What is Echo Cancellation, and What is "Hoth Noise"
Solution Your Echo Canceler and You: What is "Hoth Noise?"

Hoth Noise is that "schussing" sound that you hear when an echo canceler is in action.  Normally, this is a good indication that the echo cancellation software is active and working.  The alternative to the Hoth Noise is the echo itself, since the echo canceler replaces the echo audio with Hoth Noise.  This is becuase the echo canceller "user" needs to have some sort of audio feedback to assure that the call is still active.

On the technical side, Hoth Noise is used to model indoor ambient noise when evaluating communications systems such as telephones. It is named after D.F. Hoth, who made the first systematic study of this. The official definition of Hoth noise is IEEE standard 269-2001 (revised from 269-1992), "Draft Standard Methods for Measuring Transmission Performance of Analog and Digital Telephone Sets, Handsets and Headsets."  The Main use of Hoth noise is by the NLP in conjunction with the Echo Canceler. The noise too many sounds like a woosh when you start or stop speaking.

If you desire to get rid of the Hoth Noise, there are options in our echo canceler routine.  The Hoth Noise can be replaced with muted audio.  Disabling the NLP (hoth noise) is NOT recommended as it is used by the echo cancellation routine and can result in poor audio quality/echo.  What is recommended is switching the NLP type to mute (from Hoth), so that it is still active, but you do not hear the hoth noise over the head set.

This can be done by following these instructions.

1) Stop Asterisk. In most cases, this is done with the "amportal stop" command or "service asterisk stop"

2) Unload the rcbfx module. Depending on your configuration, this may be "service dahdi stop" or "rmmod rcbfx"

3) Edit the driver configuration file. This will be a file in /etc/modprobe.d/ with "rhino" in the name, eg: rhino.conf or modprobe.rhino

3b) If no such file exists, then create one named rhino.conf

4) Look for the line that begins "options rcbfx" or if no such line exists, add one (eg: options rcbfx debug=0)

5) Change the value of nlp_type= to 1 (eg: nlp_type=1)

5b) There are several possible values for nlp_type. For more information, run "modinfo rcbfx"

6) Save the file and exit.

7) Reload the driver. Typically this is "service dahdi start" or "modprobe rcbfx"

8) Finally, restart Asterisk.

Note: These instructions can also be applied to Rhino digital cards with EC modules. Simply substitute rcbfx for r1t1 or rxt1 as appropriate.

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