How to record telephone conversations

Sometimes it is useful to record telephone conversations. The easiest way is to go to a shop and buy a telephone conversation recording adapter which is connected between telephone line and the tape recorder. Those recording adapters are usually connected between telephone line and the telephone, but there are also other type available.

The adapter has two functions: it connects the audio from telephone line to recorder input at correct leveal and it isolates the recorder electrically from telephone line. The isolation is very important part of the adapter, because connecting tape recorder directly to the telephone line is against telephone regulations, causes all kinds of noises to telephone line and will damage your recorder. Some adapters have also tape recorder controlling function enabling recorder to start automatically when phone is picked up and stop when phone conversation ends.

How the audio interfacing in the adapter works

Adapter in parallel with telephone

The audio interfacing is usually done by connecting the audio recording circuit to telephone line in parallel with telephone. The audio circuit is designed so that it shows high impedance to the line, but pics up enough signal to feed the tape recorder microphone input. Because the circuit is high impedance and does not pass any DC cirrent, it can be always connected to the telehone line and does not disturb the normal operation of the telephone. In the picture below there is a typical schematic of one telephone recording adapter.

Circuit diagram

The circuit is quite starighforward. The capacitors C1 and C3 make sure that no DC passes the circuit, but the audio can go to through the circuit. Resistors R1 and R3 provide high impedance and attenuation to the audio signal. Audio isolation transformer T1 provides isolation between telephone line and tape recorder. Components R2, C2, F1, F2 and R4 provide extra filtering and protection to the circuit. Remeber that the there can be great difference in the levels of the sound signals coming from distant telephone and local telephone. The local signal is about 20 dB hotter (normal value in USA telephone system) than the distant one, but they're both on the same pair of copper. So you might find that the voice coming form local telephone is ramatically louder than the voice from the other end of the telephone line.

Adapter in series with telephone

There is also another way to do the audio connection. It is possible to connect an audio tranformer primary to the telephone line in series with the telephone to pick up the audio signals from the line.

    To        <----------------------+
    Tape                             |
    Recorder  <------------------wwwww
                                 mmmmm    Transformer
                                |     |
  Line >------------------------+     +----------> To Phone
  Line >-----------------------------------------> To Phone

This can be done without disturbing the telephone if the transformer has low primary DC resistance and low impedance so that it does not cause too much attenuation and impedance mismatch to telephone line. The transformer must be also capable of handling the DC current which passes through telephone line when telephone operates without saturation. This circuit work by just using one transformer, which is connected between telephone line and tape recorder. The disadvantage of this circuit is that it must be connected in series with telephone to works. This also needs a special transformer which has low primary imedance and can withstand telephone line DC current without saturation.

One idea syggested in news articles is to build this circuit easily by clipping a ferrite transformer core around one of the wires in the telephone line wire pair. Twist another winding of 100 turns to the same core. This forms a 1:100 transformer which can be amplified by a differential op-amp configuration. In this way the circuit makes no connection to the telephone line and does not have any problems from telephone ring current.

How the tape recorder controlling works

Many tape recorders have some sort of way to externally start and stop them. The most common is a small 2.5 mm jack for tape remote controlling. That plug works so that when a plug is plugged to that control jack, the tape recorder motor is stopped. When the tip and ring connections of the jack are connected together, the motor start running normally. And when the connection between tip and ring is removed, the motor stops again. In this way the recording can be controlled from external circuit very easily and the tape recorder buttons are all the time in record position. Some tape recorders have this control function in their microphone or spacial control connector, but the basic idea of the controlling is the same.

