Field Report: SBS Guardian 2

The one rack-space Guardian 2 stereo audio and composite signal-fail control unit produced by Sound Broadcast Services (SBS) is designed to provide management of analog audio and composite signals. The unit has a main stereo audio input, an alternate stereo audio input, and a reserve stereo audio input. If one of the main audio input channels fails, the Guardian will derive a mono signal to both output channels from the remaining input channel. If both main audio input channels are lost, the unit will switch to an alternate audio source such as a secondary STL system. In the absence of the main and alternate audio inputs, the Guardian 2 will switch to and start a reserve audio source such as a CD player. The unit utilizes high-quality sealed relays in its audio switching circuits, thereby eliminating active components in the audio path.

Performance at a glance
  • Compact size (1RU)
  • Baseband audio or composite signal operation
  • Adjustable thresholds
  • Front-panel status indicators
  • Automatic switching of backup sources
  • Available from Broadcasters General Store
  • In addition to monitor and control of baseband audio, the Guardian 2 can also monitor a composite signal. When loss of the primary composite signal is detected, the unit will switch to an alternate composite signal source. There is no active circuitry in the composite signal path of the unit. The composite detector threshold level is front-panel adjustable via a LED and a recessed trim pot and can be defeated for use at stations that broadcast in mono. The composite monitor includes a pilot detector that will switch to the backup system immediately if the pilot fails. An adjustable PLL pilot lock circuit is part of the composite detector configuration.

    Smart sensing

    The Guardian 2 discerns between fault conditions such as line hum versus program audio via the use of 300Hz to 3kHz bandpass filters that are followed by separate dynamic and static level detectors. SBS states the possibility of false triggering is nearly eliminated by using this combination of filters and detectors. The static (constant level) detector looks for fixed audio levels while the dynamic detector looks for varying overall levels. The static detector threshold at which the Guardian 2 will switch to its fault mode is adjustable from -25 to 0dB and is adjusted by utilizing a 2kHz tone, a front-panel recessed trim pot, and a front-panel LED indicator. The dynamic detector adjustment establishes the dynamic program level that will cause a fault condition and is adjusted using program audio, a front-panel recessed trim pot, and a front-panel LED. The time before a fault is detected is adjustable in various steps from 5- to 90 seconds via a front-panel rotary switch. The front-panel composite detector recessed trim pot control along with its associated LED indicator sets the level at which the detector operates. The audio detectors can be defeated for stations that only wish to use the unit to switch between audio sources via external commands.

    The Guardian has additional front-panel indicators. They include left and right audio, composite signal, alt/main, and pilot. Each LED indicates green when in the normal mode and red when in the fault mode. The pilot LED shows only green when the pilot signal is present.

    The rear-panel includes a DB-25 connector. Several status functions, including power supply and audio status as well as multiple latching and momentary contacts for the control of external devices, are present on the connector. Other functions found on the DB-25 include the ability to remotely switch the system to the alternate or reserve audio source or to feed both audio outputs via either the left or right inputs only. Outputs for all detectors are included in the connector pin out for interfacing the Guardian to an external logging or alarm system. All OK status outputs are open collector, capable of sinking 100mA with a maximum-switched voltage of 30V DC or 50V AC.

    More connections

    Other connectors on the rear panel include audio I/O as well as composite signal I/O. The main, alternate and reserve audio input connectors and the audio output connectors are balanced XLR with switchable 600V terminations on the main and alternate audio inputs. The composite signal I/O jacks are BNC female. The power connector is a filtered IEC 320 male connector with an integrated fuse. The Guardian 2 appears to be well shielded for use in high RF environments and has a rear-panel ground lug. The internal construction is good to excellent. A single high-quality printed circuit board is used and all integrated circuits, of which there are many, are on sockets. The accompanying manual is well written and includes schematics and a complete parts list with the names of the parts manufacturers. Overall, the unit is well-constructed for long-term use in unattended installations.

    Installation and setup is made easy using the front-panel adjustments and LED indicators. The switching between the main, alternate and reserve audio sources was smooth and quiet. We configured the Guardian to start a CD machine and to report an alarm condition when, by using the circuits available at the DB-25 connector, it was forced into the reserve audio mode. Both functions worked well as planned. The reserve audio input includes a switchable 10dB gain amplifier for use with equipment having lower output levels. I also successfully tested the loss of one audio channel function whereby the unit switches the remaining audio input channel to both outputs. The composite signal detector is an added bonus rarely found on similar units of this type and switches rapidly to a back-up composite signal source when the primary composite signal is lost. The circuitry of the unit appeared to be stable even when subject to several rapid power outages under test conditions. The SBS Guardian 2 is a practical and yet unique product. Its many uses, such as automatically switching between STL systems or audio processors, are only limited to the creative minds of engineering personnel.

    Ron Bartlebaugh is director of engineering for the WKSU Stations, Kent, OH, and president of Audio and Broadcast Specialists, Akron, OH.

    Sound Broadcast Services

    Available from Broadcasters General Store

    P 352-622-7700
    F 352-629-7000

    Editor's note:
    Field Reports are an exclusive BE Radio 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 BE Radio to publish the results of any device tested, positive or negative. No report should be considered an endorsement or disapproval by BE Radio
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