How to Listen In by Q This article relates to the field of surveillance. I will not digress into an explanation as to the great importance of surveillance to the serious hacker or phreaker, nor will I attempt to delve into the many legalities regarding this field, as a whole book could be written on this fascinating and important topic. While reading this article, the question might arise as to what surveillance has to do with the field of hacking, phreaking, and computer security. Without getting technical, the answer is simply "everything." As a professional in the surveillance and countermeasures field as well as being an avid telephone phreak and "network traveler," I have found that my professional line of work in surveillance greatly complements my explorations in hacking and phreaking. The following information is only a partial listing of the many devices that are available to the general public. There are many more advanced methods developed and utilized by federal agencies with one sole purpose, and that is to spy upon innocent Americans. Long-Range Listening Devices Shotgun Microphones: A shotgun microphone consists of a long tube either of metal or plastic with a length of 12 to 36 inches. One end of the tube is open while the other end consists of a super-sensitive microphone. The microphone is surrounded by a damper to eliminate vibrations of the tube being picked up. The microphone is connected to a powerful handheld amplifier that usually contains a low pass audio filter to cut out low frequency sounds such as wind and vibrations. The shotgun microphone is extremely directional. A top of the line model can pick up ordinary voices from 3/4 of a mile away. Parabolic Microphones: A parabolic microphone consists of a "dish" composed of metal or plastic with a diameter of 12 to 32 inches. The dish focuses sound waves onto a center focal point an inch above the reflector dish. This sound is picked up by an extremely sensitive microphone and is sent to an amplifier with a low pass audio filter to eliminate wind noise. A top of the line parabolic dish can pick up ordinary voices from over one mile away. As a note, the pattern of pickup is much wider with a parabolic dish so it picks up more background noise than a shotgun microphone would, however the range is considerably greater. Laser Listeners: This is a truly remarkable and complex device that picks audio by demodulating the interference patterns in a laser or microwave beam. A simple system consists of a 15-milliwatt laser. The laser beam is aimed at a piece of glass such as a window. Whenever someone talks, the audio waves vibrate the window a minute amount. As the glass vibrates, it modulates the laser beam much in the same manner that a transmitter modulates voices onto a radio wave. A collector on the receiving unit captures the reflection from the light bounced off the window and an electronic circuit demodulates the collected light and amplifies the audio producing the voices of the subjects under surveillance. Low-end units have a range of 60 feet, while top-of-the-line units can pick up audio from over 500 feet away. High-end systems utilize multiple laser and/or microwave beams to cancel out noise caused by wind. In addition, mylar reflectors are utilized. These reflectors are an inch wide and allow an increased reception range. Through-Wall Listening Devices Contact Microphones: A contact mike is a sensitive microphone utilizing a unique principal that listens for vibrations rather than sound waves. It usually consists of a piece of piezoelectric material that produces an electric current that is modulated by vibrations caused by audio. The contact microphone is coupled to a powerful handheld amplifier either as an integral or separate unit. Contact microphones can clearly pick up a voice through up to 12 inches of concrete or 3 inches of solid wood. Spike Microphones: A spike microphone consists of a supersensitive crystal or electret microphone, and is coupled to a 2- to 12-inch metal spike. This metal spike is driven into the wall and picks up resonations from the wall very clearly. The audio signal from the microphone is then fed into a powerful handheld amplifier. Tube Microphones: A tube microphone consists of a small 2- to 12-inch hollow metal tube approximately 1/8th of an inch in diameter. The tube microphone is placed into a hole in the wall or through an air duct, etc. and picks up sounds coming from directly in front of it. The sound resonates inside the small diameter tube and is amplified by resonation. The audio then reaches a sensitive microphone on one end of the tube. The electric signal from the microphone is then amplified by a powerful handheld amplifier. Hardwired Room Microphones Occasionally the placement of a transmitter aka "bug" is impossible, impractical, or unnecessary. In certain situations it may only be necessary to use a wired remote microphone. Police often use this technique in hotels when engaged in sting operations. Typically, one hotel room is used as the setup room, and an adjacent room contains the surveillance listening post. Microphone with In-line Amplifier: This technique simply consists of a miniature microphone hidden about the target's room. This microphone is then wired into the adjacent surveillance room via an air duct or a hole in the wall. When the microphone is to be placed over 50 feet from the listening post, a miniature in-line amplifier is used to boost the audio signal, and increase the microphone s sensitivity. Hidden Wire-Line Microphone: This is a clever technique similar to the above method, only a pre-existing wire is utilized so as to avoid detection. Usually an electret microphone is hidden inside a splice block, modular phone jack, coaxial cable, intercom