The term hybrid has been knocked around here a lot lately. My understanding is that is a device that allows you to transmit your own speech on a pair of wires that also sends speech you listen to. The hybrid allow your signal to travel down the line without jamming your own listening.
A hybrid's operation is based upon the principle of a balanced bridge. Input across two opposite corners of a balanced bridge produces no signal between the other two corners. One side of the bridge is the transmission line, An adjacent line is used to match the electrical properties of the line in order to balance the bridge. At best, a lumped circuit is only an approximation to a distributed line. The other sides are used to complete the balance and adjust impedance levels. A slight imbalance can be used to introduce sidetone.
There is nothing to prevent using transformers and their windings as elements in the bridge. There often were addktional advantages to doing so. In my active ham days, I handled phone patch traffic using a patch with hybrid transformers in it. It enabled audio signals from my speaker from getting to the microphone input line.
These days, transistor based circuitry makes it easy and economic to have nonreciprocal circuitry so that bridge circuits are not essential. That was not the case in vacuum tube days.
While correct as such, that is far too narrow a definition. Hybrids are used at all frequencies of the spectrum, and are very common in transmission systems of all kinds. At microwaves it is commonly referred to as a "directional coupler", for example. (Heh heh... you've used an SWR meter?? That's a hybrid! So are duplex couplers to allow VHF and UHF repeaters to send and receive on the same antenna.)
Here is the basic circuit diagram of a "hybrid". Notice that it does not involve a transformer (and despite all the noise made by at least one poster, transformers are used in hybrids to provide isolation, and virtually nothing else).
Load 1 Load 2 +---/\/\/\/\/---+---/\/\/\/\/---+ | | | | o | | | In >---+ Port 1 +------+ | | | | o | ----- | Load 3 | Load 4 | / / / +---/\/\/\/\/---+---/\/\/\/\/---+
But a more useful diagram is slightly less easily looked at, though it is the same thing (with balanced ports).
then any signal applied to Port1 or Port2 result in 0 volts across the other Port. (Basically if all Loads are equal, any voltage applied to Port2 would show up equally on each side of Port1 at half the applied voltage, so both terminals for Port1 would be at the same voltage, and the signal across Port1 is 0 volts.)
If Load1 happens to be transmission line rather than a simple resistor, and Port2 is attached to a telephone transmitter, while Port1 is attached to a telephone receiver, then (if a balanced condition exists) the receiver will not produce any sound from the transmitter. To maintain a balance requires that Load2 be made to look very much like the transmission line. (Alternately, Load2 could remain a simple resistor, and Load3 could be made to look like the transmission line.)
In this case input on Port2 will cause no signal into Load1, but will cause signal at Port1; while a signal on Port1 will cause signal to appear at both Load1 and Port2.
The first diagram is basically describing the outgoing signal when a person talks into a telephone transmitter, while the second description applies when the distant end talk and a person hears what they say coming from the telephone receiver.
Any hybrid is some form of the above (regardless of what type of components or of what frequency it operates at)
Rather than "an adjacent line is used", that was meant to be "an adjacent port".
There are two basic types of circuits used to balance such a bridge. One is called a "Precision Balance Network", it does just exactly that: it precisely balances the network. But of course it requires adjustment to match precisely *that* line. The other type is call a "Compromise Balance Network", and that is what is being referred to above (even though both actually are a "lumped circuit". A Comp Net is close enough to be suitable for almost any typical telephone loop, and requires no adjustement.
Generally speaking, using an "imbalance" to produce sidetone will result in horrible sounding (usually very tinny) sidetone. An actual feedback path is more common.
Also, while impedance matching is in fact available in some units, the typical telephone loop does *not* have options for different impedances. Units are typcially designed for one impedance only, and are only used on circuits that are expected to be close to that impedance. (Hence a telco might have a lot of plant with 600 Ohm lines, and even more with 1200 Ohm lines... and they will have and use equipment designed for both, assigning them as appropriate.)
The bridge circuit is indeed still essential. The transformers are not! (Or, at least for a phone patch they aren't.) Each leg of the resistive hybrid shown above can be replaced with a transformer... Or the resistor can be either the output load for an OP AMP, or the high impedance input to an OP AMP can be bridged across the resistor. The problem there is of course that a transformer provides isolation and an OP AMP does not. There are now optical isolation devices available, which provide the same DC isolation that transformers do.
The discussion caused me to "go digging" and refresh my memory of the subject:
"BBC Engineering instruction ST6 - lines and telephone equipment" first edition 1945.
Appendix A - "The hybrid coil"
Three pages of diagrams and maths with an explanation of how two are used in a bi-direction repeater circuit to isolate the amplifiers from each other.
I found one of the paragraphs printed on the inside of the front cover interesting.
"The contents of this book must be treated as confidential and must not be divulged to any person not employed in the engineering division of the BBC. Non-observance of this ruling constitutes a serious offence which may amount to contravention of Defence regulations (The official secrets act)"
Thank you for a more complete description of a hybrid.
I did not imply that bridges were mot useful outside telephone practice. Hybrids are used this way and take many forms from the Wheatstone bridge at dc to Magic tees for microwaves to interferometers for light. For all I know, someone may have implemented one for x-rays.