Eurorack loves Lasers!

While we all wait for the completion of the Eye Of The Beholder or "the most creative eurorack-to-laser interface on the market that is also remarkably fully analog", I'd like to explain to newbies some basics of this wonderful world.

Use your laser the good old way!

Basically, lasers projectors are composed by: galvo scanners and laser diodes. The galvo scanners are rotating mirrors capable of directing the laser beam between a “minimum” and a “maximum” position along both the X and Y axes, generating a rectangular projection plane, the surface you are allowed to draw on. Of course, if you move this plane closer to the laser source or farther from it, you will shrink or enlarge this rectangle respectively, in this second case probably loosing definition because the laser beam will be larger too.

In a laser projector, the emitting diodes can either be a single one with few discrete levels of intensity, or three (red, green, blue) with continuous adjustment from black to full color brightness. Needless to say RGB is our preferred case.

By the way, have you ever seen a laser projector before? The following one is a very common Laserworld device.

On the rear panel you can spot a couple of old DB-25 connectors labeled ILDA (International Laser Display Association). We really want to use the ILDA input on this laser to receive our fully analog signals and control both the galvos and the RGB colors.

Eurorack compatibility

Now that we’ve covered the basics, now the question is: how can I drive lasers with my eurorack system? Luckily, the ILDA standard is fairly compatible with the eurorack one so, you can literally connect the two using a DIY adapter. I did it and it works quite well, adding an extra touch of indie to my performances (using the name "Backwash").

Do I need a specific interface?

Well, let me be concise, here's a summary:

• Lasers don't like high frequencies. Mainly because of the galvo structure itself, so you need to filter out those nasty frequencies. I won't go into transfer functions and other engineering stuff here, let's keep it simple and say you need to heavily reduce everything above 2~3.5 kHz. In my experience however there's a margin - up to 5 kHz MAYBE your galvos won’t explode, but try at your own risk! A specific interface should include a low pass filter at least.
• You may want to have stable signals (or long cables). ILDA standard optionally require a "differential signal couple" for each voltage sent to laser projector, this is for stability reasons and your DIY tuna-box adapter could be cool, but maybe doesn't have this kind of delicacy.
• On-board utilities. For example X/Y inverters are very handy, especially when your laser projector is hooked 10m up on the ceiling and you need to flip the invert switch on its rear panel. Onboard attenuverters are also really useful to save a couple of HP in your rack.

To Be Continued...