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Steve E.
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Making a strobe disc pattern "stop"--formula

Post: # 18925Unread post Steve E.
Sat Mar 31, 2012 12:48 pm

Inspired by the awesome "star spinning" animation of our Kris D's "Electric" label:

http://groove-o-matic.com/

I wondered how one determines what frequency one needs to make a pattern stay put on a rotating disc. I figured it out.

[NOTE: see corrections further down the page. These only work for a KAB-style strobe]

If [rpm] = rotations per minute....

the formula is:

**********

In countries where strobes are 60 hz, due to the electrical standard:

[60 (seconds) x 60 (hz) ] ÷ [rpm] = x

x being the number of icons to space evenly around the disc.

**********

In countries where strobes are 50 hz:

60 (seconds) x 50 (hz) ] ÷ [rpm] = x

x being the number of icons to space evenly around the disc.

**********


So, for example....60hz strobe on 33.3333 rpm

3600 / 33.33333 = 108

for 50 hertz,

3000 /33.33333 = 90

Now, interestingly, both of these are divisible by 18, leaving factors of 6 and 5, respectively. If you put 18 evenly-spaced, narrow images on a disc, they would appear to stay in place under EITHER strobe at 33.333 rpm. You would get 10 images per second. They would flicker, and different numbers of repetitions would appear at other spots around the disc. (Our persistence of vision would make it look like 90 images at 50 hertz and 108 images at 60 hz.) I think the flicker would be too extreme to make for an effective animation. I tested this by printing a B&W image of an 18-point star and putting it on my turntable.

These formulas, by the way, are a major factor in the standardization of speeds. In the 60hz US, the 78 rpm standard is actually 78.26, because:


3600/78.26 = ~46

and so you can put 46 images around a disc to test your speed.

(the pattern on that 78 rpm label is 46 * 2 or 92. ah ha.)

78 rpm in 50 hz countries is actually 77.922,

3000/77.922 = ~38.5

hmmmm. Then 38.5 x 2 is 77.

I'll bet they put 77 images around the disc, and the strobe reads every other one.

(The actual standard is probably 77.922077920....etc.)


45 is easy for 60 hz....80 icons. But strange at 50 hz: 66 2/3. You have to put 200 around for an even number. maybe 100 works and just flickers a bit?
Last edited by Steve E. on Mon Apr 09, 2012 11:25 am, edited 1 time in total.

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markrob
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Post: # 18960Unread post markrob
Sun Apr 01, 2012 3:34 pm

Hi Steve,

Nice info. Keep in mind that 78's are really 78.26 RPM which works out nicely for 60hz mains frequency. A typical 1800 RPM motor would be reduced by 23:1.

Also note that the flash/flicker rate of a lamp is actually 2X the mains freq. So at 60 Hz. you are using a strobe of 120Hz. This is true for incandescent as well as fluorescent types.

Mark

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Steve E.
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Post: # 19124Unread post Steve E.
Mon Apr 09, 2012 11:23 am

markrob wrote:
Also note that the flash/flicker rate of a lamp is actually 2X the mains freq. So at 60 Hz. you are using a strobe of 120Hz. This is true for incandescent as well as fluorescent types.

Mark
In that case, my ending numbers all get doubled, and that explains why that 78 rpm record label put twice the images on the label than I found necessary.

I measure using a KAB strobe and disc, and my 60 hz numbers reflected the numbers found on my disc. BUT perhaps that strobe is a special one that flickers at half the standard rate of a lamp.

So! Reflecting this new info:

If [rpm] = rotations per minute....

the formula is:

**********

In countries where strobes are 60 hz, due to the electrical standard:

[60 (seconds) x 120 (hz) ] ÷ [rpm] = x

x being the number of icons to space evenly around the disc.

**********

In countries where strobes are 50 hz:

60 (seconds) x 100 (hz) ] ÷ [rpm] = x

x being the number of icons to space evenly around the disc.

**********

For 60 hz countries:

7200 / 33.33333 = 216

7200 / 45 = 160

7200 / 78.26 = ~92
(78.260869... = 1800/23 = 78 6/23)

for 50 hz countries,

6000 / 33.33333 = 180

6000 / 45 = 133.33 --> (would need 400)

6000 / 77.922077920 = ~77
(77.922... = 6000/77 = 77 71/77)

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Steve E.
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Post: # 19355Unread post Steve E.
Tue Apr 24, 2012 5:54 am

It occurs to me that not everyone knows that the 78 rpm standard is different in 60 hz countries and 50 hz countries. Not the same speed! Read the above post carefully.

And check this list to see if yours is a 60 hz or 50 hz country:

http://www.powerstream.com/cv.htm

andybee
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Post: # 19368Unread post andybee
Wed Apr 25, 2012 3:30 am

yes, that´s right, also the lyrec motor has not the correct speed in some countrys (I don´t know 60 or 50hz), because of the fixed loopys inside....

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