Category Archives: sequencer

Various updates

So a few things have happened in the two months that have passed since my last blog post.

I built a MIDIBox Seq V4. It doesn’t have a proper front panel yet, is just Wilba’s PCB mounted on a piece of acrylic. It is, however, functional and great as it is, so for the moment I’m crossing sequencing off my list. I’ve also got the parts for a MIDIbox FM synth which is polyphonic, so maybe I’ll have my first piece of open hardware music soon.

The sampler is progressing in various ways. A friend of mine is designing a board based on the STM32F4 with a few megabytes of external SRAM. It will be ready, hopefully, within a few months. This should be a good platform for a sampler, and also for effects which need more working memory than microcontrollers have on-chip.

Another interesting development is the port of the MIDIbox software to the STM32F4 Discovery board. This is a cheap development board based on the same Cortex M4F from ST that my friend is using for his board. The chip on this is faster than the LPC1769 used in the regular MIDIbox core, and has a number of instruction set extensions as well as hardware floating point that make it much more suitable for DSP tasks than the LPC1769.

The Discovery board also has an onboard audio rate DAC that the MIDIbox software can address, so it’s already a good platform for making synthesizers if you can tolerate C. As mentioned previously, there’s already a SD card based sample player that could provide a starting point for a proper sampler. This is my summer project. It’s not the variable-clock, 12-bit S900 recreation I really want to see, but it would certainly be useful for certain things, like a PCM drum machine.

Speaking of drum machines, I was pleased to see the Sonic Potions LXR drum machine sell out the first 100 kits in hours. This community is doing great things, and I look forward to seeing the developments.

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First steps: A sequencer

The first part of the project that I’m tackling is the sequencers. I have hardware synths and a sampler, but I’m currently using a Boss drum machine for all my sequencer. It works but it’s a bit limited, and its interface is not very immediate.

The obvious choice for a DIY midi sequencer is something based on the MIDIBox platform. The platform is based around a Core board which includes an ARM processor and the relevant MIDI/USB ins and outs. It allows easy interfacing with custom control surfaces through a number of simple input/output modules, most of which can be daisy-chained. In addition to being a modular hardware system, it’s also a set of software components based on the FreeRTOS realtime operating system that allows easily writing MIDI controllers, processors, and sequencers in C. All source and hardware schematics are available under an open source license, although commerical use is prohibited.

What I’m specifically going to go for to begin with is the MIDIBox SeqV4L sequencer/MIDI looper, which is small and optimised for live usage. It’s basically 48 buttons, 16 corresponding to the steps of the sequencer and 32 to select functions, and 64 LEDs, 16 that show the current step when the sequence is running, and 48 to show which buttons are activated.

There’s a more fully-featured sequencer, which has two displays, rotary encoders, and a lot more buttons, but I prefer the V4L for its more immediate design. It only allows two tracks at a time and requires an external controller to enter MIDI notes, but on the other hand it’s cheap enough that you can just build more as needed. In its current state it’s not ideal for drums/single-shot sounds, but the software is shared with the larger sequencer, which has a nice drum track mode, and a drum mode is forthcoming in the V4L software.

I built my first V4L a few months ago. It was a fairly easy build, but I’m not too happy with the control surface. It uses small, cheap buttons, and I think the panel is a bit cramped. So my first project is to build a bigger front panel. I’ve scrounged some very decent Alps switches from old Apple Extended II keyboards that I’ll use. I’ve made my first experiments with printing caps on a 3D printer at my local hackerspace and early results are encouraging, though for producing larger quantities it might be more feasible to cast them.

The MIDIBox platform also includes a module for a digital-analogue audio converter, and there is an app that allows you to use it as a simple sampler. I’ll get back to this as a sampler is the next project I expect to tackle.