4. TELEPHONE SYSTEMS

4.1 Basic Operation of a Telephone

The word telephone means "sound at a distance". The most basic form would consist of a microphone at one end (A) connected to an earpiece at the other end (B) by means of a pair of wires.
The above circuit would work, and indeed it would work both ways - from A to B and at the same time from B to A. However, the output would be very small and require an amplifier, making it a one way device and requiring a second pair of wires for the return path (two-way conversations over one pair is achieved by using what are known as hybrid circuits at each end to isolate signals flowing one way from those flowing in the reverse direction).

Thomas Edison, who discovered the principle of the telephone in 1876 didn't start off with amplifiers. Basic telephones employ a carbon granule microphone, where sound waves striking the microphone diaphragm are transmitted to a piston held against a small chamber containing the carbon granules. When a battery is connected across the carbon granules, a current will flow because carbon is a moderate conductor of electricity - not as good as copper, silver or aluminum but not an insulator either. The speech waves alternately compress and release the carbon granules, thereby reducing or increasing the resistance across the microphone in sympathy. Today a telephone at B will receive the sound waves at the remote end with very much greater volume.
The above diagram represents the basic telephone circuits as still in current use. Of course, there are many additions and improvements. A two-way telephone consists of a telephone and earpiece at both ends

In the above systems, the voice IN at end A will be heard in the earphone at B and also in A (and also at any earphone an eavesdropper places in the line along the way). A's voice coming out of his own earpiece is known as sidetone: all telephones include a controlled amount of sidetone to stop people from shouting into their telephone microphones. Other improvements include allowing the earphones to be connected to the battery DC potential (by the use of capacitors and inductors), but not restricting the AC voice frequencies.

4.2 Basic Telephone System

In a phone system it is necessary to be able to switch the phone off when not in use (by means of a hook switch), be able to talk from A not only to B, but by individual choice to C, D, and all other phones. So each phone is connected directly to an exchange or switching center.

In each switching center, each telephone line terminates at its own central equipment unit, which, as a minimum, consists of an on/off hook relay, ringer relay, and a call charge facility.

In order to standardize on a worldwide basis, the basic technical services provided by individual telephone companies is much the same everywhere, and is shown below.

The telephone exchange normally has a central 48 volt DC battery which is constantly being trickle charged from the main electricity supply. Some countries use different voltages, but they normally lie in the range of 36 to 60 volts. This supplies all the subscribers individual on/off hook relays, and through them, the subscribers individual telephones. Thus, a standard telephone is operated by the telephone company central battery, and it continues to function even in the case of a power outage at the subscribers premises. Of course, if he has fancy phones which require additional power from the wall receptacle to operate, a power outage will affect some, if not all, of his phone facilities.

In the "On Hook" condition, the subscriber's phone consumes no power. His bell is connected to the line through a DC blocking capacitor, and no current flows. To DC, his phone looks like an open circuit, as if it was disconnected. Under these conditions, a voltmeter will measure the exchange battery voltage, that is : 48 volts DC.

If the exchange wishes to ring the subscriber, they put an alternating potential of about 100 volts peak to peak at about 17 to 20 hertz down the line. This passes through his bell capacitor and rings the bell. In former times, an exchange operator pulled their "ringing key" down for that subscriber: now the interrupting ringing voltage is put on automatically. At the same time, an interrupted audio tone is passed back along the line to the calling telephone to indicate this chosen subscriber's phone is ringing. Nowadays, the called telephone's circuit is split at the ringing relay: the caller is not actually hearing the 20 Hertz ringing tone itself but a simulated tone of approximately 400 Hertz passed back to him. This is an important point to note since a direct connection between caller and called line is not established until the called phone is lifted off hook. So called "infinity transmitter" taps can no longer be activated without the co-operation of the called telephone, as was originally the case.

As soon as the called telephone lifts his phone off hook, his ON-OFF hook relay in the exchange pulls, which switches off the ringing tone at the exchange and switches in the distant caller. The reason for his ON-OFF hook relay pulling in the exchange is that when the subscriber goes off hook, the line is transferred from his bell circuit to his handset (earphone and microphone effectively in series), which appears as a resistance of approximately 150 ohms. The voltage across the subscriber's phone will now be

When the subscriber lifts his phone off hook to make a call, his off hook relay at the exchange pulls. As there is no incoming call, it puts him through to dialing tone and at the same time "busies" his line to other callers.

