Turntable optical illusion - how does it work?

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I'm referring specifically to the way a row of dots, blobs or whetever on a turntable spinning at 33 1/3 rpm can appear to be stationary. My decks have 4 rows of embossed circles around the edge of the bit you put the record on which are all blurred when it is spinning at any speed other than 33 1/3. When it is playing at this speed, they appear to be stationary.

How does this work?

― hmmm (hmmm), Monday, 19 September 2005 15:27 (twenty years ago)

It's a strobe light

― walter kranz (walterkranz), Monday, 19 September 2005 15:35 (twenty years ago)

yeah, like refresh rates on a TV. if you see a TV screen shown on a TV then the one on TV has dark lines on it. kind of the same thing. and the same thing as spokes on a spinning wheel - at certain speed they may appear to be stationary or spinning backwards.

― gspm (gspm), Monday, 19 September 2005 15:40 (twenty years ago)

It still bends my mind how the edge of the record spins faster than the center, even though both ends are part of the same object.

― Pleasant Plains /// (Pleasant Plains ///), Monday, 19 September 2005 16:09 (twenty years ago)

It's not really a strobe light. It's a normal light, which like all lightbulbs flashes 60 times per second (or 50 in Europe). When the turnable is going at the correct speed the flashing gets in sync with the marks on the turntable making them seem stationary.

― everything, Monday, 19 September 2005 16:20 (twenty years ago)

It doesn't spin faster, the trip it has to make is shorter, which is why it looks like it's faster.

― Tuomas (Tuomas), Monday, 19 September 2005 16:21 (twenty years ago)

(x-post)

― Tuomas (Tuomas), Monday, 19 September 2005 16:21 (twenty years ago)

But, Tuomas, the edge of the record travels a further distance in the same amount of time as the center! Covering more distance = faster, right?

― Pleasant Plains /// (Pleasant Plains ///), Monday, 19 September 2005 16:27 (twenty years ago)

the edge's trip is longer but it does it in the same time that the centre does its trip

crosspost

― RJG (RJG), Monday, 19 September 2005 16:30 (twenty years ago)

Just for the sake of saying so, the circumference of a 45 is 22 inches, and the circumference of a album is 37.7 inches.

― everything, Monday, 19 September 2005 16:32 (twenty years ago)

If I ran sixty yards in a minute and you only ran one yard in the same minute, which one of us would be faster?

― Pleasant Plains /// (Pleasant Plains ///), Monday, 19 September 2005 16:32 (twenty years ago)

ts: linear velocity vs. angular velocity

― petesmith (plsmith), Monday, 19 September 2005 16:36 (twenty years ago)

linear. its so much easier to work with.

― AaronK (AaronK), Monday, 19 September 2005 17:12 (twenty years ago)

juts out of curiosity, has anyone ever found this feature useful (except when perhaps tripping and being glued to the trippy platter, man)?

― stirmonster (stirmonster), Monday, 19 September 2005 17:16 (twenty years ago)

Yes. I use it all the time.

― everything, Monday, 19 September 2005 17:17 (twenty years ago)

http://images.google.ca/images?q=tbn:amBbMlNQcl8J:www.geog.ucsb.edu/~joel/g110_w05/lecture_notes/rotating_motion/uniform_rot.gif

― MindInRewind (Barry Bruner), Monday, 19 September 2005 17:30 (twenty years ago)

It's a normal light, which like all lightbulbs flashes 60 times per second (or 50 in Europe)

I had no idea. I'm staring at the lamp and my mind is being blown 50 times per second right now.

has anyone ever found this feature useful

Very useful when you suspect the speed isn't constant on a faulty turntable.

― blunt (blunt), Monday, 19 September 2005 18:05 (twenty years ago)

...or the acceleration/deceleration via pitching.

― blunt (blunt), Monday, 19 September 2005 18:06 (twenty years ago)

it's cause the records don't actually turn. IT'S ALL AN ILLUSION MAN! THE SOUND IS IN OUR HEADS AND YOUR PICTURE DISK IS TRYING TO CLUE YOU IN!

don't look away from the truth!!!!!

man i need some pie.
m.

