If the quality of an idea can be measured by the number of independent inventors that have hit upon it, then the idea of a next generation Optigan® must be pretty good.

A few years ago when I started making discs with Pea, we discussed making a portable Optigan/Orchestron MIDI disc player, sort of like a DJ turntable for Optigan® discs. I got suggestions from other folks in the Optigan® community, and even received a sketch for a proposed cabinet design. Thus began the Optigan® Jr project.

I worked on the project on and off trying to figure out how I wanted to do it. I knew I wanted to drive the disc directly instead of using a pinch roller system. I also wanted to control and modulate the speed precisely, including the ability to use the pitch bend wheel from the controller keyboard. This drove me to the idea of using a stepper motor and sinewave controller like some DIY record turntable enthusiasts have done. I didn’t want the disc to drag on a flocked surface like the Optigan/Orchestron, so I went with supporting the disc on a cushion of air, like an air hockey puck (this idea was suggested by a member of the community).

I’ve been keeping Pea posted on progress, and had recently told him that I was going to start laying out the circuit. Yesterday, Pea told me to check out the Randomness section of the forum. There was a post by Gan about an Optigan/Orchestron disc player he made.

You can see further details on his web site. There are some similarities and some differences when you look at our designs.

1. He cleverly made the disc support out of a clear acrylic, which allows the light to pass through and prevents the disc from sagging or rubbing on a surface. I tried a similar idea using a glass support disc but I had problems keeping the disc clean (dust adds noise).

2. His drive system uses a record turntable, which is a great platform to hack. The Optigan® Jr will be using a stepper motor and motor controller that is connected to the microcontroller. If you’ve wondered what the strobe lines are on the new discs, the intent was to use an optical sensor to servo-lock to a reference. Also there is a start-of-loop mark on the discs, which can be used to synchronize starting and stopping the disc.

3. He made the exciter lamp out of an array of LEDs. This runs cooler than the original Optigan/Orchestron lamp, and it’s easy to source the surface mount LEDs. This something I want to do too.

4. He made a sensor array out of phototransistors. The original Optigan/Orchestron used an array of photodiodes (i.e. solar cells), which was what old sound film projectors used. Phototransistors are preferred because they have more gain (less amplification needed later, so less noise), and they can be sourced in very small packages. I tried to find photodiodes in footprints smaller than 0805 and found myself turning to phototransistors too.

Optigan® Jr Sensor Array

5. He made a collimator out of abs plastic on a CNC. The Optigan/Orchestron required this as well. I fabricated one out of printed circuit board material (it’s actually a break-off board from the photo senor array).

6. His general circuit concept is almost exactly the same as the Optigan® Jr. The signal from the photo transistors is routed through analog switches to summing amplifiers. A microcontroller decodes the MIDI data and controls the switches via shift registers with latches (I’m just using latches, but I like his scheme better).

So, congratulations to Gan on his project!

The good news for the community is that you’ve got at least two of us on the case of making an Optigan/Orchestron disc player (and one is already working!)

It is my hope that in the near future I can produce a sellable product.

-Robert-

Optigan® Jr Prototype