How do you connect a telephone to a mixing console?

How do you connect a telephone to a mixing console? Here's a hint - it's more difficult than you think...

If you have a radio station, you will need to be able to handle phone-in shows. So an incoming telephone line has to be connected to one of the inputs of the mixing console. This should be easy - just make up an adaptor cable.

Actually no. It's not so easy at all - unless you know how...

Telephone lines are very dissimilar to audio cables. An audio cable carries a single signal from an output to an input.

A telephone however carries two signals simultaneously. There is only one connection between the two people talking in both directions.

Between two telephone handsets, this works fine. However, in the studio situation, the caller is at one end of the line on a handset; the presenter is speaking into his normal microphone, and the radio audience needs to hear the conversation.

Add to that that the radio audience needs to hear the presenter in full hifi sound quality, not telephone quality.

So how do you do that?

The answer is to use a device called a telephone hybrid, also known as a telephone balance unit (TBU).

This sits between the telephone line and the mixing console. It supplies the caller's voice to an input of the console, and at the same time accepts an output from the console, which is of course the signal that is put onto the telephone line and sent to the caller.

The signal from the console to the telephone hybrid must be a clean feed (also known as mix minus) signal. This means that it consists of the studio output, minus the caller's voice, which is present on the line already.

The telephone hybrid also cancels the studio output signal from the line, so that the signal presented to the console input consists only of the caller's voice.

In practice, separation is not perfect but lies somewhere between 20 dB and 30 dB. The presenter's voice is degraded slightly by some of the signal from the line bleeding through, but you would have to listen hard to notice.

The telephone hybrid also isolates the mixing console from the DC voltage on the telephone line.

When you go for your job in radio broadcasting, don't forget to ask about telephone hybrids. Don't pretend you know it all already, but ask an interesting question that shows you have thought about these sound engineering technicalities.