Newsgroups: sci.engr.television.broadcast
Subject: Re: Digital audio over 75 ohm coax or 110 ohm tp
References: 
"ArneS"  writes:

> I've seen that different manufacturers (and indeed sometimes the same
> manufacturer) uses different connectors/cables for transmission of AES/EBU
> digital audio.
> 
> Some Panasonic DVCPRO machines have a BNC connector for this signal, while
> others have a 3-pin XLR.
> As far as I know the normal standard is 3 pin XLR with 110 ohm impedance
> cable, balanced, "shielded twisted pair"
> But for the BNC connector can one use normal 75 ohm coax, RG-59 or similar?
> 
> I asked a technincian from NRK (Norway State/Public Broadcaster) and he had
> never seen digitalaudio over BNC until I showed him on the back of one
> machine.
> The college's SPG also had digital audio testsignal output on BNC
> connectors.

There are two digital audio interface formats widely used in audio word:
- AES/EBU for professional use
- S/PDIF for consumer use (IEC-958)

There are three different medias used for carrying the signals:
- 75 ohm coaxial cable (uses BNC or RCA connectors)
- Balanced 110 ohm cable with XLR connectors
- optical 1 mm fiber with Toslink optical modules

S/PDIF uses 75 ohm coaxial cable with RCA connectors (sometime BNC)
or optical fiber.

AES/EBU was originally designed to use 110 ohm balanced XLR cable.
Later option to use 75 ohm coaxial cable was added to the standard
(the 75 ohm coaxial cable uses similar signal levels as consumer
S/PDIF). The connector for 75 ohm coaxial cable used for AES/EBU is
BNC.

You can find more information on S/PDIF and something about AES/EBU
at http://www.epanorama.net/documents/audio/spdif.html

> How about when you need to connect two machines with differing inputs /
> outputs??

Some companies make impedance and level matching transformer to
convert XLR outputs to BNC (and maybe to other direction also).
Also an active converter is an option, circuits for that can be found at
http://www.epanorama.net/documents/audio/spdif.html
("S/PDIF to AES/EBU" and "AES/EBU to S/PDIF signal level converter")

> Can you connect the common and negaitve pin of the 3 pin XLR as one can do
> with analog audio?

The system is not designed to work like this. I would recommend
a matching transformer designed for this. 

> Of course you would get a raise in signal level, but nonetheless?
> How about some sort of balancing transformer?
> 
> We have several pro-tools machines that have digital input on XLR, as well
> as 2 Yamaha O2R mixers with digital inputs on XLR.
> 
> What is the normal/nominal signal level for AES/EBU signal? Same for both
> connectors/cable types?
> 
> 
> Thanks for any input!
> 
> ARNE
> 
> 
> ABOUT ME:
> I'm a 26 year old local-tv technical operations manager for 5 years, now
> attending 2 years of college (www.hil.no) in technical television studies.
> The station that I have worked in for 5 years (www.abtv.no) is completely
> analogue, but we might do a transition to digital video (and audio?) in a
> years time.
> 
> 
> 
> 
> 
> 

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/