# How were four wires replaced with two wires in early telephones?

In this video, when A wanted to talk to B, a microphone was connected to a distant speaker using two wires (A → B).

When B talked to A, a copy of above was used in the other direction (B → A) using two more wires.

This required four wires:

But then it says the two-way communication could be made by using just two wires:

How was this possible? Wouldn't the current from the microphone affect the speaker on the same side?

• You actually want the local speaker to respond to the microphone, otherwise the user keeps raising their voice, because they feel like their voice isn't carrying. But you don't want it to give respond too strongly or the user will keep lowering their voice until the far end user can't hear them. Commented Jan 19, 2021 at 2:15
• I was just reading about this. They call it a hybrid circuit. Googling that may shed some light. In this day and age it is certainly possible to subtract out the local contribution from the microphone using op-amp circuits. In the old days they used transformers. Commented Jan 19, 2021 at 2:17
• This may be of interest: sound-au.com/appnotes/an010.htm Commented Jan 19, 2021 at 2:19
• Old phones had transformer. When signal comes from microphone to line it also added in reversed phase to speaker, so you do not hear youself. On line signal is mix from two sides signal. Commented Jan 19, 2021 at 2:19
• In fact the "hybrid" circuit still exists in semiconductor form - these days it's called a "2 to 4 wire converter". The Elliot sound link above goes into the deatils very well. Commented Jan 19, 2021 at 11:25

How was this possible? Wouldn't the current from microphone affect speaker on the same side?

The modern telephone is wired in a Wheatstone bridge arrangement like this: -

So, if you ensure that the telephone network line impedance ($$\Z_{LINE}\$$) is controlled then, theoretically, any signal produced by the microphone is dramatically reduced into the local earpiece. It won't be a perfect cancellation but it'll be pretty good.

Amended picture originally from here. It might be easier to understand this diagram: -

Picture from here.

And, all throughout the network there are line amplifiers that need to translate from 2 wire to 4 wires so that amplifier circuits can be added: -

Picture from here. Then another hybrid transformer is used to reconvert the 2-way (4-wire) amplified signals back 2-wire: -

Here's an example of an early telephone anti-sidetone circuit using the same principle as the hybrid transformer: -

Picture from this website.

• so the speaker is placed at the center of a balanced wheatstone bridge, then the signal from the mic would never reach the speaker. Clever! Commented Jan 19, 2021 at 12:34
• @across correct Commented Jan 19, 2021 at 12:36
• Not the very first ones. But OpAmps were build with tubes since the 40s. Commented Jan 19, 2021 at 15:00
• @Andyaka Normal telephones didn't have amplifiers until the 1970s or later. Commented Jan 19, 2021 at 17:48
• @grahamj42 correct and, the hybrid transformer was used before amplification. Of course these needed carbon granule microphones to get anything like a line level signal from the microphone. Not great but, as we who remember, know it worked (distortion and all). Commented Jan 19, 2021 at 18:33

Remember that the telephone was invented over 30 years before the triode vacuum tube, and over 70 years before the transistor. Changing, combining, and subtracting audio had to be done with resistors, capacitors, and transformers - only. Early phones had a "network" inside, of which the major component was the hybrid transformer. This had four-to-six windings, connected such that the signal from the microphone both drove the phone line and was subtracted from the signal going to the earpiece. The subtraction was not perfect intentionally, allowing the talker to hear themself in their own earphone. This let the talker's brain act as an automatic volume control. The leaked audio signal is called sidetone.

Search for telephone network schematic to see examples.

It is done with a circuit that sends the microphone signal to wires, but removes your own microphone signal from the wire signal before sending it to the speaker, so that only thing that can be heard from the speaker is the signal from the remote mic.

Such a circuit is called a telephone hybrid, so you can find more info if you like.

Ah yes Plain Old Telephone System or POTS is a thing of beauty. Who still uses it?

In order to have two baseband signals over shared 2 wires, one way is current modulation and the other uses voltage-modulation over a controlled impedance using hybrid centre-tapped series-parallel transformers to perform this.

The early mics were very near-field sensitive carbon-diaphragms that modulated impedance.