― msp (mspa), Monday, 19 September 2005 18:28 (twenty years ago)

By far the coolest non-musical thing about turntables. Hahhaha, when i first got my decks, despite the fact that there was a lag in one of them, I would just stare at it the whole time.

― Tokyo Ghost Stories (Tokyo Ghost Stories), Monday, 19 September 2005 23:41 (twenty years ago)

juts out of curiosity, has anyone ever found this feature useful (except when perhaps tripping and being glued to the trippy platter, man)?

Yes, my turntable is kind of fucked and the motor goes wonky sometimes. If I'm listening to something new, it's nice to be able to tell if it's supposed to go all warbly or if it's the turntables fault. Plus I have to adjust it occasionally to get it to play at the proper speed.

― walter kranz (walterkranz), Tuesday, 20 September 2005 00:14 (twenty years ago)

So who made those optical illusion cards that are meant to be put on a turntable? Man Ray, Duchamp? Someone like that.

― walter kranz (walterkranz), Tuesday, 20 September 2005 00:15 (twenty years ago)

They go stationary at 45, too, or maybe it's a difference row of dots that does.

― Tracer Hand (tracerhand), Tuesday, 20 September 2005 03:55 (twenty years ago)

Aye, 'tis. Most players have 4 rows.

― wombatX (wombatX), Tuesday, 20 September 2005 04:14 (twenty years ago)

Wait, I never thought about that. Why does mine have 4 rows if it only has 2 speeds?

― walter kranz (walterkranz), Tuesday, 20 September 2005 04:38 (twenty years ago)

33rpm 50hz
33rpm 60hz
45rpm 50hz
45rpm 60hz

― wombatX (wombatX), Tuesday, 20 September 2005 04:45 (twenty years ago)

It's because one set of two is for 50 Hz and one is for 60. I think.

Oooh, xpost. Beaten to it.

― sleeve (sleeve), Tuesday, 20 September 2005 04:46 (twenty years ago)

The reason I ask is that my dad has a nice Bang & Olufsen turntable that has equidistant black rectangles dotted around the edge of the platter. Obviously, these blur when it is in motion. When it hits 33 1/3, stationary rectangles appear amongst the blur, in a manner similar to the way in which helicopter blades sometimes appear to be stationary on TV.

I'm keen to press up a record which has a similar effect arising from a series of shapes printed around the edge of the labels in the centre of the disc. Can any of you point me in the right direction regarding the maths I would need to do in order to calculate the size and spacing of the shapes in order to achieve this kind of effect?

― hmmm (hmmm), Tuesday, 20 September 2005 10:01 (twenty years ago)

in a manner similar to the way in which helicopter blades sometimes appear to be stationary on TV.

Which happens for the same reason, of course, with the strobe @ 24 or 30 frames per second.

― Mark (MarkR), Tuesday, 20 September 2005 10:21 (twenty years ago)

I use it all the time when spinning records backwards to hear the secret satanic sayings.

― zaxxon25 (zaxxon25), Tuesday, 20 September 2005 10:56 (twenty years ago)

"Just for the sake of saying so, the circumference of a 45 is 22 inches, and the circumference of a album is 37.7 inches."

Ignoring the question of 12" singles for now, this means that the outside edge of a 7" single, being rotated at 45 RPM is (45 x 22) 990 feet per minute; whereas the outside edge of a 12" LP being played at 33.3 RPM is (37.7 x 33.3) is 1257 feet per minute.

Therefore.... um.... hang on a moment.... when did ILM become I Love Maths?

― Stewart Osborne (Stewart Osborne), Tuesday, 20 September 2005 11:11 (twenty years ago)

11.25 mph and 14.28 mph respectively.

― hmmm (hmmm), Tuesday, 20 September 2005 11:16 (twenty years ago)

That's not particulary fast, is it?

Not for something with a motor, anyway.

― Stewart Osborne (Stewart Osborne), Tuesday, 20 September 2005 11:23 (twenty years ago)

http://www.pen.k12.va.us/Div/Winchester/jhhs/math/humor/comics/trig/needle.jpg

― Mr. Snrub (Mr. Snrub), Tuesday, 20 September 2005 13:46 (twenty years ago)

The look on Calvin's face in the last panel is priceless.

