Constant Strobe Synced To DMX Output

[Mathieu]

Strobe effects are special beasts in that the timing needs to be very precise in order to be successful. In Lightjams, by default, you may generate a strobe effect using a formula like pulse(0.05, 0.02). This will output 100% for 0.02 second every 0.05 second. So that's a 20 Hz strobe.

But exact timing is a tricky thing. So if you're using this kind of formula, the pulse may be late or not visible at all from time to time.

If you go in the View/Configuration and general tab, you'll see the output rate and the input multiplier. Lightjams evaluates your formulas at a rate of output X input multiplier. So if you output at 40 fps and the input multiplier is 2, your formulas are evaluated 80 times per second. However, only one frame out of two is outputted to DMX. The input multiplier is needed since processing real-time inputs like a person playing the drums need to be faster than the DMX output rate in order to not miss anything.

So the idea is to avoid using any time parameter for the strobe formula. Instead, we'll count the frames. This formula is a good starting point:

counter(0,3,1) < 2

The counter gets incremented at each frame (3rd parameter is 1, meaning it always increment) and goes from 0 to 3. The output is 100% when the counter is less than 2. So we'll get a constant sequence of 100%-100%-0%-0%. 

If you're using an input multiplier of 1, then the formula is even simpler:

counter(0,1,1) == 0

We can add some fun and allow dynamically changing the strobe speed:

counter(0,200 / grid.speed,1) < 2

So the higher the grid speed, the faster the strobe. Feel free replacing the grid.speed by a midi value or any other value.

Have fun!

[Chris Watts]

Hi Mathieu

My output settings are 40fps and an input multiplier of 2. When I used the formula counter(0,3,1) < 2 for power, I got a stuttered output, even when the grid speed was really slow? 

Chris

[Mathieu]

Are you looking at the grid monitor or the real lights? In Lightjams (the monitor and grids), the preview will not be representing the real world. The reason is the screen refresh rate isn't the same as the DMX output rate. 

I've tested the formula with real fixtures and it seems to be good. Let me know if it's not your case.

By the way, this is a case where using the Enttec Open USB causes problems. Since its output rate is so slow (around 20 fps), you can't really generate a strobe with it. In this case, using the fixture built-in strobe effects is the way to go.

[Chris Watts]

Yes that must be it - wasn't using actual fixtures.. just preview grid.

[Robin Larsson]

I really need to test this as soon as I can, have been using a combination of pulse function and built in strobe functions in my fixtures, neither have provided a perfect strobe for many of our fixtures.

Thanks for the tip Mathieu!

[Mathieu]

Just a follow up. A new strobe function has been added in the version 401 - http://www.lightjams.com/history.html. So it's now built-in.