406 lines
28 KiB
Plaintext
406 lines
28 KiB
Plaintext
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Another Safe Cracker Production
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Hacking Ma Bell
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Part One
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Spencer Whipple, Jr.
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c/o 73 Magazine
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Peterborough, NH 03548
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Basic Telephone Systems
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Part One
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Lifting Ma Bell's Cloak of Secrecy
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----------------------------------
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Though telephones predate radio communications by many years, they
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aren't nearly as simple as they appear at first glance. In fact, some
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aspects of telephone systems are most interesting and quite ingenious.
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In this article we will describe some of these more interesting and perhaps
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less well-known areas of telephone systems.
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But before going farther, let me explain and apologize for the fact
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that some of the information in this article may not be altogether complete,
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up to date, or even correct. I do not work for any phone company, and therefore
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do not have access to internal telephone company literature. Moreover,
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there is very little material available in books or magazines which describes
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how US telephone systems work. Much of the information in this article has
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been obtained piece-meal from many different sources such as books, popular
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magazines, computer data communications journals, handbooks, and sometimes
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just plain hear-say. I have tried to correlate as much as possible all the
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little bits and pieces into a coherent picture which makes sense, but there is
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no easy way to be sure of all the little details. So think of this article as
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if it is a historical novel - generally accurate and, regardless of whether it
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is completely true or not, fascinating.
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With this out of the way, let's go on. Figure 1 shows a simple diagram
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which explains how your home telephone fits into the overall picture. You, as
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the customer, are generally referred to as the 'subscriber'. Your telephone
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connects to the Central Office through a two-wire cable which may be miles long
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and which may have a resistance on the order of hundreds or even thousands
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of Ohms. This cable is essentially a balanced line with a characteristic im-
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pedence of around 900 Ohms, but this varies greatly with different calls. (This
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is why it is so hard to keep a hybrid phone-patch balanced.)
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The main power in the central office comes from 48 volt storage batteries
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which are constantly kept trickle-charged. This battery is connected to your
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line through a subscriber relay and a balanced audio transformer. The relay
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is sensitive enough to detect even quite small currents through your line.
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The buttons which stick up out of your telephone case when you lift the
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handset accuate the hook switch. The name probably dates back to the days when
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the handset (or even earlier, the earpiece) hung on the side of the phone from
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a hook. In any case, when your phone is hung up it is said to be on the hook;
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when you lift the handset to make a call it is said to go off the hook. With
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the phone on hook, the line is connected only to the bell (called the ringer).
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Because the bell circuit has a capacitor in it, no dc current can flow through
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the phone. As a result, the subscriber relay back in the central office will
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be deenergized, indicating to the central office (let's abbreviate that as CO
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from now on) that your phone is hung up. Since there is no current through
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your line or phone, there is no voltage drop anywhere, and so if you measure
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the voltage across the phone line at your home you will see the entire 48 volts
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(or even more if the CO batteries are well charged). The positive (grounded)
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lead is called the tip and negative lead is called the ring; these names cor-
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respond to the tip and ring of a three-circuit phone plug.
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Now suppose you want to place a call. You pick up the handset, and the
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phone goes off the hook. This completes the dc circuit through the dial, micro-
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phone, and the hybrid network which is basically a complicated transformer
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circuit. At this point current starts to flow from the battery through your
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line and phone, and the subscriber relay back at the CO pulls in. The line
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voltage across your phone now drops to just a few volts because the line is
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loaded down by the low resistance of the phone. The CO now searches for some
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idle dialing circuits, and when it finds them, connects a dial tone back to
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your phone. When you hear this, you start dialing.
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The dial shown in Fig. 1 is a rotary dial of the type which you turn with
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your finger (we will talk about Touchtone dials later). When you dial a number,
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the dial acts as a short circuit until you release the dial and let the built-
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in spring return it back to the resting position. As it is returning, it starts
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to open and close the circuit in sequence to indicate the number you dialed.
