Field Report: Comrex STAC

October 1, 2010

When I started to plan new studios for the nationally syndicated Tom Joyner Morning Show nearly five years ago, the Comrex STAC was a fairly new product. It debuted in 2004 and was looking to make a name for itself in a field dominated by the competition.

The STAC had several positives for my needs. The fact that it had a built-in Web server for call screening was a huge jump over a third-party or stand-alone software platform. The price for two hybrids that had an internal mix-minus between the callers and could handle up to four control surfaces connected with a standard CAT-5 cable were all pluses as well.

The negative (at the time) was that it was still fairly new. Putting it on a caller-intensive morning show would put the system to the test quickly. And failure was not an option.

A look inside

The STAC is based on a small form-factor, custom PC-style motherboard in a 2RU chassis that runs on Linux. Booting is quick, only taking around 45 seconds. The STAC actually boots from the flash card to run, so no hard drive is necessary.

Firmware updates are as easy as removing the lid, sliding out a compact flash card and inserting a new one. Starting with firmware version 3.0 and newer upgrades can also be done via the Ethernet connection.

Over the years we've gone through a few different firmware versions and it's apparent that Comrex has been paying attention to user feedback. Most of the updates were to roll out minor fixes or upgrades to existing functions. The GUI has progressed greatly and now the setup functions that used to be buried in Linux are found in the GUI.

The GUI itself is clear and easy to use. The chat window (looking at it from an instant messaging world) needs more user control but adequately takes care of passing quick messages to and from talent (and anyone else who is logged into the GUI). The Web server login is password protected. On screen, the graphical representation of the lines has plenty of room for caller information and line selection, and disconnect can be made with a mouse click if the talent is out of the studio and away from a control surface.

The control surfaces come in six- and 12-line units. We have all 12-line units because of the number of inbound lines. The buttons are large and easy to press, and with LEDs under each button it's hard to miss the line status of each line. The LEDs are multicolor and can flash or burn solid to represent line status. The control surface is dip-switch selected to be either a screener phone or on-air phone – the difference being the screener is only able to screen calls and not put them on air. The STAC and surfaces are field upgradeable from six lines to 12 lines.

The control surfaces don't take up much space and have a handset built in to them so there's no need for a stand-alone phone to screen calls. The talent can switch a call to and from the handset at any time. The surfaces are also powered from the STAC so there is no need for power supplies at the control surface.

Good connections

The connection between the STAC and the control surfaces is via CAT-5 cable and RJ-45 connectors. Dedicated cables are necessary from the STAC to the surfaces as they run tip and ring, power and serial data through the cable.

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The Web interface duplicates control surface functions on any network-connected computer.

You'll find all the user settings and level control recessed behind a cover on the front of the STAC. The cover comes off without a screwdriver, and adjustments are made with your favorite greenie screwdriver. Nomenclature is located on the back of the removable cover to help identify the adjustment trimmers. The AGC can be turned on or off by one of the dipswitches but there's no provision for any control of the AGC.

You'll find all the standard PC connections of keyboard, mouse, network and VGA monitor. Once set up you can run without any of those connections but you'll find it is easier to have your primary talent watching the VGA output and have the screeners logging into the Web server.

Performance at a glance
Six to 12 phone lines
Two hybrids in one unit
Connect up to four control surfaces
Built-in Web server for call screening and remote line control
Built-in mix-minus between two callers

All audio inputs and outputs are analog, on the back and XLR balanced connections. The telephone line input and loop through connections for the analog telephone lines are on the back. The telephone lines are input on four RJ-45 cables (three lines per cable). The lines can be looped through to additional STACs or other phones. By enabling the clustering feature, calls can be isolated between STACs so that one studio doesn't interrupt an interview in progress in another studio.


Overall the Comrex STAC has been a reliable system for the Tom Joyner Morning Show. It's grown into a well established system that will be a benefit to any studio.

Kramer is director of engineering of Reach Media/The Tom Joyner Morning Show, Dallas.

Editor's note: Field Reports are an exclusive Radio magazine feature for radio broadcasters. Each report is prepared by well-qualified staff at a radio station, production facility or consulting company.
These reports are performed by the industry, for the industry. Manufacturer support is limited to providing loan equipment and to aiding the author if requested.
It is the responsibility of Radio magazine to publish the results of any device tested, positive or negative. No report should be considered an endorsement or disapproval by Radio magazine.

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