DIY Digitally-Controlled Analog Filter Design

In the Beginning

For the Rockit Synth, I designed a digitally-controlled analog filter.  I went down this path after a good amount of thinking, learning, and design attempts.  First, I looked into cribbing a filter design from some reputable synthesizer.  I found a few problems with this approach.  Primarily, voltage-controllable analog filters tend to be part-count heavy.  Look at most designs and you’ll find dozens of components, many of them requiring trimming to work properly, and many of them requiring many dollars to purchase.  The designs also tend to require high rail voltages, like +/- 15V.  And some require all of the above. Take a look at an old Moog filter.  It’s got boatloads of parts, high rail voltages, and in the end is only a low pass filter.  Sure, it sounds sweet, but my goal was to make an affordable kit synth.  Maybe one day, I’ll splurge and build a no holds barred filter, but it turned out to be interesting to live with some limitations and make my own filter.  Click through for a detailed discussion of the filter in the Rockit Synth.

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All Kickstarter Has Shipped!

I finished shipping all the Kickstarter orders. Altogether, I’ve shipped about 175 Rockits in some form. I’m happy to say that no problem that I encountered along the way was unresolvable. Everyone that has one built has it working. That was a lot of work and it’s kind of weird to be on the other side of such a monumental effort. I’ve been working 60+ hour weeks between my real job and Rockit for as long as I can remember. The real lesson I learned from doing this has been persistence, sheer dogged persistence. There were several times I sat at my workbench (kitchen table) and thought that if I didn’t figure out what was wrong pretty soon, I was going to cancel the Kickstarter project. But I just kept coming back and trying again. I did two board revisions between the beginning of the project and when it was funded and then spent an entire more month tweaking the software. I added the drone/loop feature after the Kickstarter project started! All in all, Rockit took two years of my evenings and weekends to design, and several more years of learning before I had the courage to start it. All in all, there were six Rockit prototypes from breadboard to the present board. I have a hard time estimating the amount of time that I put into it, but it must be a couple thousand hours.

Rockit is now available for sale in the HackMe Store. The case is coming along and should be finished and available in December.

So, what’s next? I’m going to be working on the software a bit to come out with a 1.1 release with some fixes, some changes, and some new features. I’ve received some requests and I’m going to do my best to incorporate those requests. Some people would like some envelope changes, some LFO changes, and I think I could figure out some surprises too. There are different oscillator modes that leave open some clock cycles that could be put to use. That’s all I’ll say on that for now.

I’m also starting work on the next project. It’s not super-secret. I just want to have gotten a little further before I show it and why not have a little surprise. I’m thinking it’ll be ready to show in a few weeks. Now that Kickstarter is over, I’ve got some time on my hands.

Rockit First Case Prototype

I got the first case prototype in today. It’s definitely a work in progress, but the general idea is there. Click through for a big picture and a discussion of the changes I need to make before I call it done.

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Who’s Soldering on a Friday Night?

The majority of the kitted synths shipped this week, not all, but most. The first one was built this week by someone other than me. I’m glad to see that the build was successful. I love the smell of validation. At the very least, it means that everything is coming out correct. And I couldn’t be happier than that. Built synths start to ship next week. Tomorrow, I’m having a little solder party with some friends to hopefully get close to finishing the Kickstarter orders entirely. I’ve got about 10 finished, and another 10 in some state of finish. Tomorrow, we’ll be taking down a good portion of the last 10.

If you hadn’t read in the forum, the pcb house neglected to cut slots for the DC power jack. This means you’ll have to basically flush cut the tabs and solder it on. I haven’t noticed any mechanical problems with that, but you’ll want to have it close to the board when the case comes together.

On that note, I’ve finished the case design and will be getting samples next week. I’m waiting for my wife to finish jazzing up the engraving to order it. It’s going to be made from black acrylic. I think it’ll be pretty sweet

Case Decision Time


Rockit Face

After a bit of feedback, I’m gonna call it. I’m going to pursue the Sammich/Shruthi style of case. It looks decent and won’t be hard for me to manage. I’m working out the CAD files presently and I’ll cost it out pretty soon.


Rockit Ready for Launch!

Rockit #001

Alright! The PCBs have arrived. I built Rockit #1 and it worked the first time I powered it up. It just doesn’t get any better than that! I apologize for the blog silence for most of September but it was a hectic month: bachelor party, trip out of town, wedding, birthday party… It was stressful trying to fit in work and waiting for the boards to come in, but it’s all over now. The wave of relief when the first Rockit worked was awesome. I’m getting ready with shipping materials (not sure why none of that occurred to me before now!). Look out for a notice from Kickstarter because I’ll need to get people’s address. I don’t want everyone to get too excited to fast because I’ve got a lot of work to do to get it to you. I’m going to mix in all three versions into my schedule, shipping some bare bones, some kits, and some assembled units as I go. It’s going to take some time though, since there’s just one of me. I’m enlisting help and if you’re in the Chicago area and feel like helping, hit me up. I’ll get the beer and pizza. The software for version 1.0 is pretty much a wrap. There will be future versions with upgrades and updates, but like my friend says, “At some point in a design, somebody’s gotta shoot the engineer.” Well, that time has come. Time to put on my manufacturing hat and let these beasts out in the world. Cheers again and thanks for all the support.

Rockit Progress

The journey continues. I’ve put in some good time with the firmware this week and developed some good stuff. I’ve been tweaking wavetable playback to make some dynamic sounds. I’ve made similar changes to the LFO waveshapes that have made those pretty interesting. I’ve tightened up the filter envelope timing and found a handful of little bugs. This work will continue. More waveshape stuff. Tweaks to the pitch shifting. There have been requests for portamento which is just a subcategory of pitch shifting. I’ll try to squeeze it in.

The PCBs are on order. I’m ordering parts pretty soon.

Here’s a little gem I made to tide you over. I’ve got only oscillator 2 on. The LFO is working the Detune, so it’s swinging the pitch. This is one of the new LFO waveshapes. Then, I’m messing with knobs while its running. The bandpass filter is on and it’s perfect for this sorta thing.