Is The Intercom An Ear Of A Spy?

Back in the days of analog telephone, there were rumors about listening to the line, even about the ability to listen to the situation without picking up the phone and talking on it. With the advent of push-button telephones, the problem seemed to go away, moreover, in connection with the principles of switching signals on twisted pair, this was problematic.


The article describes the version - what if, today, this opportunity migrated to intercom networks, moreover, it is used for tacit interference in private life?

In the digital age, nothing should seem transcendental. Perhaps in your home, you should also carefully choose words and topics for everyday conversations, no matter how harmless they would be, if not today, then in the near future.

Today, almost every apartment has a door phone, in particular, a subscriber unit (apartment intercom), made in the form of a telephone receiver, commonly referred to as the common people - an audio handset.

First, a little theory about the types of intercom

According to the principle of operation, intercoms are analog and digital .
By assigning, they are divided into individual and multi-tenant (multi-unit), which, as a rule, are installed in the entrances of multi-apartment buildings, providing communication between visitors and apartment owners.


The multi-door intercom operates according to a mini-automatic telephone exchange (analog or digital).

Analog (coordinate addressing system) works on the principle of conventional telephone communication using an analog switch (mini telephone exchange) and a special multi-core cable connecting the switch to apartments, they are more common and less expensive.

Digital wiring on the porch use only two wires (two-wire cable) to communicate with all apartments on the porch and the digital intercom has the ability to perform additional functions, such as internal communication (intercom) between the tenants of the porch and the concierge serving this device. There are still video intercoms, as well as GSM with a call forwarding function.

About audio handset

In addition to the speaker and microphone, it also contains a control button for the electromagnetic/electromechanical lock of the access door.

It works on the principle of so-called duplex communication - it is a two-way communication that can be carried out simultaneously.

When you press the "talk" button, as well as picking up the handset of the apartment intercom, the telephone line resistance changes, which is perceived by the microprocessor device (switch) and it sends the corresponding signals.

All circuit diagrams of analog audio tubes are almost identical, simple, but at the same time cleverly executed. The bottom line is that the speaker is always connected directly to the line, however, like a microphone through a resistor.

For those who do not know how to read radio circuits, I will explain.

To de-energize the entire circuit, you can only turn off the device with a switch, if any. But anyway, when you pick up the tube, the circuit is again powered. The speaker is turned on directly, and when answering - through a resistor (voltage divider). Trimmer resistor R5 is designed to adjust the balance between the speaker and the microphone. Another radio component that affects the audibility from the speaker is the capacitor C4, but not the point.

Here the problem is not even whether the microphone is listening or not, but that the loudspeaker is the ear in absolutely all audio tube designs. Not many people know that the same speaker (telephone capsule) may well act as a microphone, even if not with the best sound quality and features. In this case, the considered scheme uses a dynamic SD-150 capsule.

About capsules

Telephone capsules are electromagnetic , electrodynamic , piezoelectric.

Electromagnetic contain a permanent magnet and a ferromagnetic membrane, electrodynamic contain a permanent magnet coil with a movable element, and piezoelectric based on the piezoelectric effect.

Unlike the loudspeaker head (speaker), the capsule does not have a diffuser or horn, so its use as a microphone, which is problematic (but still possible).

So, our capsule has good electro-acoustic characteristics. Sensitivity above 90dB in the range from 200 to 4000 Hz, R ~ 120 Ohms.

What do we have?

The telephone capsule of the handset is constantly included in the intercom line.

The tube with the capsule lies on the body of the device and is turned to it, which has openings, thereby the body is an enlarged area of ​​the diffuser, and in addition the entire wall during wall installation of the device.

Suppose someone has the intention to listen, record and analyze conversations. Doorphone networks here are an ideal tool for this role: tenants themselves pay for infrastructure support, put it into their house voluntarily.

Moreover, digital models make it possible to solve such problems much more efficiently, more selectively, their activation will be invisible, and it is already almost impossible to understand the average person in the logic of the program sewn into the chip, in general, it will allow you to shoot full-fledged home video.

You can find stories on the network when people receive calls to the wrong apartments, they turn out to be parallelized or intercom conversations are duplicated to other apartments. All this can be attributed to mistakes, but when you live in the era of digital technology, nothing should seem transcendental.

This is not a crazy idea, but for some reason, no one is touching or studying this topic, there are no articles about this, you will not read in any amateur radio forum.

The theme goes at the level of speculation.
In any case, having a prying ear in your house, you should carefully choose words and topics for everyday conversations, no matter how harmless they would be.
All of the above should be considered - an assumption not without meaning. There is no evidence of the intentional use of special equipment for tracking tenants here until everyone checks this if allowed.

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