Tag Archives: eegsynth

Timing and jitter in DMX512 signals

My previous post on building an Art-Net to DMX interface using an ESP8266 seems to be getting a lot of attention. However, from the comments it is clear that a lot of people that build it themselves have difficulties to get it to work, or don’t get it to work at all. This post investigates this in more detail.

We have not been using these interfaces in our performances for quite some time, and started wondering whether there is something wrong my firmware. My implementation goes back to April 2017. Over the course of time there have been some updates to my code. Furthermore, the Arduino IDE has been updated, as well as the ESP8266 core for Arduino.

Recently I received all three interfaces back that I had built for my 1+1=3 collaborators and decided to update the firmware and to test them. One of them did not work at all due to a broken connection between the power supply and the Wemos D1 mini; two of them started just fine. After fixing the broken wire and updating the firmware on all three of them; they started up just fine, showing the green light (indicating a connection to the WiFi network) and on the monitor page of the web interface I cold see that Art-Net packets were being received. However, with my DMX controlled light it did not work at all.

Testing and initial diagnosis

Using an Enttec Open DMX interface and the very nice JV Lightning DmxControl software (which supports both Art-Net and the Enttec Open DMX), I set out to debug the issue. Since DMX is all about timing, I connected my DS203 mini oscilloscope to pin 2 and 3 of the DMX connector.

I found detailed schematic information about the timing of the DMX protocol on this page. Searching for oscilloscope images of DMX signals, I also found this page with information.

Comparing the output voltage with the DMX512 schematics, it became clear that something was wrong in the signal. To make it easier to see the full signal on the oscilloscope, I configured only three DMX output channels, all set to zero. The oscilloscope shows 5 similar blocks; changing the value for DMX channel 1, I see that the 3rd block changes – that is apparently the first channel. Prior to that should be a “start code” with value 0, so the last 4 blocks make sense. But the first block is too short; there is also a very short pulse all the way at the start which does not match the specification.

Output voltage with the initial firmware:

By connecting the Enttec Open DMX to my DMX controlled light, I could confirm that the combination works and that the DMX interface to my light is not broken. Connecting the Enttec Open DMX to the oscilloscope, I see that it has a much longer 1st block which is the “space for break” (labeled 1 according to this), and slightly different 2nd block which is the slot with the “start code”. Following is a whole series of blocks/slots corresponding to the DMX channels. In this case I cannot limit the number of DMX channels, so the series contains a full universe of 512 channels, each of them set to zero. Changing the values of the first few channels in Lighting DmxControl, I can see the corresponding change in the signal on the oscilloscope.

Since a static photo of the oscilloscope screen does not show that there is quite some jitter, especially in the first part of the signal, I uploaded a short video recording of the Enttec Open DXM signal.

Output voltage of the Enttec Open DMX:

As the initial “break” signal was apparently not correctly implemented in my firmware, I changed the code to use the low-level implementation of the break that was contributed here. Using 3 DMX channels again, this results in a good DMX signal.

Output voltage with the firmware, using the low-level break:

I investigated the problem with the original code, which consists of switching to a slower baud rate, sending a single byte as the break signal, and switching back to the original baud rate of 250000. Given that the “break” appears just as long as the other bytes, the switching of the serial port speed apparently fails. On the Arduino forum I found a post that suggested to flush the serial port prior to and after changing the speed; I implemented this and now the break signal looks fine again.

I think that the reason that it broke is due to updates in the Arduino version that I am using, and/or updates to the ESP8266 Arduino core. I guess I must have been lucky with my buggy initial implementation; this may explain why it failed for some people and worked for others.

Output voltage with the firmware, using the slow serial break:

DMX512 universe size

Connecting my Art-Net to DMX512 adapter with updated firmware to the DMX controlled light initially did not work; increasing the size of the DMX universe that is is passed on by my Art-Net to DMX512 adapter solved this. After some trial and error it turned out that – although I had configured the DMX light for three channels (R, G, B) – it would not work if it would receive less than 8 channels. I suppose this is because the light has a 3, 4, and 8-channel DMX mode, and that 8 channels is apparently the minimum for DMX input to be properly handled.

Comparing the timing of the three systems

Looking back at the videos, I see that the timing of the Enttec Open DXM controller is the worst, especially in the space for break and the mark after break (MAB). This is actually a known characteristic of the Open DMX, since timing is controlled by the serial (USB) port of the computer rather than by a dedicated microprocessor; that is also the reason why it is not recommended for serious applications.

My own EXP8266 firmware with the low-level code for the break shows less jitter for the break. Using the “slow serial byte” the jitter is totally gone for the break itself, but there is still some jitter in the duration of the MAB.

