So, it heats the string, lowering the pitch. This would tune a string that\'s been detuned sharp. But how does it tune a string that starts flat, as it seems is the more common out-of-tune condition?

@Scott
I understand from the video the piano is factory tuned mechanically at a string temperature of 35 degrees C
In that case each string must be heated somewhat to be in tune. If a note sounds flat there is about 10 degrees temperature drop to environment temperature to correct it.
I love the technical detail in this article and the open inspired way the inventor shares his work.

Considering he taught himself electronics, and then used an FPGA... Well done! Not a real fan of piano music but I\'m sure it\'ll catch on somewhere. well done Don!

You are very talented, However, This is sort of not needed or not for any professional pianist who plays a Steinway etc. A piano is in tune to concert A, to a human ear not a computer as it will sound sort of out of tune. After the notes are in tune then the difficultly is the relative tuning 4ths 5ths and octaves. I have the Steinway video, and they very clearly state that their pianos are still the best (I agree) due to the fact that everything even tuning is done by hand, Yamaha uses computers and robots to aid in their completion. But still, well done, there is a market for it:)

The tuning you get with the self-tuner is a hand-tuning. A master technician tunes the piano by hand when it is warm and this tuning is \"stored\". No matter what the technician does with inharmonicity, harmonics, intervals, stretching, etc., the fundamental pitch of each string will be at a specific, repeatable frequency. The piano will return to that same tuning every time. If you don\'t like that tuning, it also has a retune feature. You simply flip a switch and the piano warms to a median temperature, have your favorite tech tune it perfectly, then flip the switch back and the new tuning is saved.
Don A. Gilmore Kansas City

Awesome! A simple but brilliant way to keep a piano tuned, and I like the idea of storing your favourite tuning. Well done Don:)

A wonderful invention, Mr. Gilmore!
I have a question that you have surely solved, and I am curious what the solution is. With repeated use of this system, it seems that all of the wooden parts of the piano would be subjected to substantial swings in relative humidity. Doesn\'t this cause the bridges, sound-board and pin-block to develop cracks? Wouldn\'t these changes in relative humidity also affect the regulation of the piano, especially in humid climates?
Also, what happens when the system is on and your cat jumps in the piano? :)

The environment itself poses a much more severe hazard to your piano than the minuscule temperatures of this system. The keys in your pocket are hotter than these strings.
And as for your kitty, he will be happy to learn that only 5 volts are used to warm the strings, which cannot be felt to the touch.
Don A. Gilmore Kansas City

Sitar owners should love this.
Is the tuning unique to each piano ? One would think that if each string were tuned to the desired frequency, a different piano tuned to the same set of string frequencies, would also be "in tune" but still sound different. This could allow portability of the tuning algorithm from one piano to another.
I may be missing something here. Different pianos would require different tension on each string to get the right frequency for each string. And different pianos would sound different with all the strings tuned to the same set of string frequencies, some better than others. Would there be a case where you want to untune or miss tune a piano to get it to sound proper, or at least the way you think it sounds the best ?
Instead of using the 'original professional tuning' as the gold standard for retuning could you use the mathematically perfect frequency for each string as the gold standard. Starting with all the strings tuned sharp it would take many iterations (thousands ?) to get all the strings in tune, but the processor is very fast. The response time for each string to adjust might slow tuning down some, say to five minutes ?, or even ten ? Would you have a good sound when done ?

You can have anything you want. Any tuning can be saved and repeated. That\'s really all it is: a tuning storage and retrieval system.