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If you dial a 1,it opens the circuit once; if you dial a 9 it opens the circuit
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nine times. As the dial is returning it causes the subscriber relay to open and
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close in step. This enables the CO to recognize the number you want. When you
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finish dialing, the dial becomes just a plain short circuit which passes
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current through the microphone and the hybrid network. Since the mike is a
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carbon unit, it needs this current to work.
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When the CO receives the complete number, it starts to process your
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call. If you dialed another subscriber in the same area, it may connect you
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directly to that subscriber's line. Calls to phones a little further away may
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have to be routed through another CO, while long distance calls may go through
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one or more long distance switching centers (called tandems) and possibly
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many other CO's before arriving at the destination. At the completion of this
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process, you may get either a ringing signal, indicating that the phone at
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the other end is ringing, one of several types of busy signals, or possibly
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just silence, if something goes wrong somewhere.
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When you talk to the person at the other end, the cable carries audio
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in both directions at the same time. Your carbon microphone varies the current
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in your circuit, and this current variation is detected by a balanced trans-
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former in the CO.At the same time, audio coming back to your phone goes through
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the hybrid network to your earphone. (In phone company lingo they like to call
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the mike a transmitter, and the earphone is called a receiver.)
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You may be interested in the makeup of the various tones you may hear
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on your telephone; these tones are important to people such as computer com-
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munications designers who have to build equipment which will recognize dial
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or other signalling tones:
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Dial tone in older exchanges may still be a combination of 120 and 600 Hz
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but the newer exchanges use a combination of 350 and 440 Hz. There is often
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a slight change in the dc line voltage at the beginning of dial tone, and
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this may also be detected.
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Busy signal is a combination of 480 and 620 Hz which alternates for 1/2
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second on and 1/2 second off (i.e., 60 interruptions per minute) when the
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party you are calling is busy. The same busy signal may be used for other
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conditions such as busy interoffice or long distance circuits, but would then
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be interrupted either 30 times a minute or 120 times per minute. This is a
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standard agreed on by an international telecommunications organization called
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CCITT (and I don't offhand remember the French words it stands for), but
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occasionally other frequencies up to 2kHz are used. A siren-like sound varying
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between 200 and 400 Hz is often used for other error conditions.
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The ringing tone, which you hear coming back to you when the phone rings
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on the other end of the connection, is nowadays mostly a combination of 440
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and 480 Hz, but there is a great variation between CO's. Very often a higher
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frequency such as 500 Hz is interrupted at 20 Hz, and other tones are used as
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well. The tone is usually on for two seconds and off for 4 seconds.
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The ringing current, actually used to ring the bell in a telephone, is an
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ac voltage since it has to activate a ringer which has a capacitor in series
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with it.Different companies use different ringing currents, but the most common
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is 90 volts at 20 Hz. Since a typical phone may be thousands of feet away from
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the CO, the thin wires used may have a fairly high line resistance. Hence only
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a relatively small current can be applied to the bell, certainly not enough
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to ring something like a doorbell. This problem is solved by making the bell
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resonant mechanically at the ringing frequency so that even a fairly small
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amount of power is enough to start the striker moving hard enough to produce
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a loud sound. This is the reason why a low frequency ac is used. Although this
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raises some problems in generating a 20 Hz signal at a high enough voltage,
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it has the advantage that a bell will respond to a ringing current only if
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the frequency is quite close to the bell's naturally resonant frequency.
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If you build two bells, one resonant at 20 Hz and the other resonant at 30 Hz,
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and connect them together to the same line, you can ring just one bell at a
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time by connecting a ringing current of the right frequency to the line; this
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has some useful applications in ringing just one phone on a party line.
