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Telephone System

As any other radio station in the US and the world for that matter, we often take phone calls and requests. In order to do this there is some specialized technology involved.

Mix Minus

rather than try and explain this, I will draw a diagram....

telephone diagram

This should give you at least a guess as to how the whole mess works, but I will break it down. The telephone subsystem is broken down into a few simple-er stages, the telephone interface, the mix-minus mixer, and the audio editing computer.

Telephone Interface

The inferface, often referred to as a 'hybrid' is the device that tries to seperate the audio that is coming from the phone line from the audio that is going into the phone line. It does this by subtracting the signal going into the line from the signal coming off of the line. This is where most of the technology comes into play, beause you want the greatest seperation between these two signals as possible. Modern hybrids use DSP technology to get and increadible null off of plain old telephone lines. Earlier hybrids used primitive parametric equalizers in the nulling loop to try and better null the send audio out. The main reasons these systems did not work very well is that they just can't cope with common phone line quirks, such as line echo, and other abnornmalities caused by various things, including the weather! You can now see where this powerful DSP technology comes into play.

Several other devices work with the hybrid to delivere caller audio. The line selector selects the phone line to pass to the hybrid, while a hold device loads the phone line down to seize the call, and prevent the loss of the call when the jock puts the caller on the air, or hangs up the phone. Along with this is a device to interrupt the phone line to drop the call, when needed.
A compressor is placed on the outputs of the hybrid to control the widely varying dynamics or volume of the caller from call to call, making a consistant sound on air. Another compressor/limiter configuration is used on the input of the hybrid to prevent overmodulation of the phone line, which causes distortion and seperation isuses in the hybrid.

The Mix-Minus Mixer

This device is the simple-est of the devices in the telephone audio chain. Its purpose is simply to combine together the sources of audio we want to send down the phone line. The audition audio, both left and right channels, and all microphone channels are mixed down, and sent to the the hybrid, and the editing computer. The other side of the mixer combine the left and right editor outputs, and the caller audio together, then sends then to the board. Simple, right?

Editing Computer

The editing computer's main purpose is to time shift calls. Because we have a very loose language policy, we rarely edit caller audio, but in the real world of radio, this serves a good purpose. Many stations use a similar setup called a Voxpro system, in which the left channel represents the jock, and the right channel represents the caller. By editing this way, the DJ can fix his f*ckups, or the callers' independant of each other. before the call is aired, it is combined to a monaural signal to blend seemlessly into the rest of the station audio.