Re: Bi-directional audio over power cable
- From: Eeyore <rabbitsfriendsandrelations@xxxxxxxxxxx>
- Date: Mon, 15 Dec 2008 11:53:46 +0000
David Bourgeois wrote:
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.
Convert to the audio to SPDIF / AES3 or I2S Mind you that's in Mbps land.
Graham
.
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