Elliott Sound Products AN-010
2-4 Wire Converters / Hybrids Rod Elliott (ESP)
The analogue telephone system is commonly known as the PSTN - public switched telephone network, but is also called POTS - plain old telephone system. It is characterised by the operating voltage of 48V DC supplied from the exchange when the phone is 'on-hook' (not connected to the local exchange), and around 5-12V when 'off-hook' (during a call). It's a 2-wire system, with simultaneous bidirectional communication. Dialling is either by DTMF (dual tone multi-frequency, aka 'Touch Tone' in the US) or (rarely now) pulse (aka decadic), where the line is connected and disconnected to create pulses that signal the dialed number to the exchange. One pulse signals the digit '1', two pulses for '2', etc. The details for DTMF signalling can be found on the Net if you want to know.
Ringing is provided by an AC voltage superimposed on the line, at a frequency of about 20Hz, and with a voltage of 90V RMS. The ring current is 'cadenced' which is to say it's interrupted to create a ringing pattern. This differs in different countries, but part of the reason is to minimise the risk of electric shock. When the handset is lifted (off-hook), the exchange sends 'dial tone' to signal to the user that dialling may commence. Like the ringing cadence, dial tone differs in different countries. When the called number is ringing, 'ring tone' is sent to the calling party to indicate that the remote phone is ringing. If the remote phone is busy (off hook), the caller hears a 'busy' tone.
While the specifics of all these functions are subject to individual country's standards, the principle is unchanged. Mobile ('cell') phones operate completely differently, and are not included in the above. Communication (dialling, speech, etc.) are all digital, dial tone is usually not provided with modern systems, but ring and busy tones are still supplied so the caller knows that the call did or did not get through. In some cases, special tones are used to signal network congestion when no spare radio channels are available or the exchange is at capacity.
While the PSTN is being superseded worldwide by mobile/ cell phones and VOIP (Voice Over Internet Protocol), the principles are no less interesting. They are also no less important, but many of the principles are (or can be) 're-purposed' to suit particular requirements. One of these is 'talk back' radio, where the requirement for a hybrid are still essential, regardless of the type of phone system in use. The adventurous experimenter may also find other uses for a system that can use full duplex (simultaneous 2-way information over a single pair of wires). To be useful, the individual signals need to be separated at each end, and that's what a hybrid does.
This article does not cover the signalling or power systems, or the main infrastructure, but concentrates on one small but vital part of the system as a whole - the hybrid circuit (as it is commonly known by telephone engineers). A hybrid is used to convert a bidirectional 2-wire circuit into separate 'send' and 'receive' channels, commonly known as a 4-wire interface. More information is available in Reference 4, which is a fairly comprehensive overview. It's based on the US system, but those used worldwide are similar, and the general ideas are representative of other systems.
NOTE: It is an offence in most countries to connect non-approved equipment to the phone network, and the information here is not intended to allow you to make any connection to your phone line. This material is for your information only. Obtaining approval is a costly exercise and it's highly unlikely that any 'home made' equipment would even be considered.
Hybrids are the heart of the analogue telephone system. They allow two people to speak and listen simultaneously over a single pair of wires, with little or no interaction. This Application Note is not about producing a telephone system or even a part thereof, but is intended to introduce the concept of a hybrid, and explain how it works. In one form or another, hybrids have been used since the early telegraph days, and they are an essential part of the telecommunications system.
You can also build a pair - not because it's inherently useful for anything, but to experiment and learn. There's nothing especially critical about the principle, but it does become a lot harder (and there is inevitable degradation) when transformers are included. While these are not used in your home telephone, most exchanges (aka central offices) use transformers to ensure complete isolation from the outside world and all the dangers it represents.
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