from the line when the receiver is down. After a $5 installation charge, it only adds 50c a month to your bill.
If you intend to record off the phone, re member that the law requires that a "beep" be sounded on the line, or that the other party know right from the start that a record is being made.
There are a number of low-priced induction pickups on the market which work by detecting the magnetic field of the autotransformer or of the earphone. These not only work, but they will also feed a signal into the line if the level is high enough. However, unless the other party is aware of it from the beginning, it may prove to be a violation of the federal wiretap laws. ,
The diagram shows all the necessary
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