Software CW Filter

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CW and Music make a great match….

One of my other hobbies, along with amateur radio, is music.  I probably have as many music keyboards as I do straight keys.  But the last few years I have been pretty dormant playing music.  In a recent issue of MAKE Magazine I was reading an article for a free piece of software called VCV Rack which emulates a vintage modular synthesizer in software and for me re-opened exploration in music.

A modular synthesizer generates it’s sound by taking basic building blocks such as oscillators, amplifiers, and, filters then patching various combinations of it all together with cables.  The number of sounds you can make is limited to how many modules you have and your imagination.  Some famous rock bands have even taken real instruments or their voices and patched into these vintage synthesizer modules for some interesting effects.

Image 1 – A Moog modular synthesizer from Wikimedia Commons

So what does a music synthesizer have to do with CW?  There are 2 hints I mentioned in the paragraph above.  I realized VCV Rack has several styles of audio filters and it also has the ability to accept audio input from my radio via a sound card.  Can I use this virtual music instrument as an audio CW filter with variable frequency and bandwidth?  Only one way to find out.  The first attempt was to just get audio in, adjust it with a simple filter, then audio out the speakers.  The filter initially sounded “choppy” and found changing the block size to 1024 when selecting the audio input gave it a smoother sound.  The spectrum analyzer tool shows how well the filter is working.  Below is a screen shot to get started with the spectrum analyzer red trace the input from radio and green trace the filtered audio.

Image 2 – Patch #1 – A Simple Filter experiment

At the time of experimenting the bands were not too crowded with just a single signal and background noise.  The next step was to simulate a crowded noisy band.  Apparently there must be hams using VCV Rack as there are a couple different modules to create CW signals available to download.  So getting carried away I made a filter simulator using 5 CW signals and lots of static band noise (bottom row in Image 3) then going into a limiter and 3 filters (2nd row right side).  The simulated filter worked so well I was able to pull out a signal -10db down from all the other signals and noise.  Without the filter I could barely tell the signal was even present.  Mixers were used to adjust the various levels as well as bypass the filters.

Image 3 – CW Simulator & Filter

The current patch as of this writing for the  filter was to keep the limiter and 3x filter combination and added sliders to change the bandwidth and frequency of all three filters at the same time.  You can probably get away with only 1 filter module but the extra modules make the skirts of the filter steeper.

Image 4 – The current version being used

VCV Rack, basic instructions, and various modules are available at https://vcvrack.com/.   You need to register and there are some paid options but all the downloads and registration used to date are free.  The VCV Patches in this article can be downloaded from KC9ON (Links in patches below).  You will also need to download modules from the library as noted but patch 5 contains stock modules.

Patch Section:

Patch 1 – A Simple Filter:

Modules:

          • Bogaudio: VCF
          • LOGinstruments: Speck
          • Stochastic Telegraph: Fermata
          • VCV (built-in modules): AUDIO

Patch 2 – CW Simulator and Filter

Modules:

          • Autin: NON
          • Bogaudio: VCAMP, MIX4, MIX8, & VCF
          • cf: LABEL
          • Count Modula: Manual DPDT Switch
          • Digital Apothecary: Rue
          • LOGinstruments: Speck
          • Stochastic Telegraph: Fermata
          • Tonecarver: Morse
          • VCV (built-in modules): MULT, VCO, NOISE, AUDIO, & Scope

Patch 3 – Current configuration  (2023-08-10)

Modules

          • Ahornburg: Flying Fader
          • Alikins: Reference Voltages
          • AS: Mixer 2ch
          • Autin: NON
          • Bogaudio VCAMP, VCF
          • cf: LABEL
          • LOGinstruments: Speck
          • Stochastic Telegraph: Fermata
          • VCV (built-in modules): Audio, MULT, Scope

Patch 4 – Improved configuration (2023-08-14) – A shortcoming with the above patches is the incoming audio is only at 1Vp-p while the synthesizer operates closer to 10Vp-p.  A multiplier was added to boost the audio to the 10V range.

Modules

          • Ahornburg: Flying Fader
          • Alikins: Reference Voltages
          • AS: Mixer 2ch
          • Autin: NON
          • Bogaudio VCAMP, VCF
          • cf: LABEL
          • Count Modula: Quad Trace Oscilloscope
          • kocmoc: MUL
          • LOGinstruments: Speck
          • Stochastic Telegraph: Fermata
          • VCV (built-in modules): Audio, MULT, Scope

Patch 5 – Filter using only the built in modules.  However, I’m not impressed with the standard LP/HP stock filter and prefer the BogAudio filter.

Modules:

          • VCV (Built-in modules): Audio, VCA, VCA MIX, Compare, VCF, Scope, Logic, Mult, Notes

Patch 6 – Another simple setup to experiment with filters.

Modules:

          • Bogaudio: VCF
          • Daniel Davies: Samuel
          • LOGinstruments: Speck
          • VCV (built-in modules): LFO, VCO, ADSR, VCA, Scope, Audio
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