Regardless of the jitter, all three are working fine with my DMX controlled light! Having seen the timing of the signals in detail, I am also more confident that they should work with other DMX equipment. One thing I noticed however and don’t have a solution for right now, is that the differential voltage of the Max485 modules that I am using is considerably lower at ~3.5V than that of the Enttec at nearly 8V.


To summarize, I was glad to have the Enttec Open DMX USB adapter for testing, and especially for having the DS203. If you are doing electronics like this, I can really recommend to get an oscilloscope!

Besides testing with the Lightning DmxController software, I was also able to confirm that it all works smoothly again with our EEGsynth, using a patch consisting of the redis, inputcontrol and outputartnet modules.

Touch-proof enclosure for OpenBCI

The OpenBCI Cyton and Ganglion boards are open hardware and maker-friendly biosensing systems. Although there are alternatives, such as Bitalino and OpenEEG and some companies and/or projects are currently working on new hardware (see e.g. EEG.io), the OpenBCI boards are in my opinion at this moment still the best.

The maker-friendlyness of the OpenBCI boards is somehow also a disadvantage: the OpenBCI systems come as bare PCB boards with a Dupont-style header. OpenBCI (the company) focusses on using it in combination with dry electrodes mounted in a 3D printed headset. I personally don’t value dry electrodes that much; I don’t see the problem with a little bit of gel in the participants hair, and I don’t like the pressure needed on dry electrodes to provide a decent signal. Electrodes with gel or Ten20 paste usually provide better and more robust signal quality. However, it depends on the situation: dry (or saline, like the Emotiv Epoc) electrodes are great if you quickly want to swap the EEG system from one participant to the other.

For the 1+1=3 performances using the EEGsynth setup, we not only use EEG recorded from the scalp, but also EMG recorded from muscle and ECG recordings from the heart. The standard in research and clinical applications is to use touch-proof connectors, technically known as DIN 42802 connectors. These are available in many versions, such as cup electrodes for EEG and snap electrodes for EGC and EMG.

The Dupont-style headers are ubiquitous in the Arduino scene, therefore I previously designed an 8-channel head-mounted system based on a sweat band with the amplifier mounted at the back. It is comfortable and works quite well during performances, but it is still a bit fragile, especially when replacing the battery (see below). Furthermore, after prolonged use the gold-plating of the electrodes wears off, and replacing the electrodes is a hassle. The advantage of touch-proof connector is that it is much easier to switch between different types (cup versus stick-on) and to replace worn-out electrodes. I guess this is also one of the motivations for OpenBCI also selling a Touch Proof Electrode Adapter. Connecting the adapter to the correct pins of the 11×2 header is not trivial, and results in a relatively fragile and bulky setup, i.e. not ideal in demonstrations/performances where I want stuff to be robust.

Another issue that I have with the OpenBCI boards is that they use a two-pin JST connector to connect the LiPo battery to the board. These JST connectors are not designed for frequent connect/disconnect cycles. To disconnect the battery for recharging, you have to pull the cable and I have accidentally pulled off the header from the cable more than once…

Based on these experiences I decided to make an enclosure for the OpenBCI boards that is robust in performance/demonstration settings, that uses touch-proof connectors so that it can be used with EEG/EMG/ECG equally well, that is compatible both with the Cyton and Ganglion, and that includes an easy to charge LiPo battery.

The 8-channel Cyton board exposes a lot of the flexibility of the ADS1299 analog frontend like common reference versus bipolar, and normal ground versus active bias, but I typically use it with a common reference and the normal ground. Consequently it needs 10 connectors (8x active, REF, GND). The Ganglion board has 4 channels and can be configured with jumpers for either unipolar and bipolar reference schemes. It hence needs 6 (4x active + common REF + GND) electrode connectors, or 9 (4x active + 4x bipolar REF + GND) electrode connectors. An enclosure design with 10 connectors (4x active, 4x bipolar REF, 1x common REF and 1x GND) therefore supports both reference schemes for the Ganglion.

The external dimensions of the enclosure are 100x100x30 mm. The height is needed for the 10 connectors, but also has the advantage that it should be possible to mount a WiFi shield on top of the board.

The internals of the enclosure are shown here. At the top you see a 850 mAh LiPo battery, connected to a LiPo charger/protector module with micro-USB connector. The on/off switch is this one and the LED is 5 mm diameter. I used a RGB LED, since that was the only that I had available, but I am only using a single color (green) connected through 470 Ohm resistor to the on/off switch. Both the OpenBCI board inside and the lid are secured with 2.5 mm screws. I purchased the touch-proof connectors from Medcat; these are actually the most expensive component of the enclosure.

