Patchbays and Matching Boxes (was [Tig] Grouping XLR outputs to a stereo signal)

Ted Langdell ted at tedlangdell.com
Sun Nov 5 20:06:50 GMT 2006


Ted Langdell
Ted Langdell Creative Broadcast Services
Marysville, CA
Main:   (530) 741-1212

Now with 3GHz MacPro powering Final Cut Suite



Hi, Jean-Francois...

On Nov 5, 2006, at 8:17 AM, Jean-Francois Panisset wrote:
> It's a bit of a pain when you have to deal with a mix of BNC and  
> XLR audio connectors in
> your equipment racks, especially if you have a manual patch panel.

Most AUDIO patch bays use so-called "1/4-inch phone" plugs and  
jacks... named so because the telephone company (at least in the US)  
used them.

They're quite similar in appearance and function to full-sized stereo  
headphone plugs and jacks.

XLR connections are "balanced" with the signal on a pair of  
conductors surrounded by a shield that connects to ground. They  
better resist noise, offer better frequency response at length  
compared to "unbalanced" systems using a single conductor surrounded  
by a shield.

A "Balanced" patch bay uses "tip-ring-sleeve" plugs and jacks to  
connect the two signal-carrying conductors and the shield (ground).

> Should you convert all XLR connections to BNC/coax and use a video
> style patch panel, or convert the BNC/coax connections to XLR and use
> an audio-style patch panel?

Neither.  Use the standard tip-ring-sleeve (TRS) style of audio patch  
bay, which can be mounted below your video jack field (patch bay).

Video and audio inputs and outputs from each machine or audio and  
video source have a corresponding set of jacks in the patch bay,  
sometimes "normaled through" to another piece of equipment.

"Normalled through" means that signal appearing on the back side of  
the top jack of a patch bay is normally routed through to the back  
side of the jack right under it... so no patch cable is normally  
necessary.

This makes for a simple and less cluttered method of routing signals  
from source A to destination B.  If you needed to send a signal from  
Machine A to Input Q, you'd stick a patch cable in the jack coming  
from A and insert the other end in the jack feeding Q.

TO HANDLE AN UNBALANCED MACHINE like a consumer video deck, you can  
use one of the "bi-directional" transformer boxes like the Jensen  
one... or an active box like Henry Engineering's "Matchbox" or a  
number of others.

These take care of converting the unbalanced connections from the  
consumer equipment to balanced line connections.

Use one at each machine for convenience. That keeps everything in the  
plant on balanced lines and makes the wiring of things consistent and  
simple.

Boxes like the Matchbox are "active" units that do several things.

First, they often use transformers to isolate the consumer gear from  
the balanced line side of the box, which can be a big help in  
avoiding ground loops, resulting hum and other nasty effects.

They are called "Active" because they have amplifiers. These increase  
the consumer gear's "-10db" signal output to the "0db" or "+4db"  
levels that professional and broadcast grade gear uses.

The "active" boxes also have pads to drop the incoming +4 signal down  
to -10 so that the input amps on the consumer gear aren't  
overdriven... which causes clipping and awful sounding recordings.

Most active boxes have level controls so you can adjust the signals  
to more closely match what's needed.

IF THE STUDIO IN QUESTION is more like a home-based one-room facility  
with a lot of "pro-sumer" gear generally within arms reach of the  
operator, and most of the gear has RCA inputs and outputs... I'd  
suggest an audio patch bay that may use RCA jacks on the back and Tip- 
Ring 1/4" Phone or RCA jacks on the front.

Keep the audio cable runs short to avoid noise and high-frequency  
losses due to capacitance, and then use the matchbox at the equipment  
whenever need to tie in an XLR equipped broadcast grade piece of  
equipment.

Hope this is helpful...

Ted.








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