RockIt, part 2


In part 1, I mentioned that the LEDs were turning on and off really slowly, and that I was missing the two side panels for the case. I contacted the designer, and he replied back that he may have set a bit wrong when programming the Arduino EPROM so that it would run on the internal clock rather than from the external 19.66 MHz crystal. After about a week, I received the side panels and a new Arduino. My first thought was that I could desolder the current microcontroller and simply pull it out of the board, but there were two problems with that. First, the pin holes are too narrow and the solder sucker couldn’t get all the solder to set the pin free from the hole. Second, a couple of the pin paths, specifically the ones for the crystal connections, are wide enough to act as heat sinks, so the solder wasn’t melting enough for the sucker to have any effect at all. Instead, I had to resort to cutting the pins off the old chip and pulling them out of the holes one by one with a needle-nose pliers. Definitely sub-optimal, and it still took close to an hour for all 40 pins.


(The Rockit, fully boxed up.)

After replacing the Arduino, I still had the same problem with the kit running too slow. Thinking that the crystal was bad, I decided to first make sure that there wasn’t a hidden short or a broken path by ohming out the board around the crystal. That’s when I found the actual cause – three capacitors, C1, C2 and C3, are in a tight little cluster and the silk screening identifying them is very confusing. The marking for C2 is next to C3, and the marking for C3 is almost half an inch away. C1 and C2 are 22pf, used for balancing the crystal. C3 is 0.1uf and is connected to the Arduino reset pin, for initialization on power up. Since I didn’t have an ohm meter when I bought the Rockit, I had to guess which holes were for C2 and which for C3, and I had a 50% chance of getting them wrong. So, the crystal wasn’t running, and the Arduino wasn’t resetting on power-on. That was easy to fix, and the Rockit is now running more or less correctly. I say “more or less”, because there are so many parts that even if there is a problem with one or two of them, it may not become apparent right away.


(Side panel assembly system.)

As mentioned in part 1, the first version of the kit had a design flaw that prevented MIDI from working correctly with some instruments. The designer included a replacement chip with the kit just in case, but the original chip works fine with the Roland A-300 Pro keyboard. The Rockit has 3 operating modes: normal, drone and loop – selectable with the Drone/Loop button. In normal mode, the box requires a MIDI-capable keyboard to be plugged in, and all of the Rockit controls work as marked on the cover. In drone mode, you can still play from the keyboard, but the box generates its own Gate signal and the ADSR envelop controls get remapped. In loop mode, the keyboard is disabled, and there’s a 16-step loop that’s applied to all of the controls on the box; movement of any control is recorded and played back as part of the loop. The ADSR controls are then remapped to specify the loop rate.


(Close-up of the box assembly hardware. I’m tempted to epoxy the top cover down to the side panels, and then figure out a way to take the cover off as all one unit.)

Even with all of the pots on the box top panel, there are more controllable parameters than controls. There are 2 LFOs (Low Frequency Oscillators) that can be used to vary OSC1 or OSC2 pitch, the LFO cutoff frequency, Resonance, etc. Only 1 LFO is selectable at a time, and each LFO can only be sent to one destination out of 6 available. So, using the box to create the instrument voices is a bit restricted. On the other hand, you can save all of the parameter settings to 1 of 16 memory slots and restore them later, which is useful. But, the best part is that all of the Rockit parameters are accessible via MIDI CC messages, and the A-300 keyboard has 20 programmable sliders and dials. So, the Rockit and the A-300 should be a pretty good match (I need more time to figure out just how good the match is).


(Back panel.)

The Rockit is marketed as being open-source, for both the hardware and software. I’ve asked the designer for instructions on how to update the software, but haven’t gotten a complete reply back yet. He’d mentioned that several people have asked about hacking the software before, but he hadn’t heard back on what, if any, changes had been made to the code. Personally, I’d like to see an Arpeggiator mode added to drone/loop and available via the A-300, and a change to OSC1-OSC2 offset. Right now, OSC2 is tied to OSC1, and the OSC2 pitch is varied in integer note steps. If possible, I’d rather set OSC2 as a percentage of OSC1, which provides a lot more noise for the VCF (voltage-controlled filter) to work with, as well as adding more beat frequencies.

 

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