Building a Modular Synthesiser Part 6: Finishing Touches and Putting It to UseFollow article
Adding some refinements to the Eurorack case and using an oscilloscope to explore how the synthesiser modules operate.
There are a few finishing touches to add to my synthesiser and I still have a fair way to go to familiarise myself with how all the modules work. To help with the latter I decided to have a look at the various waveforms it produced using my oscilloscope.
Some finishing touches and useful resources
I added 2 more railsat the rear of my Schroff case to support the power supply and the USB connector for the Bela Pepper – this freed up some space at the front of the case and kept the power supply switch out of harm's way.
I found a useful Eurorack panel plugin for Inkscape and put that to work to make some laser cut blank panels also for the rear of the case.
I had been using M2.5 nuts in the rails but found they were very fiddly when fitting the modules. The addition of the proper Schroff threaded insertsimproved matters greatly!
If you are planning to build a modular synthesiser of your own I can recommend the Modular grid web site. This has an online graphical interface that lets you select an appropriate sized rack and plan your synthesiser by adding modules to it. It gives you a running total on power consumption and approximate cost! The screenshot above shows my modules with a couple extra that are on the wish list.
Setting up PicoScope on Ubuntu 18.04
As I was working on a PC running Ubuntu 18, I needed to install PicoScope before I could use the oscilloscope. I did this following the instructions on the PicoTech page, but the application would crash every time I tried to start it. I eventually tracked down what was causing the problem thanks to some information I found on this post on the PicoTech forum: I was missing the libpng12 library. “libpng” is the reference library that handles .png graphics files in Ubuntu and version18 (and newer) only comes with libpng16 installed. Version 12 that PicoScope needs is not available from apt package management system, so I downloaded the necessary files from here and installed them that way. After that was done PicoScope worked fine.
Now I had PicoScope up and running I made up some signal splitter cables so that I could connect the oscilloscope at different stages of the patch. These were made from a length of screened audio cable, 2x mono 3.5mm jack plugs and a cable mount 3.5mm jack socket . The strain relief boot on the jack plug would not accommodate a double thickness of cable so I replaced it with some heat shrink sleeving on that plug.
I next set up some simple patches and connected the oscilloscope using my new cables.
In all the videos the untreated sound is represented by the blue.
I first took a saw wave from my Product module and fed it through the EQ Filter.
In this video, the saw wave is being fed through the voltage-controlled filter on the Product module. The cut-off is being adjusted and then the emphasis. As the emphasis pot is turned to almost fully clockwise you can hear it start to self-oscillate. Self-oscillation is a form of feedback commonly produced by VCFs. If the resonance value is turned up sufficiently high, the self-noise of the filter circuit will cause the positive feedback to "ring" the circuit and produce a tone. The resulting waveform is usually fairly close to a sine wave.
The sound here is produced solely by the VCF self oscillating.
The blue line on this one is an equal mix of saw and triangle wave, this is being shaped by the Envelope generator and then chopped up by the Bela pepper’s Granulator project.
The video above shows my synth as it is at the moment. In addition to the modules I have built, there is a Mutable Instruments “Braids” module that will produce all sorts of different waveforms, along with a Music Things Modular “Turing Machine”, that is a random looping sequencer — both of which I picked up fairly cheaply on a well-known Internet auction site. These two provide me with some additional “voices” and an element of chaos for producing the organically evolving drones that I am aiming for.
In the patch above Braids is generating three sawtooth waves. The tuning of two of these is being controlled by the Turing Machine, which in turn is being fed a control voltage by the Erica Synths modulator. The Turing Machine is also controlling the Envelope Generator on the Product Module. Product is also producing the 2 other oscillators that you can hear being faded in via my mini-mixer at about 30 seconds into the video. The Bela Pepper is providing a touch of Tape Echo and the EQ Module is filtering out some of the higher frequencies.
As my fellow DesignSpark member @anthonywilliam commented on a previous blog post, this can become very addictive! I am already planning what other modules I would like to add and how I might expand my case, as I am running out of room. As can be seen from the back of the case there is plenty of space to add features inside, even though the front rack is full. I am thinking of adding a small amplifier and speakers, and also a Raspberry Pi that I can use to load Bela Pepper projects and possibly run the oscilloscope through an external Pi touch screen. I am also looking to upgrade the power supply and install a bus to carry the power, to replace the “flying” version I have now.
And I definitely need to design and make some kind of tidy rack for all the patch cables.
I just have to keep telling my partner that it is cheaper than taking up golf (so far anyway).
I have shared a few of my sound experiments on social media and have been invited to go along to the local “synth club” as a result, so I must be doing something right.