Sonifex RB-SD1
Silence dectect unit


The RB-SD1 Silence Detection Unit is used to monitor an unattended stereo studio feed and in the event of the signal going 'quiet' after a given period the unit will switch through an alternative stereo audio signal. This signal could be a recorded message (e.g. normal service will be resumed. etc), a feed from a CD player or MiniDisk machine, or an alternative recorded program. Controls are provided to start external equipment and to provide remote status indication. It has a passive signal path so that it will still allow a signal to be transmitted in the event of a power failure.

It has two balanced stereo audio inputs that are user-defined as either the main source or auxiliary source, with the maximum input being -28dBu. Both sources are monitored for failure and each has a remote failure alarm. The main source is constantly monitored and in the event of it dropping below a pre-set level for a pre-determined amount of time, the unit will automatically switch through to the auxiliary signal.

The silence detect level is user definable and adjustable between -60dBu and -15dBu in 3dBu steps via a 16 position rotary switch on the rear panel. The silence interval can also be adjusted between 2 and 30 seconds in 2 second steps, or alternatively set to 2 minutes 5 seconds also via a 16 position rotary switch on the rear panel. The outputs use stereo balanced XLR connectors and the auxiliary input can have the gain increased to accept an unbalanced signal.

The unit can operate in two modes - auto or manual. In both modes the unit will automatically switch over to the auxiliary source after detecting silence. When the main signal is again detected the unit will either return to the main signal automatically or manually depending on the mode chosen. A lit LED indicates when the unit is in auto mode.

The RB-SD1 can be remotely controlled with functions that mirror the following on-board controls : mode selection and return indication, restore method and input source selection and return indication. Remote outputs provide separate relay contact closures for failure of the main and auxiliary inputs. The unit can be configured by dip-switch to alarm when either the left or right channel of the main input source fails, or if the whole stereo signal fails. Front panel LED indicators show individually left and right programme and alarm conditions for both the main and auxiliary inputs.

The RB-SD1 has been designed to have a passive signal path through the main input, so if power to the unit fails the signal input will still be routed through to the output. This is essential for applications such as installation at transmitter sites where a power failure to the unit should not prevent the audio input signal from being output to the transmitter.


  • Input Impedance: >100kohm balanced
  • Maximum Output Level: +28dBu
  • Output Impedance: As input, except when using unbalanced auxiliary input where output impedance <50ohm
  • Frequency Response: 20Hz to 20kHz +/- 0.1dB
  • Gain: +8dBu (for unbalanced input B - optional)
  • Noise: < -87dB, unity gain, ref +8dBu output for unbalanced input

Rear Panel Connections and Controls:

  • Inputs (Main and Auxiliary): 4 x XLR 3 pin female (balanced, auxiliary can be unbalanced)
  • Output: 2 x XLR 3 pin male (balanced)
  • Remotes: 15 way D-type plug
  • Alarm Threshold: -15dBu to 60dBu in 3dB steps via rotary switch
  • Silence Detect Duration: 2 sec to 30 sec in 2 second intervals and 125 second option via rotary switch
  • Detection Type: Mono or stereo, via DIP switch
  • Silence Switch Defeat: Disable/enable silence switching, via DIP switch
  • Remote Start: Latched or momentary, via DIP switch
  • Mains Input: Filtered IEC, switchable 110-120V, or 220-240V, fused, 9W max
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