[Wenting Zhang] is clearly a fan of old school STN LCD displays, and was wondering how various older portable devices managed to drive monochrome LCDs panels with multiple grey levels. If the display controller supports multiple bits per pixel, it can use various techniques, such as PWM, in order to produce a pseudo-grayscale image. But, what if you have a monochrome-only display controller? With a sufficiently high pixel clock, can you use software on the application side of things to flip those pixels in such a manner as to give a reasonable looking grayscale image?
[Wenting] goes through multiple techniques, showing the resulting image quality in a clear, systematic manner. The first idea is to use a traditional dithering technique. For each pixel, it is set to black if the grey value is below some threshold. The resulting error value, is then propagated to neighbouring pixels. This error diffusion process smears the error out over the whole display, so spatially speaking, on average the pixel values correspond roughly to the original gray values. But, the pixels themselves are still either on or off. This isn’t quite enough. The next idea is to PWM the individual pixels over multiple frames, to approximate different grey levels. But, that gives a worst case effective refresh rate of 8 Hz with a PWM period of 15 frames, at 120 fps, and that flickers. Badly. One way to mitigate that is to switch to PDM (pulse density modulation) which selects different length sequences to give the same duty cycle but at higher frequency, at least for some grey values. Slightly better, but there’s more that can be done. Continue reading “Monochrome LCD Video Hacks Galore!”→
As a general rule, it’s not nice to prank your mother. Moms have a way of exacting subtle revenge, generally in the form of guilt. That’s not to say it might not be worth the effort, especially when the prank is actually wrapped in a nice gesture, like this ever-changing e-paper family photo frame.
The idea the [CNLohr] had was made possible by a new generation of multicolor e-paper displays by Waveshare. The display [Charles] chose was a generous 5.65″ unit with a total of seven colors. A little hacking revealed an eighth color was possible, adding a little more depth to the images. The pictures need a little pre-processing first, of course — dithering to accommodate the limited palette — but look surprisingly good on the display. They have a sort of stylized look, as if they were printed on a textured paper with muted inks.
The prank idea was simple — present [Mrs. Lohr] with a cherished family photo to display, only to find out that it had changed to another photo overnight. The gaslighting attempt required a bit more hacking, including some neat tricks to keep the power consumption very low. It was also a bit of a squeeze to get it into a frame that was slim enough not to arouse suspicion. The video below details some of the challenges involved in this build.
In the end, [Mom] wasn’t tricked, but she still seemed pleased with the final product. These displays seem like they could be a lot of fun — perhaps a version of the very-slow-motion player but for color movies would be doable.
E-paper displays are unusual in that power is only needed during a screen update. Once the display’s contents have been set, no power whatsoever is required to maintain the image. That’s pretty nifty. By making the display driver board communicate wirelessly over near-field communication (NFC) — which also provides a small amount of power — it is possible for this device to be both wireless and without any power source of its own. In a way, the technology required to do this has existed for some time, but the company Waveshare Electronics has recently made easy to use options available for sale. I ordered one of their 2.9 inch battery-less NFC displays to see how it acts.
Running a server completely off solar power seems like it would be a relatively easy thing to do: throw up a couple of panels, tack on a charge controller and a beefy battery, and away you go. But the reality is somewhat different. Most of us hackers are operating on a relatively limited budget and probably don’t have access to the kind of property you need to put out big panels; both pretty crippling limitations. Doing solar on a small-scale is hard, and unless you really plan ahead your setup will probably be knocked out on its first cloudy day.
So when [Kris de Decker] wanted to create a solar-powered version of his site “Low-tech Magazine”, he went all in. Every element of the site and the hardware it runs on was investigated for potential power savings, and luckily for us, the entire process was written up in meticulous detail (non-solar version here). The server still does go down from time to time if the weather is particularly poor, but in general it maintains about 90% uptime in Barcelona, Spain.
The solar side of the equation is fairly simple. There’s a 50 watt photovoltaic panel charging a 12V 7Ah lead-acid battery though a 20A charge controller. With an average of 4 to 6 hours of sunlight a day, the panel generates 300 Wh of electricity in the best case scenario; which needs to be split between charging the battery and running the server itself.
As for the server, [Kris] chose the Olimex Olinuxino A20 Lime 2 in part because of it being open source hardware, but also because it’s very energy-efficient and includes a AXP209 power management chip. Depending on processor load, the Olimex board draws between 1 and 2.5 watts of power, which combined with charging losses and such means the system can run through two days of cloudy weather before giving up the ghost. A second battery might be added in the future to help improve the run time during low-light conditions, but for now its been working pretty well.
Perhaps the most interesting part of the whole project are the lengths to which the website itself was optimized to keep resource utilization as low as possible. Images are compressed using dithering to greatly reduce their file sizes, and the site eschews modern design in favor of a much less processor intensive static layout. There’s even a battery capacity display integrated into the page through some clever use of CSS. Even if you aren’t looking to set up your own sun worshiping website, there are tips here for building efficient web pages that could absolutely be put to use in other projects.
Marketing guys love bigger numbers. Bigger is better, right? After all, Subway called it a “footlong” not an 11-incher. So when it comes to analog to digital (A/D) conversion, more bits are better, right? Well, that depends. It is easy to understand that an A/D will have a low and high measurement and the low will be zero counts and the high will result in the maximum count for the number of bits. That is, an 8-bit device will top out at 255, a 10-bit at 1023, and so on.
The question is: are those bits meaningful? The answer depends on a few factors. Like most components we deal with, our ideal model isn’t reality, but maybe it is close enough.
To be honest, we’ve heard of dithering but that’s the extent of our knowledge on the topic. After looking through [Windell’s] post about using Dithering in Processing we can now say we’ve got a base of knowledge on the topic.
Dithering is used to produce an image out of two colors that our eyes can put together into something meaningful. The history of the algorithms goes back to monochrome displays. But now the hobby electronics we work with for fun have comparable computing power and perhaps it’s time to rediscover these techniques. [Windell’s] project implements the Atkinson dithering algorithm in real-time on your webcam. He’s doing this in Processing, which should make it pretty easy to port for your own purposes.
So why might you want to use dithering in your own projects? Because if it can be used to make very cool milled artwork there must be other undiscovered uses lurking around your workshop.