― Mr. Snrub (Mr. Snrub), Tuesday, 20 September 2005 13:47 (twenty years ago)

It's because one set of two is for 50 Hz and one is for 60. I think.

Ah! It all makes sense now.

― walter kranz (walterkranz), Tuesday, 20 September 2005 14:59 (twenty years ago)

http://learning.cc.hccs.edu/Members/cschweitzer/images/scanners.gif

― Pleasant Plains /// (Pleasant Plains ///), Tuesday, 20 September 2005 15:52 (twenty years ago)

a point on the outer edge is indeed travelling at a higher velocity than a point along the inner edge. so indeed it has a higher linear velocity than it does at the center.

― Susan Douglas (Susan Douglas), Tuesday, 20 September 2005 17:49 (twenty years ago)

and i think the arm moving in from a pivot corrects for this. but i'm not sure. maybe the recording is actually adjusted? although that cannot be.

― Susan Douglas (Susan Douglas), Tuesday, 20 September 2005 17:59 (twenty years ago)

oh it is recorded that way. i didn't know.

― Susan Douglas (Susan Douglas), Tuesday, 20 September 2005 18:06 (twenty years ago)

a point on the outer edge is indeed travelling at a higher velocity than a point along the inner edge. so indeed it has a higher linear velocity than it does at the center.

Along with the smaller grooves, this must be one of the reasons that the inner tracks sound worse than the outer tracks.

― walter kranz (walterkranz), Tuesday, 20 September 2005 18:17 (twenty years ago)

so how is the master actually scored?

― Susan Douglas (Susan Douglas), Tuesday, 20 September 2005 18:26 (twenty years ago)

Where does the anti-skating adjustment come in?

― k/l (Ken L), Tuesday, 20 September 2005 18:34 (twenty years ago)

so how is the master actually scored?

Hmm, I don't think it's as difficult an issue as you're imagining. The cutter arm used in mastering will have the same velocity as the arm on your turtable when you play back the record. There shouldn't be any adjustment necessary.

― walter kranz (walterkranz), Tuesday, 20 September 2005 18:39 (twenty years ago)

SKATING IS NOT A CRIME

― The Obligatory Sourpuss (Begs2Differ), Tuesday, 20 September 2005 18:41 (twenty years ago)

ok - god, that IS easy.
xpost-i think its a centripetal force thwarter.

― Susan Douglas (Susan Douglas), Tuesday, 20 September 2005 18:41 (twenty years ago)

BTW, to answer my own question upthread the rotating discs I was thinking of were made by Duchamp and called Rotorelief. They look like the Vertigo records label...

http://hebert.kitp.ucsb.edu/studio/md-rotorelief.jpg

Anyway, that's what I first thought of when I read the thread title.

― walter kranz (walterkranz), Tuesday, 20 September 2005 18:45 (twenty years ago)

Didn't his pal Henri-Pierre Roche, the guy that wrote Jules and Jim, help him out with those?

― k/l (Ken L), Tuesday, 20 September 2005 18:46 (twenty years ago)

I haven't a clue.

― walter kranz (walterkranz), Tuesday, 20 September 2005 18:51 (twenty years ago)

the anti skate helps keep the stylus centralised in the groove i think. apparently the needle has a natural tendency to lean towards the centre of the record and the anti-skate neutralises this by pulling the arm in the opposite direction. if you don't have it set right it can make the inner groove distortion sound worse.

― wombatX (wombatX), Tuesday, 20 September 2005 21:59 (twenty years ago)

two years pass...

informative!

― Jordan, Thursday, 20 December 2007 22:06 (eighteen years ago)

the rotating discs I was thinking of were made by Duchamp and called Rotorelief

has anyone on here made a Gysin style dream machine? I read an article once giving instructions but I don't have a 78 rpm player.

http://en.wikipedia.org/wiki/Dreamachine

― dmr, Thursday, 20 December 2007 22:17 (eighteen years ago)

hmmmm the wiki says u can try it at 45 ...

― dmr, Thursday, 20 December 2007 22:19 (eighteen years ago)

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