Here you can see it with the OpenBCI board mounted, but still without the leads between the OpenBCI header and the touch-proof connectors.

The 3D design for the enclosure can be downloaded in STL format or as Fusion 360 project from ThingiVerse.

Motion capture system

For the EEGsynth project I have developed a full-body 8-channel motion capture system. It is based on the MPU9250 9-DOF inertial motion unit, which contains a three-axis accelerometer, gyroscope and magnetometer. I have combined this with the Madgwick AHRS algorithm, which takes the raw sensor data and computes the yaw, pitch and roll.

The design is based on one battery operated main unit that is worn for example in a Fanny pack around the waist, and up to 8 sensors that are attached to the arms, legs, etc.

The main unit contains a Wemos D1 mini, which is based on the ESP8266 module. It uses the TCA9548 I2C multiplexer to connect a maximum of 8 MPU9250 sensors.

The data from the IMU sensors is streamed using the Open Sound Control (OSC) format. The sampling rate that can be achieved with one sensor is around 200 Hz, the sampling rate for 8 sensors is around 60 Hz.

For initial configuration of the WiFi network it uses WiFiManager. After connecting to my local WiFi network, it has a web-server through which the configuration can be set, which includes the number of sensors and the destination host and port for the OSC UDP packets.

For the IMUs I am using MPU9250 modules that I purchased on Ebay for about 3 USD each.The MPU9250 units fit very nicely in a Hammond 1551MINI enclosure.

I designed the enclosure for the main unit in Fusion360 and printed it on my Prusa I3 MK3 3-D printer. I made two motion capture systems so far, one with black and one with white PLA filament.

The Arduino sketch and more technical documentation can be found here on GitHub.

Art-Net to DMX512 with ESP8266

Update 6 April 2019 – I wrote a follow up post on the timing and jitter in DMX512 signals and fixed a bug in the firmware.

Update 26 May 2017 – added photo’s of second exemplar and screen shots of web interface for OTA.

Professional stage and theatre lighting fixtures are mainly controlled over DMX512. To allow a convenient interface between the EEGsynth and this type of professional lighting systems, I built an Artnet-to-DMX512 converter. It quite closely follows the design of my Artnet-to-Neopixel LED strip module.

Let me first show the finished product. It has a 5 pin XLR connector, a 2.1 mm power connector, and a multi-color status LED:

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ESP-8266 Art-Net NeoPixel module

As explained in a previous post, for the EEGsynth we want to use a neopixel array that can be controlled wirelessly using the DMX512 protocol. I purchased a number of Adafruit neopixel rings with 12, 16 and 24 elements respectively. Each RGBW pixel contains a red, green, blue and white LED. For the 24-pixel ring that means that there are in total 4*24=96 LEDs of which the intensity can be set.

The ESP-8266 module is a versatile WiFi module that comes in many versions. During development I especially like the NodeMCU version, which mounts the ESP-12 module on a development board with USB connection, and the even smaller Wemos D1 mini board. The Wemos D1 mini is hardly more expensive on Ebay than the simpler bare-bone ESP-8266 modules.

The hardware connection is simple: I connected Vcc and GND directly to the Wemos D1 mini board, and connected pin D2 to the data-in of the first pixel. Although the Neopixels are specified for 5V, in my experience the Adafruit rings also work fine at 3.3V, both for power and for the serial control signal. Each LED can take up to 20 mA when fully bright, which means that all LEDs of the 24-pixel RGBW ring can take up to 24*4*20 = 1920 mA, or close to 2 A. However, not all LEDs will be at full intensity at the same time, and driving them with 3.3V rather than 5V further reduces the current. I encountered no issues powering them over the USB port of my MacBook.

For the EEGsynth we want to map a small number of control signals to aesthetically pleasing light effects. E.g. it can control the hue, the frequency with which the array flashes, or the speed with which a bright bar rotates along the ring.

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Scalable lighting systems

The X-mass holiday is always a nice time of the year to spend studying and tinkering on electronics projects. In the EEGsynth project we have identified that it would be cool to control light with brain and body signals, besides controlling modular synthesizers which we have focussed on so far. As it is not yet clear what kind of light and what kind of control will conceptually and aesthetically work well on the EEGsynth control signals, I have been studying both small and large lighting systems. We might for example want to use small and wearable lights on a performer, or control the stage light, or use a LED strip as indicator of the EEG-extracted control signals.

In theatrical and stage performance lighting there is a clearly dominant standard: DMX512. For lighting setups there are many fixtures (i.e. lamps rigged on ceiling mounted truss) that can be remotely controlled over DMX512, not only on-off, but they can be dimmed, the color can be changed, spotlights can be moved, etc. If you look on for example on Thomann, you’ll see that many light fixtures support DMX.

