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Filter and Compressor Provide Realistic Radio/Intercom Simulation


Simulating a radio is more than just sending digital audio around the network when the frequencies match. Aside from the comprehensive model of radio electromagnetic propagation that is required, the sound of a radio is different from a telephone or intercom even when the incoming signal is strong.  In order to properly simulate the audio sound of a radio a number of audio effects must be present.

First, voice streams must be dynamically filtered as they move through the audio system.  Aircraft intercom systems usually have a wider bandwidth than radios. The following example illustrates the difference between unfiltered voice, typically used in an intercom simulation, and a radio, with typical bandwidth of 300Hz to 3000Hz.  This means that instructors can role play as a ground station, etc., and have a different sounding voice for the intercom during instruction.  This has not been available in the past, and is not available on other systems.

The second voice is the same voice passed through "VComm Radio Audio Emulation Filter" object 2055. The audio stream is filtered and squelch tail and radio background noise are added in real time. 

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Intercom Voice (Unfiltered)
   
Radio voice stream (Filtered)
 
Object 2055, "Radio Audio Emulation Filter"

This high performance digital filter object should be attached to all radios since the realism is greatly enhanced.  Due to the efficient design it doesn't require much processing time. Even though the filter is very processing efficient the response drops off rapidly below 300 Hz and above 3000 Hz. (16kHz sample rate)

 

 
Compressor/Limiter
 

Another very important effect for radio simulation is the proper use of an audio compressor/limiter.  Some radio simulation systems in military exercises have large dynamic ranges of incoming voice volumes. That is, some radios from some sites are very loud while others are not, making it difficult and uncomfortable to communicate. This is due to a common problem that has been addressed in real radios by incorporating a compressor limiter.  In addition to the AGC and squelch system of real radios an audio compressor limiter decreases the amplitude of large signals, and amplifies weak signals dynamically.  In reality the process is somewhat more complex but the effect is the same. Different microphones exhibit different gains along with the fact that some sites use different settings of the vendor equipment. This is a normal condition that should be expected. However, the vendor equipment should include a compressor limiter. SimPhonics systems include a software compressor limiter in order to eliminate this problem.

   
Object 2056 "VComm Audio Compressor"

There is a tradeoff between the compression ratio and background noise level since a larger compression ratio creates better voice smoothing, it also increase the background noise level picked up by the microphone. 

 

If you don't have a SimPhonics system there are analog compressor limiters available on the market that can perform this function external to the equipment.  Radio Design Labs produces one than can be inserted between the system output and the headset. With proper connections the large amplitude swings will go away and all communications will be at roughly the same level. The difficulty may be in that the signals are line level and the output may require amplification before application to a headset.

 
 
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