Same question as thread title.
The Master speaks:
It’s the same thing that makes airplane propellers to appear similarly, as well as the flickering band on TV screens.
Movies are shot at 24 frames per second (typically, not counting Maxivision and other uncommon formats). As a wheel (or a propeller) spins up, it will rotate at increasing RPMs. The wheel’s rotation rate will have a certain resonance with the 24 fps shooting rate of film. At some resonances, the wheel will appear to be spinning slowly forward. At RPMs closely resonant with the 24 frames, the wheel will appear to be standing still (if blurrily). Going past that rate will cause the appearance of slow backward rotation.
Imagine a household fan with four blades. Assume its blades are spinning exactly 100 times per second. If you take a picture once per second, the blades will appear to be (actually will be) in the same position in each picture. If you take a picture four times per second, you’ll see the same thing. But if you take a picture eight times per second, every other picture will have the blades at forty-five degree angles, because eight doesn’t go evenly into 100.
Now imagine if you take pictures at a rate just slightly less than four per second. In each picture, the blades will not have completed their entire rotation, and the sequence of images will show them positioned just slightly behind the previous image. If you then look through this sequence, the blades will appear to be moving slowly backward. Similarly, if you take pictures at a rate just slightly more than four per second, the blades will complete a rotation and a bit more, so this sequence will appear to show the blades moving slowly forward.
Likewise, TV screens. NTSC video has a frame rate of 30 per second (approximately; it’s actually 30 point oh something). The flickering black band is the “edge” of the out-of-sync frame being drawn sixty times per second by the raster.
(Along these lines, “24 frame playback” in movie credits refers to a technical team that electronically adjusts the television screens being filmed so they are synchronized with the frame rate of the film moving through the camera.)
Never occurred to me to check the column that this entire message board is based on. Thanks for the quick responses.
By the way, I often see car wheels appear to be turning backwards with my own eyes. It’s at night under the freeway lights, which flash, what, 120 times per second?
While I do get some work done in this office, I’ve also found time to notice that the a computer VDU looks freaky viewed through a desk fan, with strange lines like TV interference - is this the same principle, but reversed?
Why does that sound like a question a toddler would ask?..
Maybe. I’ve noticed that when a TV or video monitor appears on video, you can see the lines from the blanking interval distinctly. The sample rate of the camera and the refresh rate of the monitor are close enough that they interfere. I think I heard that when Hollywood wants to show a video monitor on video or a movie, there’s a special link they can put in between the camera and the screen to synchronize the blanking intervals, so the lines do not appear.
See Cecil’s column.
Possibly you heard it in my post directly above.
Actually, this happens whenever the speed of rotation passes Point Blimfark.
The term for this effect in sampling theory is called ‘aliasing.’ It occurs any time the frequency of a sampled signal is above the Nyquist Frequency, which is half the sampling frequency. So, if I’m digitally sampling music at 44,000 samples / second, I can effectively capture data up to 22kHz, before aliasing occurs. In order to avoid aliasing, filters are employed to cut off the frequency response so the higher frequencies don’t end up being represented as ‘aliased’ signals (i.e, a signal at 23kHz will be represented as 21kHz in the sampled waveform, which is very bad…so you have to cut off at 22kHz to avoid this).
The same thing is happening with the 24 fps sampling of the wheel rotation. - once the wheel goes more than half way around in 1/24 second (so up to 1/12 rotation / sec) the wheel will instead look like it’s moving backwards…etc, etc…
Do I remember correctly that older high fidelity record players used to emply the same concept? I never owned one but didn’t they have a strobe light flash on the platter edge so that you could tell when the speed was the same at which it was recorded and you could slightly increase or decrease it’s play time by making it walk forward or backward? My memory on this, like the LPs, has grown a tad dusty.
You are correct.
Check this out. You can change the speed of the rotation and get an idea.
You haven’t lived until you’ve seen the “spinners” now available to well-heeled vehicle owners. They look like chromed airplane propellors and are the same diameter as the vehicle’s wheels and attach to the chromed wheels. They spin backwards in the wind and even free-wheel when the vehicle motion is stopped.
I thought it bizarre (and cool) when I saw the first set on a white Stupid Ugly Vehicle. Shortly thereafter I had a car serviced at a tire store that also had chromed wheels on display. I asked if they sold the spinners such as I had seen and they said that they did and had sold a set to the owner of a local white SUV, for $3,500.
Well-heeled and well-wheeled!
Re: Television monitors/CRTs on film. As Cervaise points out, monitors operate at a different frame rate than film cameras. There are a couple ways to fix this. The easiest is to use a milliframe controller. As the name implies, it allows the camera operator to adjust the frame rate of the camera in 1/1000 frames per second intervals. (I think this is called a ‘phase button’ in video.) Another way to shoot monitors is to use a 144° shutter at 24 fps. My Éclair will do 145°, which is pretty close. I haven’t tried shooting a TV yet though. (I do have a milliframe controller port on the motor, but I don’t have the device itself.)
I heard of these being sold in the US, from these very boards about a year ago. I saw my first set here in Sydney just last week. They are the most stupid, ugly looking things I’ve ever seen. Made the car look like a toy, and hopefully took about $5k of its market value.
Back on topic, you can also get interesting effects from TVs and computer monitors simply by waving your finger (or better still, a ruler) in front of one. Another place I notice a similar effect is on certain types of bridge railings with closely spaced verticals (1960s bridges had them a lot) when viewed from the side window as you go over the bridge in a car. If there is another, similar set of railings a little behind the first, it’s even freakier.
A science master when I was in high school did a similar thing by pulling the blinds down, turning off the lab lights, and firing a strobe light at a fast-moving stream of drops from a dripping tap. Get the speed of the strobe the same as the frequency of the drips, and you get this string of water beads apparently motionless in the air. Very cool.
I was a music education major in college (not that I finished the course, but that’s neither here nor there), and in my first-year acoustics class the instructor demonstrated how drumheads vibrate with a strobe light. Coolest thing I ever saw…it was like watching a slow-motion film of parts of the drumhead vibrating in waves.