Tag Archives: artnet

Timing and jitter in DMX512 signals

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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:

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Art-Net to DMX512 with ESP8266

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Update 1 August 2019 – added the connectors to the list of components.

Update 4 July 2019 – You may also want to check out this instructable, which describes a more sophisticated ESP8266-based solution.

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.

Update 3 Sept 2022 – the code has moved to its own esp8266_artnet_dmx512 repository to improve the timing and jitter with I2S (following this comment).

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

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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

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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.