Re: Bi-directional audio over power cable
- From: "David Bourgeois" <david@xxxxxxxxxxxxxx>
- Date: Tue, 16 Dec 2008 13:06:22 +0100
On Tue, 16 Dec 2008 00:18:08 +0100, RFI-EMI-GUY <Rhyolite@xxxxxxxxxxxx> wrote:
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?
Thanks,
David Bourgeois
I know that there is no shortage of vendors making intercoms for broadcast studios, Telex-Vega comes to mind.
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:
http://www.intercomheadsets.com/binary/Handbook%20of%20Intercom%20Systems%20Engineering.pdf
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
.
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