I'm not an engineer and was reading this article to learn about echo in phone calls:
"Echo Cancellation Demystified"
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If I got it right, this part of the article says that hybrids are used to separate TX and RX signals so as to be able to amplify just the TX signal so it's powerful enough to reach the remote party and still be heard correctly even over long-distance calls.
But today, phone calls (in industrialized countries at least) are digitized at the Central office, travel in this loss-less form inside the telephone network, and are turned back into analog at the remote local switch.
So logically, loss of power is no longer an issue, even with long-distance calls, so... are hybrids still used in modern telephone networks?
Hybrids are used on _2-wire_ analog voice circuits to convert the signal into to separate TX/RX signals on separate wire-pairs.
Given that the 'last mile', to a POTS set on the customer premises, _is_
2-wire, a hybrid is needed at the 'head end' end of that 2-wire circuit, to split out the part of the signal coming _from_ the CPE, prior to digitization and digital transfer to the far end C.O.
The digital circuitry -- between C.O.s -- is the logical equivalent of 4-wire analog circuits, in that the TX and RX signals are carried on distinct, and non-conflicting (well 'usually' :), data paths.
I don't know authoritatively, but I =strongly= suspect, that anywhere any form of multiplexed signaling was used (to put multiple conversations on a single wire pair) _between_ C.O.s, that regardless of the underlying technology (digital OR analog), that the call was split into distinct components for the signal in each direction.
Indeed hybrids are still used in all two wire circuits to separate the two directions of transmission. In the phone the receiver gets the signal from the line and the transmitter sends its signal down the line. There is a fourth port for the balance network. Local loops are usually two wire to control costs.
At the CO hybrids are used on the line cards, along with the other BORSCHT (Battery feed, Overvoltage protection, Ringing supply, Signaling, Codec, Hybrid, and Testing) functions. Modern day line cards use an electronic hybrid instead of the classical transformer.
The echo related problem is that the balance network in the hybrids at each end of the two wire portion of the circuit are "compromise" networks and not exactly matched to the individual line. As a result, the separation of the transmit and receive paths is not perfect and, as a result, echo is produced. In the phone this "echo" shows up as sidetone. User tolerance to echo is a function of both the level of the echo signal and the delay (longer is worse, satellite circuits are really bad...). To eliminate the irrating effect on the user, at least 40 dB of echo return loss is required. As standard companding codec (mu-law and A-law) only have a signal to quantizing noise ratio of about 35 dB, a digital echo canceler doesn't do the whole job, thus echo cancelers come complete with a "non-linear processor" which is really an echo suppressor (insert a large loss in the echo path when speech is present).
If one uses a 16 bit linear codec on the line card, the additional signal to quantizing noise ratio allows the magic 40 dB of echo return loss to be achieved. Once the echo is removed, the sigal can be companded for digital transmission. ATT Microelectronics actually made such a device and it tested very well. It didn't sell well because the echo improvement was noticed by the far end party, not the near end party who pays for the local loop and switch line card.
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