wire, or an alarm sensor element, and is connected to a pair of alarm or telephone wires. The listening post simply taps into the wire pair and can monitor all sound within the target room. Fine Wire Laying Kits: This is an old but very advanced technique of hardwiring a microphone that was extensively used by government agencies. It utilizes ultra-thin coated wires, similar to magnetic winding wire. This wire can be laid and run throughout a room or house and remain undetected indefinitely. A fine wire-laying tool is used to spool the wire, as it is laid. This wire can be placed into cracks in the floorboard and under carpet, as well as behind moldings. After laying, a small amount of silicone or beeswax is used to hold the fine wires in place. Advanced fine wire kits utilize a three wire system, where two of the wires are intertwined and the third is run alongside. This eliminates any RF emission from the wire, making it extremely difficult to detect. Hookswitch Bypass: This is an old but very effective technique to monitor room audio by bypassing or shorting out the hang-up switch on a telephone receiver making the phone "hot-on-hook." The room audio can then be monitored by simply tapping into the subject's telephone wire pair. Telephone Line Microphone: This method is similar to the hidden wire-line technique. Only the telephone equipment is used to hide and transmit the room audio. A simple electret microphone could be placed inside a modular phone jack, or perhaps connected somewhere along the line in the target's room, picking up all of the room sounds, when the telephone is not in use. The listening post then taps into the subject s wire pair. A specialized audio filter is then used to strip off the dial tone. Coaxial Cable Microphone: This device consists of a microphone placed onto a television coaxial cable. This method is subject to interference, and there are much better methods discussed later in this article. Transmitters aka "Bugs" Transmitters, often referred to as "bugs" or, when worn on the body, as "wires," are perhaps the most commonly known form of surveillance. This equipment is also the subject of the most misinformation and exaggeration created by the media and Hollywood. Bugs come in a variety of sizes ranging from the size of a beeper to slightly smaller than your pinky fingernail. The greatest falsity created by Hollywood is that bugs can transmit at a range of miles. This is entirely false; bugs transmit on the order of feet, not miles. Typically, bugs can transmit between 75 and 2,000 feet. Another misconception is that the greater the range, the better. While a greater range is certainly more convenient, it leaves the bugged conversations open to accidental interception. Bugs are often prepackaged in various innocuous household items such as RJ-11 telephone jacks and electrical outlets, and can also be carried on your person concealed in fountain pens, calculators, watches, beepers, lighters, etc. FM Transmitters: These are the most commonly available bugs that amateurs can obtain and lawfully use. They operate at a frequency range of 88-130 MHz, and have a power output of between 10 and 100 mW. High level amateurs will usually want to transmit on the 109-130 MHz air band because that frequency can only be picked up on a wide band scanner. FM bugs use a circuit called a free-running oscillator for convenience. This allows the bugs to be tuned on a variety of chosen frequencies. The main problem with operating within the FM radio band is the strong background emissions from commercial radio stations. If the signal from the bug is too weak, it will be ignored by the receiver in favor of the stronger commercial signal. FM bugs are also subject to interference from aircraft. VHF Transmitters: VHF transmitters are occasionally used by law-enforcement personnel and amateurs. They operate at anywhere between 130 MHz and 450 MHz. They either have free running oscillators or are crystal controlled. UHF transmitters: Almost all professionals or law enforcement personnel use UHF transmitters. These operate at much higher frequencies, between 400 MHz and 3 GHz. UHF units are always crystal controlled and operate on a very narrow bandwidth. As a result of the higher transmission frequencies coupled with a narrow bandwidth, these UHF units are free from interference caused by commercial RF background signals and natural anomalies. The transmission range is typically three to five times further than their free-oscillating counterparts. Wafer Transmitters: Wafer transmitters are the most exotic devices ever designed. They are extremely small in size and do not even require an internal power source. They are specially designed transmitters that are powered by strong highly directional RF signals, usually in the microwave range. These powerful signals charge up the circuits of the wafer transmitter. The range of these devices is not very far, but they are extremely small, being no larger than the size of your pinky fingernail. There is another unique type of listening device often categorized as a wafer transmitter that operates on a principal similar to a laser listener. A strong highly directional microwave RF signal is aimed at a target's area. This type of bug simply consists of a very small special piece of material that is flexible and will be modulated by voice waves, and is highly reflective to microwave signals. When room audio is present the wafer transmitter will vibrate. This in turn will modulate the microwave signals that are being beamed into the area. The receiver simply demodulates the reflected microwave signals, producing the audio which was present in the target's room. This technique is extremely high-level and was believed to have been invented by the Russians, who