Why do Digital Numbers Blink in Videos?


I have seen several videos and I myself filmed couple near the gas station prices and wonder why this happens?

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The camera is recording at some framerate like 60hz

The screen is updating at some framerate… like 60hz

This may misalign, causing the camera to occasionally capture a frame while the other screen is updating frames.

You’ll see a similar effect in any digital display.

If the display in question is something like a flat panel display or looks like a giant television, that’s because the screen’s refresh rate is different from the camera’s recording speed. This can manifest in the form of black bars traveling up or down the display, or other visual artifacts.

If you’re talking about a screen that uses LEDs in a pattern, like a standard LED numeric display, that’s because the circuitry driving them actually pulses the current to the LEDs. This lets them be driven to be apparently brighter while not burning them out or requiring more expensive LEDs that can handle higher power or work continuously at greater brightness.

Normally our eyes don’t see the flicker, but a camera can capture the effect, again usually because of capture speed being different than the update frequency of the pulsed LEDs.

You’ll see a similar effect in some car headlights and many other devices that use LEDs, most especially with LEDs whose brightness can be varied- that’s done by changing the pulse width to make the LED be “on” more or less, changing its apparent brightness to the human eye.

Because they do blink in general, but fast enough so that your eyes don’t notice it. The blink is due to the alternative current running through it. This current oscillates at the electrical network frequency (60Hz in France, i think 50Hz elsewhere), and whenever its value is far from 0 (be it in the positive or negative) is when the LED it powers shine the brighest (hence it happens 60*2 =120 times per second).

Now, i’m not 100% sure about this paragraph because it involves eye vision, but it seems to me, from the bits of studies i had involving that precise thing, what you see with your eyes, at every moment, is an average of all the lighting that came into your eye in the last 1/24th second (Which would be why if you shake your hand in front of your eyes, you see it at several positions at the same time).

So if you take a signal oscillating at 120Hz and take, at a given instant, its average value during the last 1/24th second, you’re gonna obtain sensibly the same value, no matter what instant you consider. Because 1/24th second is enough to contain several periods of a signal oscillating at 120Hz (because said signal has a period of 1/120th second). Hence when you take several periods, the moments when the signal is near 0, and the moments where it is far from 0, compensate each others, and the average value you get is sensibly always the same.

Which is why, with your bare eyes, the lighting seems constant and non-flickering. If it was powered with a current of lower frequency, you’d maybe see it flicker with your bare eyes.

When filming it with a camera, the camera proceeds differently. Say it films at 60 images per second. Every 1/60th second, it will take a photo. But to take that photo, it won’t expose its objective to light for that whole 1/60th second. Maybe less. Maybe it will expose itself for only half of it (1/120th second) and be shut during the other half. That time is called exposure time.

Well, this way, the camera might have exposure times that are small enough that they do not contain several periods of the 120Hz electrical signal. Take an exposure time of 1/240th second. It’s half of a period of that 120Hz electrical signal. That exposure time is too short to contain a whole period. So maybe sometimes that will correspond to a moment of the period where the electrical signal is around 0, and sometimes that will correspond to a moment of the period where that same signal is far from 0. In the first case, the LED will not be shining a lot on the image, and in the second case, it will. Hence, you will have frames where the LED will be shining bright, and others where it won’t. Hence the flickering on video.

Digital displays such as LED, VFD and LCD are scanned. The numbers and letters are not on all the time. The computer supervising the display is turning on the LEDs or whatnot at a scan rate faster than the human eye can perceive. This allows the pixels in a display to be a matrix rather than 1:1 controlled. This allows you to control more LEDs than you have control lines for. The refresh rate for this is around 20Hz.

You camera, if it is digital, is also scanning pixels in a similar manner.

Because the display you are looking at and the camera you are using are not synchronized to each other, the LEDs appear to blink from time to time. This is because the camera scans at about 3 times the scan rate of the LEDs. This means that sometimes the camera will be scanning an LED when it is off, but should be on. To your eye the LED stays on all the time. But the camera sees when it is off as well.