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Grant

Combined cable construction (power & signals)

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Hello all,

 

I'm about to fabricate an expanded and this time consistently color-coded set of audio cables (length matching jacket color). The primary use is live location work around the US Capitol - lots of hallways, rotundas, etc.

 

This time, while making up individual XLR-terminated audio cables, I am also going to take an 'umbillical approach' between fiber interconnect box, camera position and correspondent position (which is up to 25 feet from camera, but rarely more than 12 feet away from the lens). Traditionally, the signals between these 3 locations include mic audio, phone audio (IFB), and HD-SDI signals, using mini-RG59 cables.

 

I would like to experiment, cautiously, with including power into the umbillical - distributing final power to the correspondent position for USB devices (5v DC) and IFB components (9v DC @ aprox 200mA). Can anyone predict if this can be done without risk of audio interference, and if so, would it be best to send out the power first @ 120 volts (say via 18/3 cable, minimal current), stepping it down at the end of the umbillical, OR first doing low voltage conversion and then distributing power via low voltage wiring?

 

If a mockup shows a clean and reliable set of signals, I will bring all cables inside a common Techflex housing.

 

The back story : US networks have now stripped almost ALL DC live crews down to single operator, so that this proposed equipment refinement will make life a little easier (one cable set instead of 2) - faster plug'n play, less taping down to the marble floors, and less entanglement with other live crew's cable runs, etc. I am proposing to use Canare L-4E5C both inside the Techflex/umbillical environment and outside of it.

 

Grant.

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If your grounds are separate and isolated from each other you should be fine. I'm a bit concerned about radiated RF or crosstalk within the multipairs. You would have to build the assembly and then test it. Even then there are no guarantees.

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7 minutes ago, Eric Toline said:

If your grounds are separate and isolated from each other you should be fine

Thanks Eric, that's a great point. Part of the power train was to send 12 volts DC to the camera. I think that means lifting the ground on that circuit, since there will be at least 2 of the cables connecting to the camera body (video and audio).

 

If after successful testing, I still ran into an issue on a location one day, I could always ignore the power-thru-umbillical cct and simply run a separate line out.

 

Grant.

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Hi Grant,

 

Some years ago I combined a signal leads with powering through a 12p hirose connector to make it easier to connect/disconnect a 'back up' recorder from my mixer. I didn't have problems with power interfering with audio but the experience taught me it's best to keep power and signal separate in some way. Whilst electrical continuity is paramount to signal and power cables alike, you may have an issue with a signal cable which is not other wise a problem for the power cable and vice-a-versa. EG signal cable has a break in the shield and you are getting interference from something (other than the integrated power element). In this age of cheap batteries would it not be easier to have all powering requirements from a localised battery?

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A cam op I worked with  a lot had two 100' snakes (Canare snake cable I think). Each had 2 balanced audio channels w/ XLRs, 2 coax w/ BNCs and a stereo audio HP return 1/8" TRS. They were real handy for the two cam interview shoots we were doing at the time. I think Markertek assembled it in their custom shop. They have a huge selection of all kinds of bulk A/V cables, snake and other wise.

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Back in the analog days I knew a few video assist operators who did this, even going the to length of using very expensive large Lemo connectors on the runs.  These guys spent a fortune on these cables, but that did not prevent them from getting abused on the set, and breaks or damage to them was very hard to fix due to the number of pairs/signals involved and the special order connectors.  I understand the impulse to combine as many feeds as possible--I too hate on-site patching, but consider carefully your what-if scenarios, make it as modular as you can and make more pieces than you think you'll need!

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It is seldom a good idea to lay AC cables next to -- and parallel with -- audio runs.  Doing so is asking for induced hum.  If you must put power next to audio, keep it DC.

 

If you include a monitor return in a unified cable, be prepared to deal with ground loop issues.  Do what you can to eliminate those with the cable grounding scheme, and also be prepared to not use the video return when (not if) an unwanted gremlin appears.

 

With taped-down cables, keep in mind that -- in respect to a tripping hazard -- one bigger cable can sometimes present more of a hazard than two two smaller taped-down cables.

 

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I have a 100' snake made from 6 100' XLRs and 1 A/C 100' 14GA. cable....   It's big and heavy, but... it works great, I can swap out or repair any one line.... As for any 60 cycle hum, or any noise or hum from that set up.... Never anything...  I seldom use it anymore...but I pull it out once and a while..

These were my fears as well, but... in this case I have never heard a thing...  Works great...for years... Go figure...

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I work NABET and IBEW local 1200 gigs, and it is true that lately my cam op brothers have been sneaking out and working sound without insisting on being accompanied by a union sound tech.

 

At the very least the cam ops could helpfully remind the assignment desk about the agreements by billing a few extra hours to wrangle all those extra cables they never had to mess with before.

 

 

 

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In the Live Sound world, with Powered Speakers (speakers with their amps built in) becoming ubiquitous, combined power+signal cable is very common, carrying line-level balanced audio and AC power together for long distances. Induced hum is rare and considered a sign of bad equipment design. Your application however is likely to be carrying mic-level signals and unbalanced (though shielded) video so I'd stick with DC power - and don't use the overall cable shield (if there is one) as one of the DC conductors or share grounding within the cable 'bundle'. Then there should be no more risk of grounding issues than individual cables. Techflex is great for short looms, but I'd avoid it for longer runs as cables tend to twist and kink inside under heavy use. There are cable manufacturers who will make up custom cables containing power, balanced audio, and Coax in one overall jacket to your specs, or you might find the right combination in one of the manufacturers ranges, and just use the color-coded Techflex to make the fan-outs at each end.

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John, Alen, Nick, Philip, Rick, Daniel and Few More Years, 

 

Thank you all for a well-rounded set of commentaries. 2 ideas that have stuck with me - the physical issues that will sporadically arise with crews running in and out of the shared location (lots of wheeled carts on the move, larger diam cable to cross), and the idea that any issue within the umbilical requires taking the whole unit off line for repairs.

 

My modified approach, which will still accomplish speedier setups and fewer discrete parts, will be to run one cct for power and another for signals. Thanks again, will post some photos when done.

 

Grant.

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