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Active Noise Cancelling

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Active Noise Cancelling removes background noise efficiently

This article describes the Active Noise Cancelling feature in the STENTOFON IP Stations. The active noise reduction is used for intercoms that should be put in high noise environments. The noise reduction feature filters out the noise and at the same time keeps the voice signal. For Inca stations, the noise reduction feature is configured from the web interface of the station its self. But for Turbine stations, the feature is configured from the AlphaWeb.

Background

Having a conversation in a noisy area is difficult enough when you are talking face to face with your partner and can be virtually impossible when using a communications system. Entry gates of car parks and toll roads particularly are difficult points to have a conversation due to the noise of idling car and truck engines. What if this noise could be suppressed, wouldn’t that make it a lot easier?

A noise cancelling microphone is one way of suppressing environmental noise. A noise cancelling microphone works on the principal of differential sound level pressure. It has 2 points of entry for sound. Environmental noise enters both ports at the same level and has only a minimal effect on the microphone diaphragm, while the wanted audio should only be present at the front of the microphone. Only when the audio source, in our intercom world a person speaking into an intercom station, is close enough will the differential sound pressure be large enough for the microphone to register the sound. Close enough is just a few centimetres, and this is the weak point of such a solution. In many occasions it is just totally impractical to be that close to the microphone, for instance in those difficult locations as mentioned above; and even if it is possible, for an occasional user of an intercom station it may not be obvious that it is necessary to be close, for instance somebody using a help point in a railway station.

Active Noise Cancelling on the other hand filters out the noise and at the same time tries to keep the voice signal. It is no longer necessary for the user to be very close to the microphone.

Active Noise Cancelling

The active noise reduction use advanced signal processing algorithms in the STENTOFON IP stations. These algorithms tries to identify the noise signal (background repetitive audio). When noise is identified, the algorithm filters out the noise and at the same time tries to keep the voice signal. If you set the noise reduction level high, more part of the voice signal will also be filtered out.

The filtering can be set in steps from 0 to 7 to set the station characteristics tuned optimally to the environment.

Inca Stations

The ‘Noise Reduction Level’ setting is done via the built-in web server of the IP station. The setting will take effect immediately after saving, making it easy to test various parameters while the conversation is active.

Setting of Noise Reduction Level for Inca stations is done in the web server of the IP station


Turbine Stations

The ‘Noise Reduction Level’ setting is done in AlphaWeb System Configuration --> Stations:

Setting of Noise Reduction Level for Inca stations is done in AlphaWeb


Simply click the station you want to change the Noise Reduction setting for and a new window opens:

Click the Noise Reduction drop-down box to select a Noise Reduction level from 1 to 7.


The setting will take effect immediately after clicking Update Station, making it easy to test various parameters while the conversation is active.


  • Level 0 means that the function is disabled
  • Level 1 gives a maximum noise reduction of 0.2 dB
  • Level 2 gives a maximum noise reduction of 6.2 dB
  • Level 3 gives a maximum noise reduction of 12.2 dB
  • Level 4 gives a maximum noise reduction of 18.3 dB
  • Level 5 gives a maximum noise reduction of 24.3 dB
  • Level 6 gives a maximum noise reduction of 30.3 dB
  • Level 7 gives a maximum noise reduction of 36.3 dB

By default the noise reduction is disabled. i.e. set to 0. In high noise environments we recommend to set the noise reduction level to 3 or 4. With this setting the audio quality is ok when quiet, and when noisy most of the noise is filtered away.

Software requirement

The Active Noise Cancelling feature is included as from IP station firmware version 01.07. The latest firmware verison can be downloaded from the download site.