Re: Bi-directional audio over power cable
- From: "David Bourgeois" <david@xxxxxxxxxxxxxx>
- Date: Mon, 15 Dec 2008 15:46:28 +0100
On Mon, 15 Dec 2008 14:12:45 +0100, <bob9@xxxxxxxxxxxxxx> wrote:
On Dec 15, 10:52 am, "David Bourgeois" <da...@xxxxxxxxxxxxxx> 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
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.
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
.
- References:
- Bi-directional audio over power cable
- From: David Bourgeois
- Re: Bi-directional audio over power cable
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