Making an FPGA-based Organ Tuner

[johnbeetem]

[I wasn't sure which section would be best for posting this, but I figured people who do their own organ maintenance and repairs would be most interested.]

Last year I scored a Rodgers Trio 321C in very good condition. At first I found the tuning good enough, but either it got worse or I got more sensitive and I decided to get in there and tune up those 158 hand-tuned oscillators.

Rather than buy some sort of tuning device, I decided it would be a lot more fun to build my own using a field-programmable gate array (FPGA). If you haven't heard of FPGAs, they're a wonderful technology for making semi-custom digital ICs at very low cost. Basically, an FPGA is an IC with an array of n-input logic blocks (typically 4-input or 6-input), each with a programmable logic function. The FPGA also has a programmable routing network to connect the logic blocks to each other and to the IC's pins. You run software on a PC that converts high-level source code into a binary bit-stream to program the FPGA, and now you have an IC with whatever function you need (within the limits of the selected FPGA).

I wrote up my project and uploaded the source code at element14.com: https://www.element14.com/community/...ed-organ-tuner

Disclaimer: I posted the project hoping that it would be interesting and useful. However, it is provided "as is" with NO WARRANTY of any kind and if you connect it to an actual organ you must know what you're doing to avoid damage to the organ or to the tuner. The user assumes all risk using this project with actual hardware.

[AllenAnalog]

Quite the impressive project John. You and I have very similar definitions of the word "fun." Who wants to buy an app when you can be creative and learn things instead?

[jbird604]

I dunno. Even though I'm an organ "fixer" by trade, given my druthers, I'd spend my time either playing or else renovating or upgrading my instrument instead of re-inventing the wheel. But I certainly DO understand the thrill of a new challenge, if that's what floats your boat!

However, I find my little phone app called "ClearTune," which cost about $4, does everything imaginable in the area of tuning. Far superior to the StroboConn I bought in 1975 for several hundred dollars. And beats the pants off a stand-alone digital tuner I paid $200 for 15 years ago. It can automatically detect any pitch, show it on a dial plus a detailed cents scale, even numerically if I want it. And can sound any note over a range of about 8 octaves, if I need to tune by pitch-matching. as may be necessary in the top octaves of 2' or 1' stops.

There are even free phone apps for tuning. If you pursue this project, you will need a flawless reference instrument to test your new device, so get a free or low-cost phone app for that purpose if nothing else.

[Admin]

I'm impressed with your skill and ability to design and build your tuner. Also impressed with your project write-up.

[johnbeetem]

I find my little phone app called "ClearTune," which cost about $4, does everything imaginable in the area of tuning... There are even free phone apps for tuning.

Thank you for your comments. As I said in my element14 'blog, I'd heard that smart phone tuner apps existed but I didn't have any specifics.

It certainly makes more sense to use one of those apps than build one's own tuner, but I had a number of reasons. First, of course, for the fun of it (though we all have our own definitions of "fun"). Second, I saw it as a nice size project to test my FPGA design software -- it's by far the largest example I've run through my software and I'm quite pleased with the result, especially since it required so little FPGA logic. Third, it was a terrific learning experience. I got the Trio since I had wanted a large organ for a long time, but didn't have time or space for one. Now I do, and the Trio is a great instrument for my needs. I wanted something that I could repair and maintain myself, which is very much the case with the Trio since it's mostly standard through-hole parts. I am very grateful to members of the Organ Forum (and you in particular) for posting so much useful lore.

My plan is to add VPO capability. Yes, I could set up a PC with Hauptwerk or Grandorgue, and buy multiplexer boards for the Trio itself. But I'd rather have the fun and learning experience of building my own, based on various single-board computers in my collection. My tuner was a nice first step, since the Direct Digital Synthesis technique that generates the reference pitches in my tuner also works nicely to generate notes for a VPO -- just add sample vectors :-)

[jbird604]

I do understand! Amazing blog about this project, though I lack the detailed knowledge of programming and logic circuits to fully grasp the theory. But I can see that it would be fun to design all kinds of things using this flexible system. In fact, I can think of a lot of practical applications for the ability to create custom IC's without having to go through all the steps involved in manufacturing those LSI's that were so common in older organs.

So keep up the good work and keep posting about your results. Best of luck with all of it!