Teensy 4.0 600MHz ARM Cortex M-7 MCU - ideal for digital MCU based theremin?

Posted: 9/8/2021 5:49:00 AM

Joined: 1/27/2021

Hi, Buggins! Please check your private messages. I can't get in touch with you

Posted: 9/15/2021 7:11:37 PM

From: Theremin Motherland

Joined: 3/16/2017

Created teensytheremin organization and theremin project on github.

Will put all schematics and firmware there.

Created github pages site with description of project - for documentation, build instructions, etc.

Posted: 10/16/2021 3:55:23 AM

From: Theremin Motherland

Joined: 3/16/2017

Project pages updated

Published Teensy 4.1 Theremin Main Board KiCAD project and schematics in PDF

Published Teensy 4.1 Theremin Display and Controls Board KiCAD project and schematics in PDF

Previous LT1711 comparator based oscillator is unstable (starts oscillating at high frequency) when hand touches antenna.

New candidate for oscillator is described in this post

TODO: BOM and soldering instructions

Posted: 10/22/2021 1:57:50 PM

Joined: 10/24/2020

Good article on mcu oscillators...

not sure if it will help, but I thought of it when I was reading this...


Posted: 10/22/2021 6:03:43 PM

From: Theremin Motherland

Joined: 3/16/2017

Good article on mcu oscillators...

not sure if it will help, but I thought of it when I was reading this...

It might be useful for FPGA theremin implementation - e.g. for generation of reference frequency in phase shift or pll based sensors.

Not for usual oscillator.

Posted: 10/22/2021 6:16:16 PM

From: Theremin Motherland

Joined: 3/16/2017

Unusual current sensing oscillator: details in this post

LTSpice model file is available on GitHub

SN74LVC2GU04 and other component models can be downloaded here

UPD: adding several output buffers in parallel provides bigger drive current. With 2 additional inverters in parallel with existing one gives 400Vpp on antenna.

LVC2GU04 model is a bit strange. Sometimes, provides empty signals - neither current nor voltage is visible in components connected with U04.
Does someone know where to find good lvc unbuffered inverter model?

Capacitor added to R7/R8 reference voltage sometimes helps.

Posted: 11/10/2021 11:41:19 AM

From: Theremin Motherland

Joined: 3/16/2017

New candidate for digital theremin sensor oscillator. Details in this post.

Posted: 11/30/2021 6:29:05 PM

From: Theremin Motherland

Joined: 3/16/2017

Based on discussions with Eric at Wallin Oscillator thread, I'm starting Teensy 4.1 MCU based Digital Theremin from scratch.

Two ideas:

1) provide descent digital PLL theremin sensors based on cheap small FPGAs.
Gowin GW1N-LV1QN48C6/I5 chip with 1K LUTs is selected as FPGA, in prototype - as $6 Sipeed Lichee Tang Nano FPGA boards.
Analog front end boards will be placed on separate PCBs, for experiments with different types of front ends.
Two FPGAs will be used - one per sensor.
To minimize CPU resources usage for sensor data transfer from FPGAs to Teensy MCU, we will use i2s data channel.
Pitch and Volume sensor values will be packed as left/right channel data for each sample.
FPGA boards will be controlled from MCU using slave SPI interface - as a set of registers.
As well, it may be possible to upload FPGA configuration directly from Teensy using slave SPI interface - if MODE0 pin of GW1N is tied to +3.3V

2) use Android tablet for UI and controls - instead of small LCD, encoders and pots.
Teensy can be connected to android tablet's USB OTG e.g. as serial port device.
Application on Android will work as GUI and control.
Teensy MCU will spend most of CPU time on voice synthesis.

Planning PCB and cabinet.

I think plywood laser cut technology for cabinet fits ok here.
Cabinet dimenions: 400mm width, 74mm depth, ???mm height.
Inductors frame: 32mm polypropylene water pipe, 60mm length, 0.1mm / 0.125mm copper wire winding 35-40mm length.
Antennas: water/gas pipes similar to Etherwave ones.
Mic stand mount on bottom of cabinet - e.g. AtlasIED AD-11B Surface Mount Female Mic Flange 5/8-27 Thread
Legs to allow put cabinet on table.
Minimal size of main PCB - 200x60mm

Front panel connectors:
    Line Out 6.35mm audio jack
    Line In 3.5mm audio jack
    Teensy MicroUSB connector

Rear panel:
    Power In DC barrel jack: 5V 2A
    S/PDIF Optical Out
    S/PDIF Optical In
    Teensy MicroSD card slot
    Expression Pedal 6.35mm audio jack

Posted: 12/5/2021 3:10:33 AM

From: Northern NJ, USA

Joined: 2/17/2012

Your layout is quite nice, I like the rotated use of the same board layout.  You might want to rotate the pitch coil 90 degrees, which will minimize magnetic interaction (not a big deal IMO, but if it is free) and place the sensitive end of the pitch coil nearer to the antenna?

I still don't know what to think about using commercial computing HW & SW as an central integral component here.  Those are some mighty shifting sands!  Many times I've seen it ending up being a mistake (the use of PCs and Windows in particular) so I'm gunshy.  But the economies of scale are hard to resist.

Vadim, I just implemented auto-unmute for ACAL and it seems to work OK.  When the player initiates an ACAL (i.e. set the pitch and volume null points automatically) the volume is muted.  After the ACAL is over, when the player moves their volume hand to a position that produces < -48dB then the volume is unmuted.  For safety, the unmute only happens if the audio was unmuted before ACAL.  Those (like myself) who play with non-traditional volume sense never even notice the action happening, as having arms back for ACAL naturally produces < -48dB post ACAL.  Traditional volume sense is (cover your eyes / ears everyone!) crap.

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