Audio over CAT5

[sleah]

Just after others experience and/or advice on a problem I had with the above.

 

Chrimbo fair at school and the organisers asked if I could 'pipe' music and announcements around the building.

Upstairs fine, long audio cable.

The foyer was a challenge - audio source in Main Hall, Foyer across the coridoor from the Hall. No way I'm running a cable....

 

Sooo... AH-HA! Thinks I. There's a double network socket by the stage, t'other end being in a comms cab near the foyer with only one line in use.

 

Pair of baluns, jobs a good 'un. No such luck :(

 

Got the audio OK, but with a continuous low/mid buzz. I abandoned the idea and told the organiser. (Basically didn't have time or inclination to investigate further)

 

Now I have time to think... I can think of two things. Baluns were dual phono to CAT5 from CPC, so should be no problem there. So...

 

1. The cabling is physically to blame, not terminated correctly, wrong order blah blah but AFAIK it's OK doing networky stuff.

OR

2. The fact it was one of a pair and the other line was an active network line causing some kind of cross-talk?

 

I've only previously used 'over CAT5' for a vga link over a good 50m or so alongside active network connections and it seems fine. Other than that I've sent audio but only on dedicated CAT5 not near active network lines, no problems.

 

Any clue as to why I buzzzzzzed rather annoyingly? :unsure:

[bigclive]

Were the baluns actual transformers or was it just a phono to RJ45 adapter?

Was the CAT5 wired correctly so the chosen circuit was an actual twisted pair?

I should think that with a proper audio isolation transformer and a twisted pair any common mode noise would be limited.

[olitee]

It could depend on the types of baluns you're using, and also on the type of CAT-X cabling the venue has installed.

 

Passive baluns don't necessarily balance the audio line, and so even with the twisted pair nature of the CAT-5 line, it's still susceptible to interference, particularly if they're using the cheaper unshielded installation cable.

 

Also, with a CAT-5 network, you've for a number of other points of failure to consider: the sockets, the patch panel(s) and patch leads.

 

Two things to check: were you getting the same interference on both audio channels? (I'm assuming this was a stereo feed). And did the "buzz" sound to be a 50, 100 or 200Hz typical mains-induced buzz, or was it higher pitched? And did the buzz disappear if you disconnected your audio source (but not the baluns)?

 

Oli.

[BlueShift]

A few things:

 

First, If youre output is XLR (ie, already balanced) then you don't need a balun! Simply wire pins 2 and 3 to a pair on the cat5, and pin 1 to another core (take your pick). you can run balanced analog audio down cat5 for aaaaages with no screen (think 500m...). Thats how matrix intercom systems work...

 

Its possible that the cat5 is badly installed and subject to interference from other building infrastructure, which could well affect network traffic also, albeit in a much less noticeable way. In which case, you may be out of luck. try another patch point?

[sleah]

Thanks folks. That's given me a few ideas to poke around with when I get chance. they'll ask for the same next year, but it won't be used in between, so I have plenty of time :D

 

Interesting points about the baluns, they were a pair of phono to CAT5 with some kind of transformer, cheap'n'cheerful from CPC though. Can't do balanced without extra wink-wonks so sadly lashing an XLR either end wouldn't help :( although I will remember that tip for future projects... :)

 

It's the school where I work so no worries prodding about with the infrastructure, which may well be to blame, as we've had problems with cabling in other areas of the school.

 

Nearly forgot, it's all UTP so no nice foil screens :(

[bigclive]

Unscreened CAT5 is the standard choice. The fact that each pair is a twisted pair with a different twist ratio is what gives the cable excellent rejection of interference. I wonder if your issue was ground borne. If you saw how the average facilities installer treated data cable (and indeed mains cable) you'd realise that very few installations achieve anywhere near the rated data capacity of the cable. The use of minimally trained labourers to do everything is standard in the construction industry now.

[DrV]

Thanks folks. That's given me a few ideas to poke around with when I get chance. they'll ask for the same next year, but it won't be used in between, so I have plenty of time :D

 

Interesting points about the baluns, they were a pair of phono to CAT5 with some kind of transformer, cheap'n'cheerful from CPC though. Can't do balanced without extra wink-wonks so sadly lashing an XLR either end wouldn't help :( although I will remember that tip for future projects... :)

 

It's the school where I work so no worries prodding about with the infrastructure, which may well be to blame, as we've had problems with cabling in other areas of the school.

 

Nearly forgot, it's all UTP so no nice foil screens :(

It might be worth checking that they are actually wired as baluns.

I got some cheap phono to XLR baluns from CPC a year or two ago and, when they produced the most appaling amount of buzz, I took 'em apart and discovered that the transformers had been put in at 90 degrees to the correct orientation. The result being a common mode choke and no DC isolation of the phonos from the line! Simple job to desolder them and turn them round. They worked a treat after that...

 

Dave

[howlingwolf487]

I think that we're assuming this run of ethernet is a homerun, from one wall panel to the other, with no switches, routers, or hubs in-between. If this IS the case, there should not be any problem sending balanced, line level, analog audio down the cable - you don't even need Pin 1, just use Pins 2 and 3. That may solve your hum/buzz issue right there…

 

However, if there are network interfaces along the line, they will need to be physically bypassed for what the OP is proposing to work properly.

[sleah]

I think that we're assuming this run of ethernet is a homerun, from one wall panel to the other, with no switches, routers, or hubs in-between. If this IS the case, there should not be any problem sending balanced, line level, analog audio down the cable - you don't even need Pin 1, just use Pins 2 and 3. That may solve your hum/buzz issue right there…

 

However, if there are network interfaces along the line, they will need to be physically bypassed for what the OP is proposing to work properly.

 

Your assumtion is correct. Direct from socket to socket, nowt in between.

I would expect no less than a good thrashing with wet straw, a tickling by a excited squirel and a slap with a wet kipper by BR members if I had suggested running anything other than network stuff down a CAT5 run with anything other than dead copper along it's route.

I'd be cap in hand to my gaffer explaining why a chunck of network was fried due to a 'p155ed off ' network switch by now if I'd tried to connect audio kit to a live network point! :** laughs out loud **: :** laughs out loud **: :** laughs out loud **:

 

You are of course correct to double check - there's every chance there are people reading this who know no different :o