What is important to us is to note that we should expect around 48 volts on hook and 9.6 volts off hook. Telephones are not built to be exactly 150 ohms, so readings will be only approximate.

A word of warning: if you try to measure the resistance of an disconnected phone "off hook" with a standard multimeter, you may find resistance readings of anything but 150 ohms. Most will lie in the range of 200 to 20,000 ohms, and this is due to the fact that resistance meters are usually driven with 1.5 volt batteries. Due to the non-linearity of diodes and other devices such as thermistors within phones they will only look like 150 ohms when driven by a higher voltage such as the 48 volts supplied by the exchange battery.

4.3 Basic Telephone Dialing System (D.C.)

Apart from possible line polarity reversals for metering purposes, use of phantom earths for two party lines and other such possibilities, the only other facility to be noted here is the dialing facility.

When lifting a phone "off hook" to initiate a telephone call, the subscriber's on-off hook relay is pulled. This relay is "slugged " so that it will only drop out after a finite period, say around 1/2 second. Short taps on the phone hookswitch should not cause it to drop out. Now, beyond the subscribers on-off hook relay in the exchange lies the common dialing selector system. When the subscribers telephone dial is swung and released, it causes a succession of short breaks in the circuit in accordance with the digit dialed. These short open circuit breaks cause the line voltage to rise for the duration of each break and this steps selector units up to the appropriate digit dialed. The breaks are not long enough to cause the on-off hook relay to drop out. Once an individual digit has finished pulsing, the exchange selector system waits a moment and then moves on ready to receive the next digit, and so on until dialing is complete. In newer telephone systems, pulsed DC dialing is replaced by audio tones to identify dialed digits.

4.4 Exotic Telephone Systems

Regardless of the subscribers personal telephones or telephone system, it has to present the standard voltages, resistances, etc. to the telephone company, the same as the lowliest, oldest standard non-electronic telephone.

In the standard telephone, the exchange line was switched right through in solid copper connections via hook switch etc., to the handset (in order to power the carbon granule microphone, among other things).

In modern electronic subscriber telephone systems, the telephone company's line pair usually terminates in a resistor or similar component on a printed circuit board and goes no further. signals from then on proceed through a series of circuits - transformers, amplifiers, clippers, solid state switches, level normalizers, etc.

In a multi-station electronic phone system, the telephone company's subscriber line end up on connection blocks which are through connected to a central electronic equipment cabinet, full of printed circuit boards fed with power via a power unit, from the regular electricity supply.

Systems vary, however most systems are normally connected to the central electronic equipment cabinet via a four to eight line cable, where two of the lines carry audio (voice) plus a fixed DC voltage, while the other two carry digital data. Some systems run the DC through a third pair of wires.

The introduction of electronic subscriber phone systems has reduced the opportunity for the fitting of infinity and other straight phone taps at the telephone instrument, but does not preclude the fitting of radio transmitting taps.

However, the standard telephone systems are far from dead and will remain with us for a decade or more.

4.5 Applications of Telephone Analyzers

The Telephone Analyzer will test all standard telephones and associated lines, normally from the telephone instrument interface plug. This is done to a point at which the telephone lines are interrupted (PBX, electronic switching equipment, telephone company switching equipment, etc.). If arrangements are made, testing of lines through manual switchboards can be accomplished as well.

Section 4.1 to 4.4 above have described the telephone system reduced to its basic elements, to enable the TSCM survey operator to understand and interpret the results of line measurements made with the telephone analyzer, or indeed with any other measuring system.

4.6 Telephone Analyzers

Telephone analyzers are measuring devices consisting of primarily a Volt-Ohm-Ampere meter, an audio amplifier, associated switching system and occasionally a polytonic tone sweep generator.

4.6.1 Volt-Ohm-Ampere Meter

The Volt-Ohm-Ampere (VOA) meter is an item of electronic test equipment utilized to determine the operating characteristics of an electronic circuit: voltage - resistance - current. In utilizing the VOA meter, safety should always be of primary importance when attaching and removing the meter leads. Whenever possible, try to insure that power is off and the power source disconnected prior to attaching the leads. When performing resistance measurements, insure that the power has been turned off as permanent damage to the metering circuit could occur.