The Disco Biscuits – City Bisco – 10/5/12 – The Mann Center for the Performing Arts – Philadelphia, PA – Photo © Dave Vann 2012

Going to the smallest systems, I considered individual LEDs. Neopixels are a very interesting type of RGB LEDs, which combine a red, green and blue (and sometimes white) LED in a single few-mm small housing together with a controller chip. The controller chip allows the individual LED intensities of the neopixels to be addressed over a serial controller by a microcontroller such as an Arduino. Furthermore, multiple Neopixels can be daisy-chained, where each pixel in the array can be addressed. LED strips consisting of 30, 60 or even 144 pixels per meter can be purchased per meter, for example on Ebay.

Adafruit NeoPixel Ring with 16 x 5050 RGB LEDs with integrated drivers

For the the EEGsynth it is desirable to have a single control module that provides a uniform interface between ExG control signals and light control. An individual neopixel can be considered as an RGB lamp, just like a theatrical stage light. The intensity of the red, green and blue can be controlled, just like the DMX channels of a stage light. Controlling a small LED jewel worn by the performer should not be different than controlling the light of the stage on which the performer acts.

An important difference in the requirements for fixed stage lighting and a small wearable LED jewel is that the first must hook up to existing DMX512 cabling systems, whereas the second should be wireless. This is where Art-Net and the ESP-8266 come in. Art-Net is a protocol for sending the DMX control protocol over a network. The ESP-8266 is a small and low-cost microcontroller combined with a WiFi chip that is compatible with Arduino.

Further details on the hardware and firmware design for the actual light controller modules will come in a series of follow-up posts.

Raspberry Pi as Eurorack synthesizer module

Processing realtime EEG data from the OpenBCI system requires software running on a computer. For the EEGSynth project we do the rapid application development using the platforms that we are most familiar with, i.e. standard laptops and the FieldTrip toolbox, which is based on MATLAB. However, in the end we want to implement as much as possible using affordable and open hardware and software. Hence we opted for the Raspberry Pi, a credit card–sized single-board computer. It runs Linux, which makes it easy to use standard programming platforms and interfaces such as Python and Redis to implement the software stack.

In the first EEGSynth studio performance you can see Stephen in the middle, operating the MATLAB-based GUI for the EMG/EEG processing, and Jean-Louis at the back operating the synthesizer. The goal of the technological development is to put Jean-Louis completely in control and to make the interface of the EEG synthesizer as similar as his other modular synthesizer modules. Hence the need for fitting the Raspberry Pi into a Eurorack synthesizer case.

Here you can see some photo’s from the construction of the front panel.

2015-11-14 15.32.52 2015-11-14 16.30.57

The front plate has holes for the various interface ports to interface with the Raspberry Pi. For a sturdy mount I glued a section of L-profile rails to the front plate.

2015-11-14 21.36.36

After mounting the Raspberry Pi, I connected the HDMI and audio port with a short cable to the front panel.


Here you can see the Raspberry Pi in the Eurorack case, next to the power supply.


Eurorack power supply

I am working on fitting a Raspberry Pi as a Eurorack module in the EEGSynth. A previous post shows the completed case. Besides the Raspberry Pi which needs 5V, it will also hold a CV/Gate controller and some other modules for interfacing between the digital and analog parts of the synthesizer. The operational amplifiers and some other ICs on those modules require a symmetric positive and negative rails.

As I am not planing any critical parts that require an very stable voltage (such as a VCO) in my enclosure, I decided not to go for an expensive linear power supply, but rather construct one myself on the basis of two 12V switching power supply that I salvaged from some old wall-warts that once served some external 3.5 inch USB hard disks. The AC-DC converters have the 220V side isolated from the 12V side. This allows to connect the positive DC rails of one to the negative DC rails of the other, resulting in +12V and -12V from either converter relative to the common ground.

The Raspberry Pi requires quite a bit of current compared to most synthesizer modules, hence converting the 12V into 5V with a voltage regulator such as the L7805 would not be very efficient. Therefore I also added a 5V 2A AC-DC converter.



For safety I made a small plexiglass enclosure using the laser cutter at the Techlab Nijmegen. All electronics, except for the connector, switch and three LEDs are completely enclosed behind the aluminum front panel.


The connector with the synthesizer modules consists of a flat cable, wired according to the A-100 system bus.

The LEDs show the status for each of the three voltage levels. Surprising is to see that it takes a good 10 seconds for the capacitors of the +12V and -12V to completely drain when switching the power off.