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Now let's look at some of the components of the phone itself. We will
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consider the most common new phone, a model 500 C/D manufactured by Western
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Electric and used by Bell System affiliated phone companies. This is the
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standard desk phone, having modern rounded lines and usually having a G1 or
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G3 handset. It was developed about 1950 and replaced the older 300-series
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phones which had the older F1 handset and had sharper corners and edges. (There
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was an inbetween phone, where they took an old 300 series phone and put a
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new case on it which resembled the 500-style case but had a straight up-and
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down back - the back of the case came straight down right behind the handset
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cradle,whereas the true 500-style telephone has what looks like a step sticking
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out behind the cradle). If you are still in doubt as to which phone you have,
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the bell loudness control is a wheel on the 500-type phone and a lever on the
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300-type. If you live in the boondocks, you may still have the 200-type phone
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(sometimes called the ovalbase) or maybe even the desk-stand type that looked
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like a candlestick, with the microphone mounted on top and the earpiece hanging
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on the side from a hook. Neither of these phones had a built in bell, and so
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you probably have a bell box attached to your wall. (If you have a phone with
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a handle on the side which you crank to call the operator, the following does
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not apply to your phone !)
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Fig. 2 shows the bell circuit, which consists of a two-coil ringer and
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a 0.5 uF capacitor. On Western Electric phones the capacitor is mounted inside
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the network assembly, which also has a large number of screws on top which
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act as connection points for almost everything inside the phone. (I have
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never been able to find out why the ringer has two coils of unequal resistance
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but it apparently has something to do with determining which subscriber on a
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party line makes which call.) In most phones, the yellow and the green wires
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are connected at the wall terminal block so that the bell is connected directly
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across the telephone line; disconnecting the yellow lead would turn off the
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bell (although sometimes the connection is made internally by connecting the
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black lead from the ringer directly to the L1 terminal, in which case the
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yellow lead is disconnected.
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You may wonder why a yellow lead is needed at all when only two wires
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are normally used anyway. It is true that only two wires enter the house from
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the outside; one of these is the tip and the other is the ring. In a non-party
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line the ringing current as well as all talk voltages are applied between the
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tip and the ring, and it doesn't actually matter which of the phone leads
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goes to the tip and which to the ring if you have a rotary dial phone. If you
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have a Touchtone dial, then you have to observe polarity so that the transistor
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circuit in the dial works, in which case you have to make sure that the green
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lead goes to the tip and the red lead goes to the ring.
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The yellow lead is commonly used for party lines. On a two-party line
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ringing current from the CO is applied not between the two lines, but between
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one line and ground. In that case the yellow lead goes to ground while the
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other side of the ringer (the red lead) is connected to either the tip or the
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ring, depending on the party. In this way, it is possible to ring only one
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party's bell at a time.
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The remaining connections inside the telephone are shown in Fig.3. The
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components labeled VR are varistors: the phone companies must be the world's
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biggest users of these devices, which are variable resistors whose resistance
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drops as the voltage across them rises. Their function in the phone set is
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to short out parts of the set if the applied voltage gets too high. For in-
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stance, VR2 is connected directly across the earphone (receiver) and acts as
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a volume limiter to lower the volume if the applied voltage gets too high -
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a great way to protect your eardrums.
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As you can see in Fig.3 we use the standard phone company way of ident-
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ifying normally open and and normally closed switches - an X in a wire is
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normally a normally open contact of a switch or relay, while a short bar means
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a normally closed contact.The arrows in the drawing show the path of dc current
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through the phone when it was off the hook. Starting at the green wire, the
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current path goes through a set of contacts on the hook switch, then through
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the pulsing contacts on the dial, through part of the network, through the
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mike, back through a second winding on the network, and finally through a
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second contact on the hook switch and back out to the red wire.
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The hook switch actually has three sets of contacts, two normally open
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(open, that is, when the hand set is on the hook) which completes the dc cir-
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cuit when you pick up the handset, and a normally closed contact which is
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wired directly across the earphone. This contact's function is to short the
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earphone during the time that the dc circuit is being opened or closed through
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the phone - this prevents you from being blasted by a loud click in the ear-
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phone.
