Nothern Utah WebSDR Logo - A skep with a Yagi Northern Utah WebSDR
The "DSP Noise reduction" "Notch2", "High Boost",
"Vari-Notch", "CW Peak" and Synchronous AM features

"Autonotch" is now "Notch1"

The filter that had been labeled "Autonotch" is now "Notch1".

Unlike the new "Notch2" filter, this "Notch1" filter operates on the server side of the signal path and its operation can reduce the strength of strong CW notes (carriers) and reduce the S-meter deflection and prevent AGC action on that signal.

Unfortunately the original "Autonotch" notch filter (a.k.a. "Notch1") is susceptible to other strong signals meaning that it can be prone to "hunting" in the presence of strong modulation - and with weaker signals, it may not reliably "lock" on CW notes at all.
"DSP Noise Reduction", "Notch2" and "High Boost"

Operational notes:

DSP Noise reduction:

Also added is a noise reduction filter that can take out some of the background "hiss" and improve the signal-noise ratio.  This is the same type of filter may be found on nearly all HF transceivers these days:
"I reloaded the page, but DSP noise reduction (or related option) suddenly isn't working"

There's a quirk related to the "tick" boxes and drop-down settings on the WebSDR - and other similar boxes on other web pages and that is that if you reload or refresh the web page, the boxes or selections may look as though they are active, but they will not be.

For example:  If you were to select a DSP setting of Low, the DSP noise reduction would start working - but if you refreshed the page, it may still show that selection, but the DSP noise reduction would not be active.

The work-around is to select Off (or any other setting than the current one) and then select Low again to reinstate the filter.

This same quirk applies to practically every option on the WebSDR page from the frequency to the waterfall setting to the volume setting - and everything in between.  This is not a bug per-se, but just the way the HTML/Javascript that is being used works.

There are currently four settings for the DSP Noise reduction filter:
A few comments about DSP noise reduction:

DSP noise reduction works by quickly "locking on" to the voiced elements of human speech and filtering out the rest - and this works because voiced human speech consists of tones.  For signals that range from good to "moderately" noisy, this works quite well - but when signals are very weak, it may actually reduce intelligibility in some cases because the "tones" of the speed are increasingly difficult to discern from the background noise.  When only noise is present or signals are very weak, the noise reduction tries to lock on to the random elements of noise causing the "swishing" (or "voices in a barrel") effect.

Because of its nature, this filter will also "lock on" to tones and it may be useful for CW use in some cases.  Note that this filter is adaptive and it takes time for it to "lock" onto a signal - and the higher settings take even more time to do this, so it may not be the best choice for rapid-fire contesting.

For casual listening, the Low setting is recommended as its effects are moderate, offering reasonable noise reduction without being too strong or causing deleterious effects on weak signals.

Use of DSP noise reduction with digital modes:

It is recommended that DSP noise reduction be turned OFF if you are trying to use any digital mode - and this is true not only for the WebSDR, but for ANY radio with DSP filtering.

The "Notch2" filter:

This filter uses digital signal processing techniques to detect a coherent signal - like pure tone like a CW note (carrier) - and block it out.  Because it is in the audio path its action will have no effect on the S-meter, but is may be more effective in removing weaker carriers that may still be annoying, but too weak to be reliably locked onto by "Notch1".  For very strong interfering signals you may wish to use both Notch1 and Notch2.

Of course, one should not try to use any notch filter with CW or digital modes as this filter's job is to remove the very signals comprising such signals!

The "Vari-Notch" filter:

This is a simple, manually-tunable notch filter that may be used in addition to the notch filters above.  It is most useful for a single, unchanging tone (e.g. carrier from a broadcast station) - but like "Notch2", it has no effect on the S-meter reading and will not prevent receiver desense from strong signals.

The "CW Peak" filter:

This is a simple, moderate-Q band-pass filter that is intended to reduce "ear fatigue" for CW operation by attenuating - but not eliminating - off-frequency signals.  Related to this is the "Ref Tone" button that, when activated, produces a tone that is exactly that of the bandpass filter to allow quick and precise adjustment of the bandpass filter.

The "High Boost" setting:

This setting boosts the high audio frequencies by 6 dB with the "knee" of the upward curve centered on 1500 Hz.  The purpose of this setting is to boost the "highs" which can boost the intelligibility of speech.

This feature may be helpful if you are using a higher setting of the DSP noise reduction, or you may have age-related hearing loss.

Synchronous AM:

While it may not seem like it's a DSP feature, doing Synchronous AM reception on the PA3FWM WebSDR really is.  Because the WebSDR server code itself isn't set up do this, frequency shifting and carrier recovery is done at the user end of the pipeline.  Because of the way that it must be implemented, there are a few restrictions:

Any problems that might be caused by these features?

The above functions are client side - that is, they are run on yourcomputer rather than the WebSDR server itself.  What this means is that they will increase processor load on your end - something that may cause intermittent audio issues on slower computers - particularly if you have minimized the browser window in which you are running the WebSDR.  In early 2022, these features were rewritten to dramatically reduce computer load and improve performance, but they can still task slower computers.

It has also been observed that in general, the Firefox browser seems to work better than most other browsers.  The Chrome browser seems to have more trouble on some computers running the WebSDR's web page, causing issues such as audio drop-outs.  If this happens, check to see if the sound card's speaker output sample rate is set to 48000 Hz (in Windows) rather than something higher like 96000 or 192000 - or simply try using FireFox.

Where did these features come from?

These added features (DSP Noise Reduction, Notch 2 and High Boost) have been added to the client side of the code, written by the folks at the Northern Utah WebSDR and not with the direct participation (but with the knowledge of) the original author of the WebSDR software, P.T. de Boer, PA3FWM.

If you have questions about these added features,  or if you operate a WebSDR system and are interested in adding them to your system please direct inquiries to  and not PA3FWM, the original author.  


As of 28 May, 2022 a complete rewrite of the above features was implemented on all of the Northern Utah WebSDR's servers, reducing processor load on the users' computers, improving performance and adding freatures, including the "CW Peak", "Vari-Notch", "Synchronous AM", and finer tuning resolution modes - See the "FAQ" page under the heading "Features of the WebSDR".

Additional information:

Go to the Northern Utah WebSDR landing page