modulate digital signal on analog intercom system

[Hieronymus]

Hi,

I'm looking for ideas, thoughts and suggestions for my project. I like to add a digital modulated signal with an analog one, on the same bus.

I've build (not designed) an intercom system as used in theaters. It's a single channel bus system useally called party-line where al users can listen and talk.
I use it to communicate in a live video recording setup (director - camera).

I'm looking for elegant options to extend the system to include tally light signals. For this you need to send signals to specific units on the bus (or make them listen to only their address). For starters it is sufficient to send only off, on preview and on-air information.

I've found an example of a domotica system which uses microcontrollers and uart codes to send signals to individual PICs (that control motors, lights, and so on)
I like this idea since it can be used for much more complex things. With a simple header you can make individual boards listen to a preset address.

The only problem and the is that I can't figure out how to get the signal ON the bus, given my situation with the audio and call light.

I'd like to hear you ideas, links to other recources or similar projects, design considerations, pit-falls, and so on... I don't have an idea about how cheaply modulate this uart signal with the audio.

Hope you guys can help!


MORE DETAILS:

The intercom basically sends audio and power over a three line cable (XLR). Each unit can listen or mix in it's own audio signal, so you'll hear yourself and all others can hear you. It also features a call-light. This is done by offsetting the whole bus with a DC voltage which can be detected and a LED will turn on (again, at all units)

The schematic of the intercom unit:
https://dl.dropboxusercontent.com/u/24013165/BluecomBeltpackSchematic_V1_2.pdf
https://dl.dropboxusercontent.com/u/24013165/BluecomPsuSchematic1_1.pdf
As you can see, there's a header with bus connection, so that could be used so connect the extension board.

For the comms system, you can find all the info on this forum:
http://www.blue-room.org.uk/index.php?showtopic=59644

A example of a bus system using UART and PIC micro controllers (dutch):
http://www.picbasic.nl/huis-automatisering.htm

Other considerations:
- When you push the call light signal, there is also a quite loud clicking sound. I don't realy like that, so sending the digital info should not be 'heard' in the audio
- I hope this can be done without modifications to the current system. Small ones can be considered. (if it works with these comms, it will be compatiple with many others)
- bus speed is not a real issue. If actions can be send within one video frame (say 30ms) that would still be acceptable
- My first thought was to modulate the call light with a voltage just lower then the call light threshold. But I guess you can hear that and off course when the call light is used, the modulation wouldn't work.
- a second PCB with this solution will be fitted in the same enclosure

[Marco]

The intercom basically sends audio and power over a three line cable (XLR). Each unit can listen or mix in it's own audio signal, so you'll hear yourself and all others can hear you. It also features a call-light. This is done by offsetting the whole bus with a DC voltage which can be detected and a LED will turn on (again, at all units)

Since it's a balanced audio this is the obvious way to do it, just modulate the common mode ... but ...

- When you push the call light signal, there is also a quite loud clicking sound. I don't realy like that, so sending the digital info should not be 'heard' in the audio

Obviously the existing circuit isn't up to the task of actually removing the common mode effectively.

[vk6zgo]

In Radio Communications,there are what are called CTCSS  or sometimes,PL(brand name),tones.

These are low frequency,low level,audio tones,which are superimposed on the speech signal of,say a Ham Radio,or a Taxi Radio,to allow access to a Repeater Station or similar.

They are so low in level,that they are usually not annoying,but in many cases are further reduced by high pass filters in the Receiver audio,after the point where they are picked off to do their job.

There are quite a number of standard tones,& ready made modules can be bought to ease their application.
As you only want a few functions ,this may be an answer.

[David_AVD]

The intercom basically sends audio and power over a three line cable (XLR). Each unit can listen or mix in it's own audio signal, so you'll hear yourself and all others can hear you. It also features a call-light. This is done by offsetting the whole bus with a DC voltage which can be detected and a LED will turn on (again, at all units)

Since it's a balanced audio this is the obvious way to do it, just modulate the common mode ... but ...

Actually, while the connectors and cable are often used for balanced audio, in this case one wire is +24V, the other is audio+call and the shield is common (ground).

[Richard Crowley]

So that is basically a "Clearcom-style" party-line (bus) style production intercom system.
The original design imposed the supply voltage onto the audio bus to signal a "call" event.
A second-generation system used an ultrasonic signal vs. the original DC-signaling scheme.
I suspect that newer generations use more sophisticated ultrasonic signaling (including digital encoding).
Since we are talking about only voice-grade communication here, frequency-domain multiplexing (with data in ultrasonic over the top of the audio) seems like a reasonable thing.

Of course all those old analog designs are more or less obsolete, replaced by digital versions.
(With the audio carried over digital communication, to which it is trivial to add signaling and other kinds of communication)

I have made many clones of the old analog ClearCom system and many others have done so also from the info on my website:
http://www.rcrowley.com/ComClone/default.htm

I'm seriously thinking about a new style that will use Cat-5 cable to carry party-line audio bus (but balanced vs. unbalanced in the ClearCom, et.al. scheme)
And another pair for power distribution, leaving two more pair for other things (like additional communication channels and/or signaling/digital communication.)
And there is also the scheme of using the power bus to send digital communication, also.

David_AVD is correct. The ClearCom, et.al. scheme is unbalnced, even though it traditionally uses 3-pin XLR.

[Hieronymus]

Hi Richard,

Your comClone is one of the inspirations behind the BlueComm design.

I like the idea of modulating some ultrasonic signal with the audio.

What would be a good frequency? In terms of:
- easy to use/cheap components 
- max cable length (or signal propagation)
- transmission speed (can be quite low for just a few bits of data within 1/50 second)

Do you know some other 'typical' applications of this so I can easily investigate and make a mock-up?

[SeanB]

Try modulating a 34kHz signal onto the audio bus. That way you can use common IR transmitter and receiver circuits to receive the data and provide an error corrected data output. The IR will just have to be at a low level added to the bus, and at the receive end you need a bandpass filter with a data slicer to recover the data. transmit will be easy, but you need to design the receive unit, and filter the signal from the audio line to the amplifier. To get the click out you just need to slew limit the signalling voltage so it is slower rise and fall wise.