I want to write code!
Modular synths are traditionally all analog but nowadays there's plenty of digital modules available too. As I'm a software engineer I have been planning to add some home made digital signal processing to my modular synth. I already have an STM32f4 processor taking care of midi2cv and two digital outputs but additional modules that are open for custom software are always welcome. |
| Cavisynth Ardurack PCB and panel |
I have been playing around with Arduino HW and got an idea to use it for eurorack module. Soon I stumbled upon Cavisynth Ardurack Module and realized others have got the same idea too! It's a simple eurorack module that uses the really popular Arduino embedded computing platform to do various things. It can be used for example as a square wave oscillator or a noise source. See Cavisynth website for full feature listing. The module is available as a cheap DIY pcb+panel kit and all software is open source so I had to order one immediately. Actually I ordered two: one to use the default features and another to fool around with. I also ordered Cavisynth SEQ-UFD sequencer module pcb but haven't built it yet so no comments on that one yet.
Not the Smoothest DIY Experience
Usually DIY synth modules are full of obsolete and really hard to find components and at least half of the work is to source all weird analog IC chips etc but because this is a digital module there's not that many parts on the board. The Arduino Pro Mini is a complete processor module that attaches directly to this board. In addition to that there's just some resistors, potentiometers, jacks and a simple power supply to generate 5V power supply for the processor. I was expecting a really straightforward build.
Unfortunately that wasn't the case. First problem was that the documentation is only available in french. Luckily there's lots of pictures so I was able to follow the instructions without understanding the text.
Another problem was the BOM (Bill of Materials). It does list all components but is missing pretty much all details related to them. I ended up ordering potentiometers that do not fit the hole in the panel without some extra drilling. It was also unclear what kind of zener diodes to use as they come in different voltages. With some reverse engineering I figured out that they are used as protection for the gate input so I chose 5.1V versions. Seems to work. There's also 3.3V model and 5V model of the Arduino Pro Mini board. Apparently 5V version is the correct one for this one but it's not mentioned anywhere in the documentation. (Btw. I got mine from Ihmevekotin web shop. It's really cheap and fast if you happen to live in Finland.)
Also the 3.5mm jacks used in these PCB's are somewhat hard to get. I was able to hack this one together using different type of jacks but had to order the correct ones from Cavisynth for the another unit and the sequencer. Pots and jacks are holding the panel and pcb together so it's important to have correct components to make it mechanically solid. So if you decide to build one you should probably order the jacks too from Cavisynth.
I really hope they will add better documentation and proper BOM in the future. Exact part numbers or links to component page in some electronics webshop would be appreciated.
Of course this is not a problem if you order a full DIY kit with all components included or a ready-made module from Cavisynth.
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| I had to drill bigger holes for the potentiometers. |
Soldering Time
After all the component sourcing headaches I finally started soldering things together. Yet another disappointment followed. Cavisynth seems to use 1-sided PCB's and there's horrible amount of jump wires that need to be soldered. Almost reminds me of my ugly veroboard modules! I really hope they will switch to 2 layer PCB's with top and bottom layers. They are really cheap nowadays and I'm sure all of the jump wires could be eliminated with the additional copper layer. But enough complaining. It's still lot more pleasant module to build than my previous veroboard hacks! And It's quite quick task after figuring out all the issues mentioned earlier and none of the issues are fatal. Here's some pictures of the build.
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| Lots of jump wires had to be soldered :( |
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| A trick to make a more common type of jack work. I had plenty of these available. |
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| Pretty much ready. Notice the jacks. Not very robust mechanically but the pots seem to hold the unit together well enough. |
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| Green is my favorite color! |
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| It's quite good looking little module! |
Oh, wait! We need software too!
As I bought the Arduino module elsewhere I had to flash the ardurack firmware into it. I was quite surprised that there's only source code available in cavisynth web page. No ready-to-flash binary image at all! So you will need to setup the Arduino environment and compile the code first. Luckily it's easy and there's lots of documentation and tutorials available online for Arduino. Anyway, I really like the open source approach! I hope I will have time to play around with the software soon.
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| And finally software in place and module in action! |
Conclusion
It's a cheap DIY module if you source the parts and do all soldering yourself and thanks to the open source software there's lots of possibilities if you know how to write C code. I still haven't used it enough to comment the sound but it seems to have a nice lo-fi character. Reminds me of the old computer games in my childhood. I need to build the other unit soon and do some retro game music with it!
The total cost was less than 40 euros and for that price I'm really satisfied with the features. I also like the innovative ideas on this module. It's great that we have people doing original module designs too instead of "yet another ladder filter copies". Cavisynth seems to be releasing interesting modules all the time. Check their website for details!
One final improvement idea would be to add a possibility to skip the on-board 5V power supply completely and just use 5V from the eurorack connector. For example the Doepfer PSU3 I'm using has plenty of 5V power available for the digital modules. It would only take the bigger eurorack connector and one optional jump wire to route the power to correct place to use it that way.
Here's two short videos demonstrating the module in oscillator modes:
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