Intro
FOK-GYEM VISINFORM was a brand of electro-mechanical flip-dot display. Their working principle is simple: An electromagnet forces a small, magnetized "pixel" to flip from the black-painted side to the other, usually white- or yellow-painted side.
A while ago, I managed to get my hands on a few 7x24 pixel modules for which we were able to create a custom controller board with a few friends. One of our first ideas, apart from running Doom on it, was to play Bad Apple!! on it. There were three attempts for this:
Versions
Version 1 was made with the original test/beta driver board with basic firmware, and as such it didn't perform great.
A year later, it was recreated as version 2 (not shared publically) with the new board and improved firmware which resulted in a substantial performance and speed uplift.
Shortly after version two, the (for now) final version 3 was released with even more improvements, the biggest of which was a parallel multi-threaded display server that enabled the use of four driver modules simultaneously, resulting in a further 4x speed increase.
Tech stack
- Arduino Uno R3 - the microcontroller at the heart of the project
- C for the custom driver board's driver software
- C# for the serial connection server
- PlatformIO for development
- FFmpeg for preprocessing the video (resolution and color)
Details
The project used the existing driver board without any modifications but had altered firmware, compared to the stable release on GitHub mainly around the serial communication, to speed up drawing.
The key part of the project was the serial connection server running on a computer: a multi-threaded app that handled all four connected drivers simultaneously. To achieve this, a master thread starts the four display threads to send the current frame and waits until all of them are done (finished sending data and got a "done drawing" message from the screens)
Quality of playback
This multi-threaded approach improves the speed drastically but also made timing the images very important. The theoretical refresh rate of a single driver board is 2500 pixels/sec since the driver boards only update the pixels that were changed, the different modules don't all finish drawing their frames in the same amount of time. Without the multi-threaded wait-for-all approach, the threads could get out of sync, and start developing screen tearing-like symptoms.
The final version was run at 28x24 pixels resolution (4 modules x 7x24) at the original 30 frames/sec speed of Bad Apple!! and only dropped about 60 of the total 6570; mostly towards the end of the clip.