Bi-directional audio over power cable

[David Bourgeois]

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

 

[Eeyore]

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

 

[David Bourgeois]

Convert to the audio to SPDIF / AES3 or I2S Mind you that's in Mbps land.

Hi Graham,

I forgot to tell that the fiber is already used for ethernet data, so
unfortunately I'm limited to IP if I want to use it.

David

 

[[email protected]]

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.

Bob

 

[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

 

[[email protected]]

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

 

[[email protected]]

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 Consultanthttp://www.abvolt.com

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.

 

[[email protected]]

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?

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

 

[Eeyore]

Hi Graham,

I forgot to tell that the fiber is already used for ethernet data, so
unfortunately I'm limited to IP if I want to use it.

There are commercial solutions for this. Check out Cobranet. It's mixed-traffic
compatible.

Graham

 

[Eeyore]

I was wondering if you could get it to run over the power side actually and do
it balanced (differential) to avoid the noise issues.

There are some IP intercom modules out there, but I think they require a
server, not peer to peer.

http://en.wikipedia.org/wiki/Cobranet

Graham

 

[David Bourgeois]

I was wondering if you could get it to run over the power side actually
and do
it balanced (differential) to avoid the noise issues.

That's an interesting idea indeed, balanced AES3 could be used but if I'm
correct that can't be used for full-duplex. I guess a custom design could
use some kind of time-multiplexed signal to achieve full-duplex.

http://en.wikipedia.org/wiki/Cobranet

Great modules there, a bit overkill for a single audio link so it's over
the budget I expected, but it's a nice product. Thanks for the links.

David

 

[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 of Intercom Systems Engineering.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

 

[krw]

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 of Intercom Systems Engineering.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.

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.

One of the systems uses balanced audio so basically I could
couple that audio line to my power bus and connect a user station

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.

to the other side, taking care of the 300V isolation of course.
I'm gonna try that.

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.

 

[JosephKK]

That's an interesting idea indeed, balanced AES3 could be used but if I'm
correct that can't be used for full-duplex. I guess a custom design could
use some kind of time-multiplexed signal to achieve full-duplex.


Great modules there, a bit overkill for a single audio link so it's over
the budget I expected, but it's a nice product. Thanks for the links.

David

Personally i would have used FM with a carrier around 4.5 MHz
piggyback on the RS-485 pairs. Just a suggestion.

 

[Eeyore]

Personally i would have used FM with a carrier around 4.5 MHz
piggyback on the RS-485 pairs. Just a suggestion.

All depends on the quality required which has not been stated.

Graham