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The dial has two contacts. One of these is the pulsing contact, which
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is normally closed and only opens during dialing on the return path of the
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dial after you let go of it. The second contact, labelled the off-normal con-
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tact, shorts the earphone as soon as you start turning the dial, and releases
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the short only after the dial returns back to the normal position. In this
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way you do not hear the clicking of the dial in the phone as you dial.
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Finally, the phone has the hybrid network which consists of a four-winding
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transformer and a whole collection of resistors, capacitors, and varistors.
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The main function of the network is to attenuate your own voice to lower its
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volume in your earphone. The simplest phone you could build would be just
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a series circuit consisting of a dial, a }ike, and an earphone. But the signals
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coming back from the other party are so much weaker than your own signals,
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that an earphone sensitive enough to reproduce clearly and loudly the voice
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of the other person would then blast your eardrums with the sound of your own
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voice. The function of the network is to partially cancel out the signal pro-
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duced by the local mike, while permitting all of the received signal to go to
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the earphone. This technique is similar to the use of a hybrid phone patch
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with a VOX circuit, where you want the voice of the party on the telephone
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to go to your transmitter, but want to keep the receiver signal out of the
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transmitter.
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In addition to the parts needed for the hybrid, the network also contains
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a few other components (such as the RC network across the dial pulsing
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contacts) and screw-type connection points for the entire phone.
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A Touchtone phone is similar to the dial shown here, except that the
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rotary dial is replaced by a Touchtone dial. In addition to its transistor-
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ized tone generator, the standard Touchtone pad has the same switch contacts
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to mute the earphone, except that instead of completely shorting the earphone,
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as the rotary dial does, the Touchtone dial switches in a resistor which only
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partially mutes the phone. The circuit of the Touchtone dial is shown in
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recent editions of the ARRL Handbook so we won't print it here, but Fig.4
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shows two possible connections of such dials for amateur use. Fig.4 (a) shows
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the connection for coupling the dial output electrically to a transmitter in-
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put, while Fig.4 (b) shows how to connect it to a 500 Ohm earphone (such as
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the earphone from a telephone handset) for acoustic coupling into a transmitter
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microphone. Fig.5 shows how the terminals on a Trimline Touchtone pad cor-
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respond to the colored wires coming from the standard desk-type phone pad.
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It is fairly common knowledge as to what frequencies are used for Touch-
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tone signalling, but a misprint in several recent ARRL publications gives the
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wrong frequency for one of the high tones, so here is a short table which
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repeats the correct numbers :
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LOW
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TONE HIGH TONE GROUP (Hz)
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GROUP
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(Hz) 1209 1336 1477 1633
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697 1 2 3 A
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770 4 5 6 B
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852 7 8 9 C
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941 * 0 # D
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Each digit is composed of one frequency from the low group and one frequency
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from the high group; for instance, the digit 6 is generated by producing a
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low tone of 770 Hz and a high tone of 1477 Hz at the same time. The American
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Touchtone pads generate both of these tones with the same transistor, while
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European pads (yes, there are some) use two transistors, one for each tone.
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In addition to the first three high tones, a fourth one of 1633 has been
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decided on for generating four more combinations, called A through D in the
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above table. These are not presently in use, although the standard phone Touch-
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tone pad can easily be modified to produce this tone, since the required tap
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on the inductor used to generate the tone is already present and only an
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additional switch contact is needed to use it; information on this simple
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conversion is found in the 73 publication 'Digital Control of Repeaters'.
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What is not generally known is that the U.S. Air Force uses a different
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set of Touchtone frequencies, in the range of 1020 to 1980 Hz. Since many
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of the phones available for purchase in stores come from Department of Defense
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surplus sales, it will be interesting when these phones become available.