developed this type of device and used it to spy on the American Embassy in the USSR. Crystal Controlled vs. Free Oscillating: Free running oscillators are always used on lower grade bugs. FROs can be tuned through a great range of frequencies for convenience. This type of circuit suffers from three main problems: the first being that the signals are untuned and can produce spurious outputs and harmonics, which allow the frequency to drift, making reception somewhat difficult if the signal is weak. In addition, harmonics allow the signal to be picked up on alternate frequencies by "ghost" images of the signal. The second problem is the weak power output of the circuit. The signal of an FRO is not maximized for any one frequency. As a result, the power output is not as high. And third, an untuned circuit is not as efficient and uses more power, resulting in a shorter operating lifetime and a higher operating current. Crystal controlled units, however, are locked on one particular frequency and, as a result, apply all of their energy to a very narrow bandwidth, making the crystal controlled circuit very efficient. This higher efficiency allows a greater power output per size ratio compared to an FRO. In addition, the highly tuned circuit produces no harmonics, spurious emissions, and no frequency drift, allowing a much greater receiving distance. The power supplies of crystal controlled units typically last 5 to 10 times longer than FROs. Mains vs. Battery Powered: All transmitters are of two types, the first being battery powered. Typically, a battery powered device will last between one day and three weeks, depending upon the efficiency, the power output, and whether the device is free oscillating or crystal controlled. Mains powered devices are powered by anything but batteries. Mains powered transmitters usually come prepackaged into wall outlets or plug adapters. But a clever surveillance expert can wire a transmitter up to anything that runs on house power producing either AC or DC electricity, such as thermostats, intercom wires, alarm wires, and anything else you can think of. Remote Activation and VOX: To extend the lifetime of battery powered bugs, the transmitter must have the ability to turn itself off when not in use. This is done in one of two ways: by remote activation or by VOX (a voice actuation circuit). Remote activation utilizes a special receiver on the transmitter. When the signal is given by the listening post, a particular bug will either turn on or off. A better method is to utilize a voice actuation circuit referred to as a VOX. When a voice of sufficient amplitude is present around the bug, the transmitter will automatically turn on. Both of the aforementioned techniques use a very small amount of current to operate the activation circuits. VOX activated transmitters can have a lifetime of up to one month. Aside from conserving power, an actuation circuit serves another purpose and is useful on both mains devices and battery powered devices. That purpose is to prevent detection of the device. If a transmitter is left running constantly it has a much greater chance of being discovered by various means, including accidental interception on a scanner. A remotely or VOX activated bug is extremely hard to detect except by using advanced countermeasures equipment. If a bug is not activated, then it cannot be detected by conventional transmitter detectors. Specialized devices such as non-linear junction detectors, or a simpler device that feeds an audio source into the room to activate the device, can be used in conjunction with a standard bug detector. Advanced Modulation Techniques: Very advanced bugs are utilized only by government intelligence agencies. Very high-level bugs operate using odd modulation techniques that cannot be demodulated by an ordinary scanner. These odd modulation transmissions also allow for a greater transmission range due to their very nature. Frequency Hopping Transmitters: One method developed to prevent accidental interception or discovery of a bug by a countermeasures expert is to rapidly alter the frequency at a preset rate. This makes it nearly impossible to receive the transmission by accident or on purpose. Even if one knew the various frequencies that this bug operated on, it would be impossible to hear any audio. The reason is that the frequency hopper alters the frequency at such a rapid rate that a modern digital wideband receiver would be too slow to lock onto the signal. All that would be heard is a popping sound for a brief fraction of a second. It takes a specialized multi-crystal, multifrequency receiver to receive this type of signal. Scrambled Transmitters: Scrambled transmitters encrypt the audio signal before it is transmitted, using various methods including the very simple frequency inversion technique, as well as utilizing much more sophisticated methods. If anyone were to intercept a coded signal, the speech would be unintelligible. A special receiver is needed to decrypt the signal. Spread Spectrum Transmitters: Spread spectrum transmission is a fairly sophisticated method of preventing interception of the signal. The RF signal is transmitted on an extremely wide bandwidth. If anyone were to intercept the bug's signal with a wideband receiver, they would hear only an extremely small portion of the transmitted audio. In order to hear the bug s signal one would need several receivers operating simultaneously, each picking up a separate band of audio. A special ultra-wideband receiver is needed to pick up transmissions from this type of bug. Wideband Transmitters: Similar in operation to the spread spectrum transmitter, this type of device operates on a slightly smaller bandwidth. The signals from this type of bug can be picked