Line Noise Reduction, Rcvr Mute & AGC ASP Print friendly page

Problems:

Air Traffic Controllers have identified four significant noise, audio level and equipment safety related problems with many current voice switches:
  • High Idle Channel Noise even when no radio receiver is active, specially when using HF radios.
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  • Noise burst coupled from a transmitter to its co-located receiver when the transmitter in un-keyed.
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  • Improper balancing of the volume of incoming calls or radio audio (inadequate or no Automatic Gain Control).
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  • Transmitter lockup due to a failure in the voice switch radio interface or a PTT switch lockup in the On state.

    The consequences of these problems are significant and may results in serious accident or equipment damage.
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  • High Idle Channel Noise in the operator headset results in unnecessary controller fatigue and could become a safety issue during peak busy times.
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  • Transmitter to Receiver coupling results in controller hearing a burst of white noise every time PTT is released. It becomes a nuisance and may even interfere with other active calls at the operator position.
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  • The improper balancing of the volume from airplanes and various callers causes controllers to constantly re-adjust volume settings for ground-to-ground (G/G) and air-to- ground (A/G) calls and increases the probability that a critical message could be missed. This increases controller's workload and induces a significant safety hazard.
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  • In case of a failure in the voice switch (Locked-up PTT switch or a failure in the Radio Control Equipment) a transmitter may be constantly keyed. This prevents other operators from using the same radio(s) and may even result in a damage to transmitters.
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    Solution

    Digital Signal Products (DSP), Inc. has developed an Audio Signal Processor (ASP) that simultaneously solves all of these problems. The ASP measures the strength of all incoming voice signals and adjusts the volume of all calls to the level selected by the AGC range selection. A total of 32 dB AGC (+/- 16 dB) can be incorporated. The AGC function is performed without reduction in quality of normal voice signals or amplification of low level noise.

    When multiple radios are selected by an operator, idle channel noise from all selected receivers adds up, and becomes significantly loud and disturbing. For removing this idle channel noise, the ASP monitors line audio levels and opens up the channel for signals as low as -40dBm, and shuts it off at -43dBm (3 dB of hysteresis is provided). A fast attack time (5-10 msec) and a slow release time (50-60 msec) insures that no part of a conversation is lost, while interfering channel noise is removed gently when a call is de-activated.

    If a transmitter and receiver are co-located a strong RF coupling results in overdrive and saturation of receiver front end. Consequently, when the PTT is released, the receiver outputs a burst of white noise while this extra energy is being discharged. If an operator keys multiple such co-located transmitters, the noise level becomes very significant and disturbing. For removing such coupled white noise, the ASP monitors the incoming PTT signal and shuts off the incoming receive channel when PTT transitions from On to Off state. Duration of this shut-off (receiver muting) can be selected from 0-300 mses in 20 msec steps. In response to incoming PTT, the ASP generates a PTT output signal toward the radio transmitter. A PTT timeout circuit (when enabled by the front panel switch) shuts off the PTT output after 1 minute timeout.

    All functions are independently selectable from the front panel DIP switch.

     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     


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