Send digital data over same wire as analog data

Question

I am trying to modify an existing circuit that I am able to send another digital signal over the same connection.

My circuit is a so called "Partyline Intercom" which is based upon the schematics of Richard Crowley's Com Clone 2.

The circuit has an audio "bus" where all the audio signals are mixed together, to every unit there is a mic and headphone hooked up and you can hear everything the others say over the mic (as well as your own voice) there is also a second signal (called "Sidetone") which is as I understood basically a DC coupling. When this signal is sent, on every unit flashes a light which means something like "Take the call".

The signals are sent over a 3-Pin XLR-Cable (+30V - Signal - GND).

I do not want to change the given functionality but add up a digital channel where I can transmit some data over the same audio bus (using an Arduino Nano for example). These data will not have a high rate, I will send something like 16 bits payload (64 bits maximum if it would work) in 50-100ms to control some status LEDs on the units (Like a camera Tally - "You are live now" - red light).

How can I implement this digital channel? Would it be possible to easily build a bandpass filter that sends everything under 30.000 Hz to the audio and sidetone circuit and everything over that threshold to the digital connection?

The maximum distances I would use are something like 50 to 100 m.

Answer 1

Your idea of using an ultrasonic signal to the audio channel seems doable. Basically you can implement something like RTTY (http://en.wikipedia.org/wiki/Radioteletype) or one of those old 300 bps modem using FSK modulation (Bell 103A or V.21 standards -- see http://en.wikipedia.org/wiki/Bell_103_modem), just using very high frequencies. Modulating the signal with an Arduino might be doable (e.g., generating square waves and filtering them to something closer to a sine wave), but the demodulation part might be trickier (possibly filtering the audio signal to only include your ultrasound FSK tones, and then detecting and differentiating them: you will need a pretty high sampling rate on your ADC).