Patchbays and Matching Boxes (was [Tig] Grouping XLR outputs to a stereo signal)
ted at tedlangdell.com
Sun Nov 5 20:06:50 GMT 2006
Ted Langdell Creative Broadcast Services
Main: (530) 741-1212
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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)
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
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
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
Hope this is helpful...
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