up on high-end scanners, which have a wide band FM (WFM) mode. Narrow Band Transmitters: Narrow band transmitters have a smaller bandwidth than ordinary RF transmissions. The signal from this type of bug can be picked up on high-end receivers with a narrow band FM (NFM) mode. Sliver Band Transmitters: This is an advanced form of bug that transmits the signal over an extremely small bandwidth. A special ultra-narrow band receiver is needed to demodulate the audio signal. Subcarrier Transmitters: Subcarrier transmitters use an advanced transmission technique to prevent accidental reception and detection of the RF signal. A subcarrier is a type of hidden signal that is modulated piggy-back style on a regular radio signal, both operating on the same frequency. One cannot receive a subcarrier signal with a standard receiver. It takes a special receiver or device connected to a receiver to strip away the hidden subcarrier signal. This makes the transmission secure from being received by ordinary persons. One of the problems with subcarriers is inefficiency. The subcarrier is only about 10 percent as strong as the main parent signal. Meaning that it requires a great deal of electric power to transmit a signal of sufficient strength. As a result, the batteries on this type of device usually do not last very long. Most subcarrier bugs are "mains" operated, meaning they operate using household A.C. power. Using utility power, the device has an infinite lifetime and can transmit a much stronger signal. An example of a subcarrier signal is elevator music. This music is transmitted by a regular radio station, on their subcarrier signal. Another example is the closed caption for the hearing impaired on television transmissions. You cannot see the closed caption words because it takes a special subcarrier decoder to demodulate them. Carrier Current Devices Carrier current devices are a combination of technologies. They are a cross between wired microphones and subcarrier transmitters. The only difference is that the signal is not transmitted via radio waves, but rather through a wire pair. A person cannot accidentally intercept or detect a carrier current signal by simply tapping into a wire like with wired microphones. A carrier current device works by picking up room audio through a microphone. The signal from the microphone is then modulated by a low frequency circuit, which produces a carrier current signal at approximately 100-200 kHz. A common example of carrier current devices are the newer wireless telephones, intercoms, or baby monitor type devices that plug into the electric socket and use the pre-existing wiring rather than having wiring run all over the house. A special circuit, which can demodulate the low frequency signal, is used as the receiver. Carrier current devices require no batteries, as they are powered by the mains. Only a sophisticated receiver with a low frequency probe can detect this sort of device. Powerline Carrier Current Device: Powerline carrier current devices are usually placed inside of wall outlets and are clipped to the powerline. These types of devices are often pre-packaged inside of wall outlets. All that is necessary is to replace the old wall socket for the "modified" wall socket. The receiver can occasionally be placed at any point along the powerline, but usually it has to be on the same side of the utility company power transformer. This is by far the most common form of carrier current device. Telephone Line Carrier Current Device: This type of carrier current device is usually prepackaged inside of modular phone jacks, and then you simply swap the old jack for the new one during the installation process. Telco carrier current devices also can be purchased as separate units that are approximately 1/2 an inch in diameter and are clipped onto the phone line with alligator clips. Piezoelectric Coaxial Microphone: This is perhaps the most ingenious method ever invented for intercepting room audio. Unlike the hidden wired-line method, which utilizes a microphone to pick up sounds and then transmits the audio down a set of wires, this device consists of a length of coaxial wire 2 to 6 feet in length, which contains a thin layer of piezoelectric shielding that is sensitive to vibrations produced by sounds. When audio vibrations are detected by the piezoelectric material, an electric audio signal is sent down the cable wire. All one has to do is tap into the coax at any point and the target's room audio can be heard. An agent simply replaces the pre-existing wire for the "special" wire. Even though the audio is quite easily intercepted, this method will escape detection by even the greatest TSCM (Technical Surveillance Countermeasures) experts, because very few people know of this method (until now!). Infinity Transmitters This is one of the most diverse and useful pieces of surveillance equipment. It is a room audio monitoring device designed to operate on your telephone line. Unlike a bug that can only receive the signal at a finite distance, the infinity transmitter can work at an infinite distance. The design of this device has varied greatly over the years with the advancement of telephones. The device is placed inside of a telephone jack or a telephone itself, and is connected in series to the line. To operate the device, you call the target's house and before the phone rings once, the infinity device answers the phone. You temporarily activate the device using a touch-tone code. This puts the device in a stand-by mode. You then have a brief amount of time to enter an access code consisting of two or three touch-tone digits. If the code is correct the device will be activated and an audio path is established. You will hear all of the sounds within a particular