Re: Bi-directional audio over power cable



David Bourgeois wrote:
Hi,

I'd like to add an intercom functionality to a broadcast camera and I'd appreciate opinions about the different solutions I have in mind.
The main restrictions come from the cable, which is a big shielded cable with optical fiber inside, a pair of 37.5Ohm/km for power (300V DC), and a pair of conductors for RS-485 control signals. The cable can be up to 1km long.

The first option is to add analog audio on top of the DC power but I see 2 problems here:
- the power cables are not individually shielded so I'll get noise from the RS-485 signals, I've no idea how bad that can be and if that can still be OK for voice;
- as I need bi-directional audio, I guess I have to shift the audio signals to higher frequencies to have both on the same line.

Second option would be to use the RS-485. It's currently at 9.6kbps but I've read that bitrates can be up to 100kbps for 1km lines. At that bitrate I should be able to use speech codecs with half-duplex transmission. Would the transmission still be reliable at 100kbps?

Third option is to use the optical fiber and stream UDP audio. Here it sounds like a more complex architecture to design, but maybe I could find commercial modules that handle that already so it might be the easiest option in the end.

Any thoughts?

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
http://www.abvolt.com


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