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.