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Another Touchtone dial presently used by amateurs is made up of a thin
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elastomeric switch pad made by the Chomerics Corp. (77 Dragon Court, Woburn,
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Mass. 01801) and a thick-film hybrid IC made by Microsystems International
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(800 Dorchester Boulevard, Montreal, Quebec). The pad is the Chomerics ER-
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20071, which measures about 2 1/4 inch wide by 3 inches high, and only about
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3/16 inch thick (Chomerics also makes a smaller model ER21289, but it is very
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difficult to use and also apparently unreliable).Microsystems International
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makes several very similar ICs in the ME8900 series, which use different
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amounts of power and generate different amounts of audio. Some of these also
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contain protection diodes to avoid problems if you use the wrong polarity on
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the IC, and there are so many models to choose from that you should get the
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technical data from the manufacturer before ordering one. There are a number
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of US distributors, including Newark Electronics, Milgray and Arrow Electronics
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in New York. KA Electronics Sales advertised both the pad and the IC in the
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July 1974 issue of 73 Magazine. In single quantities, the pad goes for about
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$9 and the IC costs about $18, although it drops in price if you order larger
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quantities.
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A simple circuit for the IC and pad isshown in the ARRL publication 'FM
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and Repeaters for the Radio Amateur'. While this circuit is perfectly good,
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it does not work in the presence of a strong rf. If you want to mount this pad
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and IC on a portable 2-meter rig, you will have to use bypass capacitors and
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chokes to keep the rf out of the IC. Bypass pins 8 and 16 of the IC to pin 13
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with small discs of about 0.001 or 0.01 uF, right at the IC, using very short
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leads. Then put small 2 to 5 microhenry chokes in series with pins 8, 13 and
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16 right at the IC. If needed, put more chokes at the other end of each lead.
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Ohmite Z-144 chokes are good but a little bulky; the small 1.8 microhenry
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chokes used in Motorola Handie-Talkies (Motorola type 24-82723HO1) are about
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the size of a 1/8watt resistor and almost as good.It may seem a little funny to
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put chokes in the ground leads,as all hams are trained to use good rf grounds,
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but the object is to keep rf out of the IC at all costs and this accomplishes
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that by letting the IC float above ground if needed,but removing any rf voltage
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which might appear across the IC leads. It is also possible to generate the
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Touchtone tones with separate oscillators or with IC oscillators (such as the
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NE566), as is done in pads sold by Data Engineering. This system may not be
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as stable or accurate as other systems, though.
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One of the problems with any current IC is that the frequency changes
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if rf gets near it. Many hams are having a hard time mounting such IC pads
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on their 2-meter Handie-Talkies. But a solution seems in sight - Mostek, a
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large IC company, is coming out with an IC Touchtone generator which has cheap
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3.58 MHz external crystal as reference, and then produces the tone frequencies
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by dividing the 3.58 MHz down with flip flops to get the required tone frequen-
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cies. This approach not only promises to be more reliable in the presence of
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rf, but should also be cheaper since it would not need the custom (and expen-
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sive) laser trimming of components that the Microsystems International IC
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needs to adjust the frequencies within tolerence.
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At the other end of the telephone circuit, in the CO, various circuits
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are used to decode the digit you dial into the appropriate signals needed to
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perform the actual connection. In dial systems, this decoding is done by relay
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circuits, such as steppers. This circuitry is designed for dialing at the rate
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of of 10 pulses per second, with a duty cycle of about 60% open, 40% closed.
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The minimum time between digits is about 600 milliseconds, although a slightly
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greater time between digits is safer since it avoids errors. In practice,
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many COs will accept dialing at substantially slower or faster rates, and often
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you will see a dial that has been speeded up by changing the mechanical gov-
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ernor to operate almost twice as fast; it depends on the type of CO equipment.
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Touchtone decoding is usually done by filter circuits which separate out
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the Touchtone tones by filters and then use a transistor circuit to operate
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a relay. A common decoder is the 247B, which is designed for use in small
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dial switchboard systems of the type that would be installed on the premises
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of a business for local communication between extensions. It consists of a
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limiter amplifier, seven filters and relay drivers (one for each of the seven
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tones commonly used) and some timing and checking circuitry. Each of the seven
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relays has multiple contacts, which are then connected in various serial/par-
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allel combinations to provide a grounding of one of ten output contacts, when a
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digit is received. The standard 247B does not recognize the * and # digits,
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but can be modified easily enough if you have the unit diagram.
