StudioTech 233 manual Intercom Channel Crosstalk, Intercom Audio Levels

to the user or installer to be connected or disconnected as desired. However repeat- ed field testing found that floating pin 1 on these outputs was the key to maintaining quiet audio. From Fenway Park, to the Orange Bowl, and then northwest to Husk- er Stadium, lifting pin 1 always did the trick.

A simple solution is available if an applica- tion does require that a ground be avail- able on the main and line-level talkback outputs’ interconnecting cables. All Model 233 XLR-type connectors have a ground connection that is made to the interfacing connector’s metal “shell.” And most XLR- type connectors have a pin or connection point available to access its metal shell. By connecting the cable shield to the mating connector’s shell terminal, the common connection typically found on audio inter- connections is created.

Intercom Channel Crosstalk

By the very nature of its design, a

2-channel party-line intercom circuit is prone to exhibit crosstalk. This occurs because the audio paths are unbalanced (“single-ended”) and are typically trans- ported on a single shielded twisted-pair audio cable. The primary causes of the crosstalk are the common signal return wire and the capacitance between the wires in the cable pair. The greater the capacitance, due to cable type and length, the more crosstalk there will be. It’s not surprising to find in sports broadcasting venues that audio from one channel can be heard “bleeding” into the other channel. Does this generally create a problem? No. But it can be a bit unnerving.

There are several ways of reducing inter- com channel crosstalk. Probably the easi- est way is to use cable pairs that are not

twisted. Twisted pairs are great for differ- ential (balanced) signals, but not so great for unbalanced transmission. This is gen- erally because the more twists in a pair the greater the effective cable capacitance.

In a stadium or arena setting, choosing standard “telco” pairs may actually work better than “high-performance” audio or data cable!

Another option is to use two cable pairs for each intercom circuit. If the pairs are not shielded the wiring is simple. Common would be connected to one side of each pair, and then signal from each channel would connect to the other side of the pairs. If the pairs also contain shields the wiring could be done somewhat differ- ently. One option is to connect common to both cable shields, intercom channel 1 (DC with audio) to one full pair, and inter- com channel 2 to the second full pair. A better option might be to have common connect to both shields and one side of the pair that serves channel 2.

Intercom Audio Levels

The Model 233 was designed to function well with intercom lines associated with standard broadcast and production “party- line” intercom systems. These systems provide DC power and one or two chan- nels of audio over standard 3-conductor cables that terminate with 3-pin XLR-type connectors. Establishing the correct “listen” and “talk” levels was critical in achieving good audio performance. In North America the two most common intercom systems are those from RTS and Clear-Com. From tests performed

in Studio Technologies’ lab, the nominal RTS TW-series audio level is approximate- ly –10 dBu. The dynamic range control provided by belt-packs such as the BP325

Model 233 User Guide	Issue 1, September 2007
Studio Technologies, Inc.	Page 35