Feedback - Why doesn't it occur?- since Mic and Speaker signal travell same line ate same time? (e.g. land phone, door answering system, etc)

[migadrenaline]

I am NOT talking about feedback using "the air" as medium but directly on the (electrical) line - MY QUESTION IS:

How (or why) doesn't feedack occur, since the microphone and speaker of a door answering system or even on a telefone land line, have the microfone/speaker signal travelling in the same (electrical) line at the same time?



(PLEASE TAKE IN MIND that telefones exist since way before complicated electronics like "real time feedback attenuators" existed - How was this kind of feedback - direclty on the electrical line which the signals travell on -avoided in the early phones since in those days no complicated electronics was used as well)

Although I am an Enginner my area as nothing to do with electronics nor audio - but you may (please) anwer in an "enginner kinda way"


PS - One idea, for instance, could the Mic and Speaker signals be of phase in any sort of way that is really simple and may be a simple rule of physics that we usually don't talk about?

 

[JerryLove]

Because only one, the mic, introduces variation in the current (only one creates a signal).

Feedback requires that a variation cause a variation that causes a variation.

The speaker doesn't add anything electrically except resistance (doesn't create a signal), so there's nothing to feed back on.

This works through the air because the speaker does create a sound (air signal) which the mic picks up and converts to an electrical signal which the speaker picks up and converts to an air signal and so on.

 

[VonMagnum]

The answer you're looking for is much simpler. Telephone lines and such have separate wires for mic and receive. Ever look at a phone jack? Two wires per line.

Feedback requires a loop back to the input before amplification (either an electrical signal from the output fed back to the input or a microphone picking up the output from the receiver in the air).

Old telephone pole wires contained many wires within. Newer optical fiber can overlay many signals on the same line by various methods, none of which involve feedback).

 

[JerryLove]

The answer you're looking for is much simpler. Telephone lines and such have separate wires for mic and receive. Ever look at a phone jack? Two wires per line.

High-school electronics suggests that those are "positive' and "negative" respectively... the "in" and "out" nodes of a single circuit.

https://en.wikipedia.org/wiki/Telephone#/media/File:Telephoneschematic.gif

If you were to call someone on a touchtone and then keep pressing the keys, you would both hear the tones. This is because it's the same wire.

Notice that the speakers on your home theater system also have two wires even though there's no mic.

 

[VonMagnum]

You're correct. It would appear the real reason is the send/receive paths are separated inside the phone itself and are isolated by a transformer (or IC circuit these days apparently). That also changes the voltage levels and pretty much negates any chance of electrical feedback. The lines themselves are bi-directional audio (the only thing carried audio wise over the line is the single mesh of both voices). https://www.jkaudio.com/article_10.htm