Telephone line current sensing method

The most straightforward way of adding telephone controlling function to telephone recording unit is to add a relay which senses the telephone line operation current and controls the recorder. That relay coil is simply connected in series with telephone in on of the wires going to telephone. The basic idea is very easy and works reliably. All you have to find is suitable relay, which will pull at normal telephone operation current, can handle the maximum line current, has good enough insulation to withstand telephone line overvoltages and low enough resistamce to not to disturb normal telephone operation. The relay should also be selected so that it does not trip at normal telephone ring currents. There is usually a few microfarad capacitor (usually bibolar electrolytic) in parallel with relay coil to allow audio to pass easily the high inductance relay coil. One commercial circuit used a relay with 150 ohm coil resistance and there was 47 uF 25V bipolar electrolytic capacitor in parallel with relay coil. Here is a schematic of one simple telephone controlling circuit:

    To        <----------------------+
    Tape                             | <==== Relay Contacts
    Recorder  <------------------o/  o
                                 mmmmm DC 3v 20 mA Relay Coil
                                |     |
  Line >------------------------+--||-+----------> To Phone
                              47 uF Capacitor
  Line >-----------------------------------------> To Phone

Line voltage sensing method

Other method is to control recorder by sensing the telephone line DC voltage. When telephone in on-hook, it does not take any current from line and there is full 48V central office battery voltage present between line wires. When telephone is picked up, the telephone takes it's operation current from the line and the voltage drops to about 6-10 V at the subscriber's end. By sensing that line voltage it is possible to see when telephone is picked up and start the recorder.

You can find a circuit which controls recorder using line voltage sensing method from That circuit controls a recorder and provides the audio interface to it, but the circuit does not provide isolation between telephone line and the recorder. This type of circuit should only be used with battery powered fully isolated tape recorders. I think that this circuit does not meet the telephone line equipment regulations of many countries.

The idea of tape controlling is the same as used in telephone line busy indicators, but the light is replaced by relay which controls the tape recorder. One circuit which present how to do this phone in voltage sensing method, check the circuit which you can find at

Voice sensing

Some tape recorders have built in voice sensing circuit which makes them to start recording automatically when they detect voice. When they do not detect any voice they will stop automatically. This type of recoders don't need any special controlling interface, because that voice detecting is enough. When the telephone is not picked up, the line is silent and when somebody is talking to phone then the voice detector can start the recording. The telephone interafacing needs only the audio interface circuit. When using voice activated tape recorders the voice detector sensitivity must be carefully adjusted so that the recorder will start every time needed, but does not start because of the line background noise (this varies from line to line).

Other methods for recording telephone conversations

Recording units connected to handset wire

Some commercial telephone recording devices are connected between telephone base part and handset. Those devices are small boxed which are placed between telephone ant the modular wire coming from telephone base part to handset. The telephone conversation can be taken from that wire, because it is carrying the microphone and speaker signas which are connected to base part of the telephone. The signals going in this wire between base part and handset are no fully standardized, which means problems to those who are trying to make devices which connect to this interface. But this interface has the advantage that it can be also used with those digital telephones, where recording from line wires is not possible.

Typically the handset of the telephone is connected to the base of the telephone using 4 wire connector. Two of those wires carry the signals to the handset spaker and two other wires on the other side of the connector carry the microphone signals. The standard handser wiring used in telephone in USA has Green and White leads connected to earpiece and Black and Red wires are connected to microphone.

There are differences on the signal levels used by deferent telephones, so typically the equipment which connect to handset connector have some adjustments to make them suit to many telephone types. Well adjusted recording interface which is connected to handset interface gives good balance between the local voice and voice from distant end (much better than recording devices which connect to telephone line).

Inductive pickups

Some telephone amplifiers use small inductive pickup coil connected to headset near the spaker using suction cup. This inductive pickup pics the small changes of magnetic fields which the telephone speaker causes. In this way it is possible to get rocording of both sides of conversation, because some part of the speech which the telephone picrophone pics can be heard from speaker side of telephone. The inductive pickups don't generally work very well and they must be placed quite carefully to get best results. Still they are quite prone to external interference and do not work well with modern telephones which have small speakers (and smaller magnetic fields).

Reocird sound from wire without galvanic connection

Retell makes a product called Watsonlinc Connector which picks up the signal through the insulation of the normal telephone line cable. The telephone wire is jut made to go thugh the connector box and it picps up the sound from it (propably listens on magentic fields).

Tomi Engdahl <[email protected]>