room. Note: If a person does not enter the correct access code then the device will not activate and calls will go through as normal. Infinity transmitters lost a bit of popularity after telephone companies switched to electronic switching systems (ESS). Under crossbar switching systems, telephone lines possessed an audio path between the calling and destination points even if the destination line had not answered the phone. Hook-Switch Bypass: This is one of the most popular surveillance devices of the past. They are not as useful today, because of the switchover to ESS. The "hot-on-hook" technique involved placing a microphone on the target s telephone line, or shorting out the hang-up switch of a phone so that it picked up room audio even when the phone was hung up, and that room audio would be sent down the line. To activate a hot-on-hook device one would call the target's house and enter a touch-tone code before the first or second ring. That code would activate a circuit, which would stop the ringing, and activate the microphone, which would send the target's room audio down the line. The surveillance technician could listen to the line without ever being charged for the call, because the phone was never actually answered. However this type is defunct, because nowadays, under ESS, a device cannot be activated on the target's line until the target answers. This is because ESS never actually connects the two line pairs together until the destination line is answered. Modern infinity devices have found ways around this limit, mainly by having a circuit that answers the phone by itself. You can create a simple hot-on-hook device by placing a microphone on the phone line and listening at some point down the line with a high impedance telephone tap. Dialup DTMF Activated: This is similar to the device described above. You can have multiple infinity devices in one house connected to each phone, each using a different activation code. Each device can be switched on at any time during the monitoring process. Slaves and Loop Extenders Modular telephone taps, often referred to as a slave unit and loop extenders (LEs) are more advanced models of the infinity transmitters. They utilize various multiple line and dial-out techniques. A slave is generally any device that bridges two lines together by a capacitively coupled circuit. Dual Line Bridge Slave: A dual line bridge is a simple connection between two wire pairs. The target's line is bridged at some point along the telephone line, such as an entrance bridge, 99 block, junction or splice box, or a crossconnect- cabinet to a pre-existing or leased line specifically ordered for surveillance purposes. Multiline Dial-Out Slave Infinity Device: This unit is a slightly more advanced type of device that utilizes two phone lines that are bridged across the line pairs at some point. There are two versions of this type of device. The first is a room monitor that is placed within the target s premises and is either built into the telephone or is hidden in a phone jack. The device is actuated by voices through a VOX circuit, which dials out to the listening post on a second line not used or owned by the target. The second is a telephone monitoring device, which can be placed at any point along the telephone line, such as at 99 blocks, entrance bridges, splice boxes, junction boxes, and cross-connect-cabinets. The target's line pair is bridged onto another line usually owned or leased by the surveillance expert. When the target attempts to use his telephone or a call is received, this slave unit automatically dials out on another line to the listening post, which enables the surveillance expert to monitor and record the target's phone calls. Advanced Dial-Out Slave Infinity Device: A third more advanced type of unit is simply a combination of the above two that incorporates voice infinity and telephone infinity transmission. Units may be a combination of dial out or dial in. Typically the dial out function is for telephone, and the dial in function is for room monitoring. Remote Listening Post Infinity Device: This is the most advanced and diverse type of slave infinity device that utilizes multiple telephone lines as well as radio receivers, and a built-in tape recording unit, which is all microprocessor controlled. This unit is an all-in-one surveillance infinity monitoring system. Loop Extenders: These devices are too complex to discuss in detail in this brief article. Telephone Taps and Transmitters Hardwired Tap: A hardwire tap, which is commonly referred to as wiretapping, is the easiest and oldest form of monitoring telephone conversations. All that is needed is a pair of mono headphones with the jack cut off and replaced with alligator clips, or a lineman's handset (often referred to as a butt set). A phreak might refer to a lineman's handset as a beige box. An individual can tap into a phone line at virtually any place along the line including entrance bridges, 99 blocks, junction boxes, and cross-connect-cabinets. This type of tap is extremely simple and can be performed by even an amateur. If a permanent tap is left in place by running a wire to the listening post, and is too close to the target's residence or office, it could be detected by physical search or with advanced equipment such as TDRs or phone analyzers, if countermeasures sweeps were performed. Inductive Coupled Line Pick Up: This is virtually the same type of hardwire tap as above, however no direct connection is actually made to the line. An inductive probe is simply clipped around the telephone wire and the emanations from the wire are picked up by the probe. Since no actual electrical contact is made during the tap, not even the most advanced equipment could detect such devices. As with the