Field Report: Broadcast Devices DAB-300

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Field Report: Broadcast Devices DAB-300

Aug 1, 2010 1:00 AM, By Doug Irwin, CPBE AMD

For years the New York Clear Channel stations transmitting from the Empire State Building used BDI 3�2 composite switches to change STL feeds as necessary. With HD Radio, then PPM and finally our studio consolidation, we decided it was time to upgrade the capability of our STL switches so that several new requirements could be met: We now feed programming to our analog and HD Radio transmitters; Arbitron was pressing us to match PPM sources between analog and our primary (simulcast) digital; and we also wanted automated "smart" switching between three different sources.

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I made up a block diagram of a switcher that would do what we needed. New York Director of Engineering Josh Hadden suggested I talk to Bob Tarsio of BDI to see if the company would build this new device for us. We needed five of them. I ran it past Tarsio and his team, and they proposed a semi-custom version of the ATB-300 Audio Toolbox, which became the DAB-300.

At its most basic, the DAB-300 is an automated AES3 switcher that will choose one of four AES3 inputs, and route it to all four AES3 outputs, based on basic configuration parameters. All inputs and outputs are 110ohm AES3. The unit occupies 1RU and is 10.25" deep. As you can imagine, rear apron space is limited, so the inputs and outputs are brought out to a breakout panel, which itself is 1RU. The remote controls are accessed via DB-25 connectors on the rear of the DAB-300.

When used in manual mode, the unit operates like any other switcher. Select the input on the front panel and press the take button. Beyond that, there are options to modify the signals that come in to and go out of it. Looking at each individual input, the following options are available:

  • Swap left and right channels
  • Generate mono from left or right channel
  • Sum L+R
  • Invert the phase of the left or right channel
  • Modify the gain in the path by +/-10dB Performance at a glanceProgrammable silence threshold
    Front-panel menu programming
    Inputs: 4 analog composite, 4 AES3
    Composite-to-AES3 converter
    Synchronous AES switching between sources
    SRC on all inputs up to 96kHz
    Selectable output SR 32, 44.1, 48kHz
    Programmable alarm contacts
    Remote controllable
    This capability exists on each input; the settings are not global. In addition, each of the four outputs can be adjusted individually for gain (+/-10 dB). The output sample rate is set globally by dip switch settings inside the DAB-300.In its auto mode, there are several parameters that must be set: silence threshold and time delay. Both are configured via the front panel. (On our units, the time delay can be set as low as 10 seconds.) The scale on the threshold is linear, so I did my best to match it with what I consider silence to be. You also tell the unit if you want it to revert or not. When the unit I set in its auto mode, it will seek a source of audio on one of its other inputs in the event that it detects silence (either composite or AES3) on its primary input. If you configure revert mode, it will seek back to the primary audio source after a timeout period (which you also configure via the front panel).By the way, you can change your selection of the primary source dynamically via the front panel. Whatever source is picked prior to selecting auto mode becomes the primary. This is handy in case what you normally consider to be the primary goes out of service or becomes temporarily problematic (for any number of reasons).-- continued on page 2

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Field Report: Broadcast Devices DAB-300

Aug 1, 2010 1:00 AM, By Doug Irwin, CPBE AMD

Taking control

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A connector break-out panel is available to simplify connections.

The remote controls of the DAB-300 are what you would expect (for our semi-custom version we added some). By way of contact closures, you can change the inputs and there is status that reflects those changes so that you can tell what input is routed to the outputs. You are also provided with an input to set the unit to manual mode, as well as one to set it to auto mode. You will have to switch the unit to manual mode before changing inputs remotely. Unfortunately there is not a remote status to tell you what mode it is in.

Our plan was to use the DAB-300 not only as an analog composite switch, but as an AES3 switch as well. The two groups of switches in the DAB-300 operate in parallel and sync with each other. We have three analog composite sources, and each of those composite feeds has an AES3 replica. All six are fed to the DAB-300. When the unit is switched to the primary STL, our analog transmitters and HD Radio transmitter are fed by the same audio processor (and same program source from our studio HQ). When switching to our backup STL, again the AES3 feed is a replica of the composite feed; this extends to our tertiary STLs as well.

The composite switch that is built-in to our unit feeds a four-output DA that has a fixed gain of unity. Another feature that we asked for was a set of status outputs (by way of relay contacts) that correspond to silence on all six STL sources.

Broadcast DevicesP

The only caveat I have for potential users: Plug the DAB-300 in to a UPS. The unit has a switch-mode power supply that detects brief ac input hits. Unless the ac power is gone longer than about 3 seconds, the power supply will not come up. We're very pleased with the functionality of our DAB-300s as well as with our experience with the team at BDI, who worked closely with us to meet our requirements. I have been told now that the engineering is done, our particular application is no longer semi-custom, and is available to other users should they have the same requirements.

Irwin is transmission systems supervisor for Clear Channel NYC and chief engineer of WKTU, New York. Contact him at

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