Gecho v2 Prototype and Korg Volca FM

Published on Apr 1, 2019 Gecho Loopsynth

"The v2 prototype is working very well so far, here the MIDI Out functionality is being tested with Korg Volca, also Line-in and Line-out connectors are actively used. All is recorded via USB sound card directly from Gecho and presented in raw form. You can find more info here: http://gechologic.com/new-batch-updat..."

"The final Gecho v2 prototype
Since the previous update about v2 that was posted in August last year, it took 3 more revisions to get here. Although the first prototype of Gecho v2 has been completed, blueprints verified by our skilled hobbyists friends who soldered their own prototypes, and it even passed CE/FCC testing with flying colours, and we could go ahead with the production - it was just not good enough - it needed a small upgrade. And, things tend to turn more complicated than they appear :)

Thanks to everyone for encouraging comments, questions and feedback during the past few months, it has been very helpful and contributed a lot to shaping the new model.

While working on Glo (the whale synth), I came to realize that sticking to STM32 platform looks nice in short-term; in theory, only small modifications are needed to port the firmware over. But there was a risk that when implementing the new features you are waiting for, like MIDI and sync modes, it would be needed to trade off something on the sound side (e.g. level of polyphony) to get the extra power for the additional processing required.

The ESP32 platform allows for a very elegant solution: it is a dual-core processor, and has more power than the one we used previously. Here, one of the cores can run sound engine while the other one handles sensors, LEDs and timings. And even the single core can work at 48kHz with more polyphony than the old CPU at 22.05kHz.

In this context, Glo can be viewed as an intermediary step; it already has the new chipset: MCU and Codec that allows for proper line-level inputs and outputs, two differentially wired MEMS analog microphones, better/faster USB driver chip, 8x more Flash, and (with an expansion board, more info about Glo DIY options is here and here) even a micro-SD card interface for expansion of sample memory (however, as it is only using 1-bit mode it is slower than in Gecho).

What's changed
The buttons were moved a bit, so they are not too close together
Gecho is a bit larger but thinner, there is a front panel, electronics is self-contained and wooden box is optional
There is a proper Line-out signal available via one of the HP connector, with a switch to select function, also there are soldering pads for fully differential Line-out
The input that used to be called "piezo/line-in" does need to use analog OpAmp chip as a pre-amp anymore and has wider range in terms of controlling input gain, from passive guitar pickup to Line levels
There is charging circuit on board, that can handle Li-Po and Li-Ion cells
Mini-USB connector has been swapped for Micro-USB, and uses faster driver now. USB-C was just too complex to fit as it has many more signals that need taking care of
Magnetic sensor has been replaced with accelerometer, it does not have full 9 DOF with gyro sensing, only reacts to tilt but is sufficient for what it needs to control
There is still a magnetic sensor somewhere in ESP32 bound to one of the ADC channels but I haven't tested it yet, will leave this to those of you who want to hack it :)
The Flash memory has been upgraded from 1MB to 8MB
There is also more RAM and as the CPU is faster, this allows for 48kHz sampling rate with similar delay echo buffer lengths as in v1
A Micro-SD card slot was added, it is wired via fast 4-bit bus to a SDMMC controller and works at about 2MByte/sec r/w with cheapo SD cards from eBay. It might be useful for expanding samples memory and the throughput should also be sufficient for storing the generated track in real time, but that hasn't been tested yet
Electret/condenser microphones have been replaced with MEMS, and are routed with differential signals, which lowers the noise/interference. They are slightly more sensitive too
All units will have MIDI, there is a switch to select in vs. out over one 3,5mm Jack which is also shared with two Sync and CV signals (both directions). There was not enough space to fit one more connector, and you usually don't need everything at once anyway; however in realms of DIY, there are separate soldering pads for MIDI in and out if you want to expand to all 3 connectors
The standard powering scheme still accepts 3 or 4 AA/AAA batteries (also Ni-Cd, Ni-MH or Ni-Zn rechargeables), while built-in rechargeable cells will be optional & easy to add. Disposable cells still make sense if you mostly use the synth plugged into the 5V USB source, and need them as a backup in case you go outside where there are no sockets
The CV inputs via 3.5mm Jack will safely accept up to 10V, but they can only emit up to 3.3V as that's the power level Gecho is operating on. I don't know yet how precise these outputs are, as the testing hasn't been completed
In sync mode, similarly the output can only go up to 3.3V which is usually sufficient for 5V powered devices too (or those powered with 9V but operating on 5V internally), this is still being tested
The extra soldering pads to override what the IR sensors do are still present, they can be used as 4 more CV voltages (within the 0-3.3V range), but this is very experimental, you would need to modify the firmware
Software-wise, the new Gecho will be able to do everything that previous one did, and since there is a bit more memory and CPU power, it allows to pack in some more advanced sound effects, or the existing effects with better resolution."