hardwire tap, if a permanent induction tap is left in place too close to the target's residence or office, a thorough physical search could find the tap. Series Transmitter: A series transmitter is a bugging type of device that monitors phone conversations instead of room audio. This type of device is connected in series to the phone line and never requires batteries because it draws its power from the phone line itself. The range is not as great with series transmitters as it is with parallels, however its virtually infinite lifetime is an advantage. The frequency and power output of telephone transmitters is virtually the same as for standard room bugs. Series transmitters occasionally incorporate an automatic activation switch that turns the device on only when a telephone conversation is taking place. Parallel Transmitter: A parallel transmitter hooks to the phone lines in parallel, which enables the transmitter to be simply clipped on without breaking any connections. The power output of parallel telephone transmitters is a bit higher than with series devices usually by 20 to 50 milliwatts. However, parallel devices must use their own power source, usually a 1.5-12 volt battery. The frequencies are identical to that of series telephone taps and room bugs. The lifetime of these devices is finite and can only operate constantly for two to five days. Higher quality models almost always incorporate an automatic activation circuit, which will turn the device on only when the telephone being monitored is in use. This additional circuit extends the lifetime of the tap from three weeks to a month. Advanced Transmitters: This advanced type of tap is a combination of series and parallel circuits and bridging. When the phone is not in use the parallel circuit "trickle charges" a rechargeable battery. The device contains an automatic activation circuit and when the phone is being monitored by lifting the handset from the base, the series circuit activates and transmits using the self contained battery. This device yields the higher RF output of a series device while having a virtually unlimited battery lifetime similar to a parallel device. Super Miniature Tape Recorders Super miniature tape recorders are extremely useful devices for surveillance purposes. They have many uses: primarily recording conversations pertaining to illegal or civil matters, which can be either used as evidence in a court of law or simply to alert law enforcement personnel. Recording devices vary greatly in size, recording quality, as well as other important features. Top of the line models designed and manufactured specifically for surveillance purposes can cost several thousand dollars. Size Specifications: Super-miniature recorders designed specifically for surveillance are generally much smaller than tape recorders available for consumer purposes. Many of the features available on consumer recorders are not necessary on covert surveillance recorders. Only the most important features are designed into these super small recorders in order to save space. High-level recorders never have built in speakers, since speakers take up a considerable amount of space and serve no purpose on a recorder. To play back the recorded media, a separate speaker and amplifier playback unit is used. Electronic Shut Off: Surveillance recorders almost always incorporate electronic shut off. The mechanical shut off buttons are too bulky, and more importantly make too much noise when the tape automatically is shut off. Should a surveillance recorder ever shut off automatically, the loud click of the mechanical button could make the subject being recorded very suspicious. Silenced Motors: In typical consumer micro-miniature recorders, the tape drive motors can produce a sufficient amount of unwanted noise. Surveillance recorders contain extremely quiet motors that cannot be heard even in the quietest atmosphere. Altered Bias Oscillator Frequency: This is perhaps the most advanced feature of surveillance recorders. When recording a subject, every precaution must be made to avoid detection and suspicion. If the person under surveillance is an expert in surveillance or if he is particularly suspicious, then the subject could use a counter-surveillance device that detects tape recorders. This antibugging device detects the emanations from the bias-oscillator of a tape recorder within a certain range. These devices can detect a tape recorder from up to several feet away. A true surveillance recorder will alter its bias oscillator frequency so that it cannot be detected by the aforementioned countermeasures device, rendering it undetectable. Tape recorders that alter the bias-oscillator frequency must contain special audio compression circuits to compensate for the effects of the altered circuit. Multitrack Recording: High-end recorders will usually have several tracks for recording. Two tracks are usually for the stereo signal and the third is for time coding or reference signals. Extended Play: Surveillance recorders often are required to record for extended periods of time. Rather than using longer tapes, the recording speed is slowed down. This results in a bit of distortion, so extended play recorders incorporate compensation circuitry. Nagra Magnetic Recorders Inc. is the leader in manufacturing surveillance recorders. Their top of the line model is the Nagra JBR, which contains all of the advanced features described. Its dimensions are 110x62x20 mm and it weighs 143 grams. The National RNZ 36 is one of the smallest units ever produced, however it does not contain several advanced features necessary in high security situations. This unit has a three-hour extended recording time. Its dimensions are 85x54x14 mm making it nearly as small as a credit card.