Archive for Science

Keeping In-Sync

Flicker, flicker, everywhere!

No I’m not referring to the regrettable boy band, nor am I going to even touch the “cloud” as it were. This is about something much more solid, more hardware.

I remember the early days of personal computing, which (for me) was the 1980s when we had the option of a spinach-green or nuclear-orange command-line interface, or, if you were lucky, one of the primitive 256 colour machines, all of which used an electron gun shot at your eye.

Back then, owning a “flicker screen” was essential. Although the monitors were capable of refreshing in sync with mains power (60Hz), sadly the computers of the day were not and you could get all sorts of eyeball-bleeding from looking at the flickering moire-patterned mess. Remember the signs they used to put on computers “Use no longer than 15 minutes”?

Last night, finding myself alone, I sat myself down to watch a movie and have a quiet evening. To watch movies, I use my media pc (an old Acer laptop with Ubuntu + XBMC) and I just love it. Such convenience at never having to fiddle with discs and control my entertainment experience. It was working fine, or so I thought.

As I start to watch the film, an art film with lovely scenery, I notice something amiss. Some nightmare born out of the 80s. Not a flicker, quite. I notice just a subtle distortion. A flickery horizontal line that starts at the bottom of the screen and slowly winds its way up to the top, only to begin again at the bottom. That just doesn’t cut it, does it?

Setting aside my initial fears that the aging laptop simply couldn’t handle high-definition footage as panic-y, I spelunk into the settings to find out a problem.

My 80s experience saved the day: it was the vertical refresh rate being out of sync with the TV’s. This meant that the computer was refreshing the screen at a different rate than the display, causing this horizontal artifact that would progress up the screen. Much like when you used to see a computer monitor filmed on television and see it vertically dance, but more subtle, more digital.

To set it back, I simply turned on the “vertical blank sync” and the problem was solved, smooth watching again.

In a strange coincidence, with refresh rates much on my mind, there was an article posted on Slashdot today concerning the flickering of LED displays. What was neat about it is that it appears some people actually notice the flicker coming off of the latest generation displays – even though the refresh rate was in sync and fast enough for a human not to care about or notice.

The type of “flicker” is a completely unrelated issue than the type we were used to from CRTs.  In order to “dim” an LED, those of you in electronics will know that you don’t simply reduce the power to the LED, you have to flash it very fast, and the frequency of that flashing gives you the apparent brightness.

Unlike lightbulbs or CRT displays, which run on alternating current,  LEDs must run on direct current. This means it is either ON or OFF not anywhere in-between. To achieve a dimming effect, you modulate the DC current in a square wave (on-off-on-off-on-off) and set the frequency (duty cycle) to achieve the desired apparent brightness. This is called “Pulse-Width Modulation” or PWM.

I, personally, have only noticed this in my own electronics projects (using a 555 timer and a LED) or on my jailbroken iPod Touch which allows me to set the brightness lower than factory. I have never noticed this on a flatpanel television or on the purdy display of my Macbook Pro.

None of this is particularly interesting of itself. Quite a dry subject actually :). What made it interesting was the aforementioned Slashdot post. Apparently a small minority can “see” this PWM flicker, and it’s driving them nuts causing eyestrain and nausea and call on the industry to fix this.

At least two comments posted on this article assert “it’s all in your head” and put it in the same category as people who wear tin-foil hats who put microwaves in the same category as gamma radiation.

One person struck a middle ground and perhaps suggested that the 60Hz flickering of the ambient fluorescent lighting is causing an interference pattern with the display refresh or this PWM dimming.

The question is: can you see it flicker? does it bother you? would you pay more money for a monitor that doesn’t do this? does anyone care?

I’m all for no flicker, but I’m damned if I can see it. Get rid of those stupid fluorescent lights first. Those DO bother me.

New Landsat satellite returns first images

Ah! I’ve been waiting for this.

When I was a boy (all boring stories start this way), my Uncle, who was keen on me getting a good scientific education (which was successful) gifted me a variety of books and experimental tools. One of these was a beautiful large-format book of satellite imagery from the Landsat 5 (I think) satellite. I was enthralled. The high resolution (for the time), top down, false-colour images kept my attention for hours as I saw the physical planet in all its awe-inspiring beauty.

The area around Boulder, CO taken 03/18 by LDCM

Just last month, the latest satellite in the series: the Landsat Data Continuity Mission (LDCM) otherwise known as Landsat 8, launched. Today we are treated with the first images while it undergoes a three month test phase to calibrate its instruments.

I can’t wait to see more!

NASA – A Closer Look at LDCM’s First Scene