4.6.2 Audio Amplifier

The audio amplifier is an item of electronic equipment utilized to amplify the low level electronic impulses developed by microphones, speakers, etc., contained in telephones or installed on telephone lines. In addition to their amplifying capabilities, they also contain a circuit which generally protects them from voltages present on the lines being examined.

4.6.3 Switching System

Many telephone analyzers contain an interface consisting of a series of switches, either toggle or rotary, which readily allows the examination and comparison of a number of incoming telephone wiring combinations.

4.6.4 Polytonic Tone Sweep

The polytonic tone sweep circuit present on a number of telephone analyzers generates 1, 2, 3 or more tones and introduces them on to the telephone pair under examination in an effort to activate "harmonica bugs" which may be installed in or near the telephone instrument. The tones are generated and are constantly sweeping up and down in frequency in an effort to generate the proper frequency or combinations of frequencies required to activate a device.

4.7 Interface Device - Telephone Analysis

As stated above, the telephone analyzer consists of a VOA meter, an audio amplifier, a switching system and, occasionally, a polytonic tone sweep generator. Individual items of equipment can be utilized just as effectively, and very often provide better results.

For best results, an interface device should be utilized which allows the connecting of the examination equipment to the system/circuit to be examined. (As most telephone systems in use today in business and government locations are of the electronic type, the capability to interface with a standard 4, 6 or 8 pin telephone connector is all that is required.) The inputs from the telephone connector is then routed through a series of 8 switches for individual and combination comparison and routed to an output jack for connection of the measuring equipment (audio amplifier, VOA meter, etc.). The use of such an interface box negates the requirement for opening the instrument in order to effect the measurements. (This is a consideration for examinations which are recurring. An instrument is examined electronically and physically, at which time a "shatter seal" is applied. An intact seal means that the instrument has not been opened since the last examination and only spot physical examinations are required.)

4.8 On-Hook Tests

In performing on hook examinations, the telephone analyzer is interfaced with the circuit to be examined. The audio pair (on an analog system) is identified by performing an off-hook test and the high gain audio amplifier is utilized to determine whether audio is present during any time in which the instrument is not in use. Particular attention should be paid to the possible presence of low level audio when the instrument is in the on-hook condition or when the phone is placed on hold. If applicable, activate the voltage test circuit associated with the audio amplifier (Used to activate voltage activated microphones).

Obtain on-hook voltage measurements for comparison to other telephones within the system and for comparison purposes upon followup examination.

4.9 Off-Hook Tests

Continuing with the above hookup, lift the handset off the cradle and perform similar examination, recording the results.

4.10 Practical Bugs

For all practical purposes, an electronic bug or wiretap requires a minimum voltage for its operation.

Because it is often not possible to rely on the polarity of a given telephone line being constant, and also to protect the device from destruction in the event of a reversed connection, a bridge rectifier is incorporated. The minimum junction drop in any semiconductor device (diode, transistor, integrated circuit) is 0.6 volts (silicon devices) or 0.3 volts (germanium devices).

A bridge rectifier always has two devices in series conducting and the circuit itself will have at least one transistor in circuit. Thus, the minimum voltage drop in three junctions is 1.8 volts (silicon) or 0.9 volts (germanium). This is before the device does any work, i.e., amplifies a signal, or transmits a signal. This is likely to take 2 or 3 volts or more to operate a transistor on a reasonable part of its characteristic curves and to generate even a minimum of power.

Practical bugs and wiretaps are thus likely to consume 5 to 7 volts; figures have been calculated for 3 and 5 volt devices.

4.11 Electronic Telephones and Systems

Electronic telephones and systems pose a special problem to the eavesdropper and to the countermeasures operator.

With single AC operated electronic telephones, and automatic answering telephones, wiretaps are possible as for standard telephones, except that "transmitting microphones" may not be possible since the exchange battery voltage may not be extended to the handset in the off-hook condition.

With electronic telephone systems consisting of several electronic telephones and a central equipment cabinet, conventional wiretaps are only possible up to the point where the exchange line terminates within the cabinet. Infinity transmitters are therefore of no value since their built in microphones (those relying on employing an extension telephone microphone are inadmissible) would hear only those noises in the vicinity of the electronics cabinets. Any other series (or medium impedance parallel) bugs placed on the exchange lines would be detected in the normal way, the bugs in such cases would be "upstream" of the electronics cabinet, between the cabinet and the exchange.
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