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The 247B decoder is not very selective, and can easily be triggered by
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voice unless some additional timing circuits are connected at the output to
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require that the relay closure exceed some minimum time interval before it is
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accepted. Slightly more complicated decoders which have the time delays built
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in are the A3-type and the C-type Touchtone receivers. Both of these are used
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in customer-owned automatic switchboards when a caller from the outside (via
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the telephone company) wants to be able to dial directly into the private
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switchboard to call a specific extension. The C-type unit is similar to the
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247B in that it has ten outputs one for each digit. The A3-type does not have
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output relays, but instead has seven voltage outputs, one for each of the
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seven basic tones, for activating external 48-volt relays. The A-3 unit is
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ideal for activating a Touchtone encoder, which can then be used to regenerate
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the touchtone digits if the original input is noisy. This might be very useful
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in a repeater autopatch, for cleaning up Touchtone digits before they are
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sent to the telephone system.
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In addition to the above,there are probably other types of units specially
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designed for use in the CO, but information on these is not readily available.
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It is also fairly easy to build a Touchtone decoder from scratch. Though the
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standard telephone company decoders all use filter circuits, it is much easier
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(though perhaps not as reliable) to use NE567 phase-locked-loop integrated
|
||
circuits.
|
||
An interesting sidelight to Touchtone operation is that it greatly speeds
|
||
up the process of placing a call. With a Touchtone dial it is possible to
|
||
dial a call perhaps 3 to 5 times faster than with a rotary dial. Since the
|
||
CO equipment which receives and decodes the number is only needed on your line
|
||
during the dialing time, this means that this equipment can be switched off
|
||
your line sooner and can therefore handle more calls. In fact, the entire
|
||
Touchtone system was invented so that CO operation would be streamlined and
|
||
less equipment would be needed for handling calls. It is ironic that the cus-
|
||
tomer should be charged extra for a service which not only costs the telephone
|
||
company nothing, but even saves it money.
|
||
Another practice which may or may not cost the telephone company money
|
||
is the connection of privately-owned extension phones. You have probably seen
|
||
these sold by mail order houses and local stores. The telephone companies
|
||
claim that connecting these phones to their lines robs them of revenue and
|
||
also may cause damage to their equipment. There are others, of course, that
|
||
hold the opinion that the easy availability of extensions only causes people
|
||
to make more calls since they are more convenient, and that the companies
|
||
really benefit from such use. The question of damage to equipment is also not
|
||
easily answered, since most of the extension phones are directly compatible,
|
||
and in many cases the same type as the telephone company itself uses. Be that
|
||
as it may, this may be a good time to discuss such use.
|
||
Prior to an FCC decision on telephone company interconnection in the
|
||
Caterphone case in 1968, all telephone companies claimed that the connection
|
||
of any equipment to their lines was illegal. This was a slight misstatement
|
||
as no specific laws against such use were on the books. Instead, each local
|
||
telephone company had to file a tariff with the public service commission
|
||
in that state, and one of the provisions of that tariff was that no connection
|
||
of any external equipment was allowed. By its approval of that tariff, the
|
||
public service commission gave a sort of implicit legal status to the
|
||
prohibition.
|
||
In the Caterphone case, however, the FCC ruled that the connection of
|
||
outside equipment had to be allowed. The phone companies then relaxed their
|
||
tariff wording such that the connection of outside equipment was allowed if
|
||
this connection was through a connecting arrangement 'provided by the telephone
|
||
company' for the purpose of protecting its equipment from damage. Although
|
||
this result has been challenged in several states, that seems to be the present
|
||
status. The strange thing is that some telephone companies allow intercon-
|
||
nection of customer equipment without any hassle whatsoever, while others
|
||
really make things difficult for the customer.
|
||
|
||
...WHIPPLE
|
||
|
||
(and Safe Cracker)
|
||
|
||
|
||
The End
|
||
|