Bi-directional audio over power cable

Decades ago there were over-the-mains intercoms that used FM with carriers around 100-200KHz. It worked reasonably well. I suspect that these days electricity distribution systems in buildings are usually very low impedance at those frequencys due to all the capacitors on the inputs of SMPSU's. Since you have access to both ends a small iron-core inductor would probably work fine for stopping the equipment on the end shorting out the carrier.

Do not overlook the 'elf-n-safety requirements of modifying 300VDC equipment that might be used outdoors and if your insurance covers anything going wrong.

Bob

Thanks for the suggestion. I could find a few websites having such intercoms. I'm gonna try one to check the noise. If I'm still alive after that experience, I'll let you know :-)

David

On one project, I used the common UART over power line for duplex audio transmission. The data was encoded to eliminate the DC. The UART was running at 1 Mbps. With the basic equalization, it was able to make about 500 meters of distance over CAT-3 cable. Perhaps, it can go even further; we didn't try it. The link was operating in a Ping-Pong manner: the packet from master, then the packet from slave. The whole thing was very simple and cheap; using AVR microcontroller.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Ac couple a couple of standard pots telephone hybrid circuits to the DC lines? Either that or FM subcarriers on the DC. Its broadcast grade so I don't think folks would like to fin their intercom is a 100 mhz FM subcarrier on the video coax.

Steve

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The old Apple Talk was a transformer coupled 485 or 422. I assumed the data was encoded to remove DC, but don't know this for a fact.

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Sure use the fiber and an eeePC on both ends, use Skype.

I know that there is no shortage of vendors making intercoms for broadcast studios, Telex-Vega comes to mind.

HThat being said, you could consider a low frequency < 500 KHz FM subcarrier (like a residential wireless intercom) to convey an analog or digitized audio (FSK) superimposed on the DC wire. Yes there are subrate CODEC's to 32 kbps and compression vocoders (IMBE, AMBE class) down to

2400 bps which may fit the bill. Keep in mind the audio environment the cameraman is operating within, if extremely noisy, vocoders can sound pretty bad.

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what\'s the reason?
For if it prosper, none dare call it treason."

"Follow The Money"  ;-P

Thank you very much Joe, looking at Telex I found that great "Handbook of Intercom Systems Engineering" which explains the different systems very nicely. Anybody interested in intercoms can get it from this link:

I was misleaded by the fact that to get full-duplex, I had to have the audio flowing in 2 directions simultaneously. But basically intercom systems use a single audio line on which each user station adds it's signal. One of the systems uses balanced audio so basically I could couple that audio line to my power bus and connect a user station to the other side, taking care of the 300V isolation of course. I'm gonna try that.

Thanks to all for your answers. David

In article , snipped-for-privacy@jaguarondi.com says...>

Well, that *is* "having audio flow both directions simultaneously". ;-) Your problem will be to cancel the sent audio in the receiver. Depending on the setup requirements and complexity of the (complex impedance ;-) load, this may be simple or not so.

Note that a balanced system needs both sides to be *balanced*. If they aren't it's easy to get some pretty bad distortion. In our case this wasn't acceptable because it made canceling the return impossible. Coupling to power and ground may be problematic. The Telex configuration (balanced with power on both sides and ground on a third wire) is the best setup, IMO.

Intercom transformers (look at the transformers designed for modems) have a much higher isolation, usually 1500V. There are also transformers designed for use as trans-hybrids that may work well for you, if your system is simple enough.

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  Keith