Operating HF Radios Unsqelched without ALE or Noise Blanker


In certain situations, HF radio operators may wish to suppress noise from their radio without using conventional means.

For example, the operator may wish to fully unsquelch their radio and disable the noise blanker to help monitor for a weak radio signal that is calling their station. Additionally, the HF radio may not support Automatic Link Establishment (ALE), which helps the operator to avoid noisy channels. Without these methods of noise suppression, the operator will always hear a audio from their radio, whether the radio is receiving a signal or only “white noise”

The syllabic rate squelch system (SRSS) was developed by Rene Stiegler’s team at ShipCom, and used by Coast Guard HF monitoring stations, to aid radio operators in monitoring weak signals. Rather than using conventional means of noise suppression by detecting the amplitude of a received signal-in-noise, the SRSS detects speech characteristics of the received signal-in-noise. This allows operators to receive a weak signal that would otherwise have been suppressed by the HF radio’s noise blanker or squelch control.

The purpose of the SRSS is not to suppress noise,  it is to recognize that voice is present in the noise, then open the audio path to the radio receiver–even when the amplitude of the signal (voice)-in-noise is only 10dB greater than the noise.

C-AT has now integrated the SRSS circuit into several versions of the ICRI- tactical radio bridge to support connection to an HF radio, without the requirement for a noise blanker or ALE. With this circuitry integrated into the ICRI, radio operators can remotely monitor the HF radio, via another radio or telephone. Without the SRSS, the noise from the HF radio would result in a VOX detect by the ICRI, preventing other users from accessing the channel.

ICRI-2PE with HF radio noise suppression circuit SRSS

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C-AT has incorporated in the ICRI the ShipCom syllabic rate squelch system, SRSS,  for use on voice radio circuits in lieu of the conventional amplitude based squelch circuits found on most HF radios. The ICRI , model “2PE” can bridge VHF, UHF, 700-900MHz portable or mobile radios to any commercial or military HF radio.  The ICRI is built into an IP67 rated Pelican case, and can operate over a 6.45V to 31V  power input range. 


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