Is there such a thing as a turntable mat for tuning guitars?

I just had a sudden thought. You know these types of disc that are used for setting the speed to record turntables?

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The idea is that you illuminate it with a light source modulated by the normal mains 50Hz, and when the turntable speed is correct is correct the circular pattern of bars for the particular speed required (eg 45rpm or 33 1/3rpm) becomes stationary.

Conn used to make a device sort of like that called the Strobotuner, where the disk speed was permanently synchronized to the AC mains (or a crystal source in later models), and it was illuminated by a neon light modulated by the signal coming from the instrument. The ones designed for guitar use had 6 circles of bars, one for each string. If you plucked the A string for example, and the guitar was tuned correctly, the "A" circle would be stationary, (or otherwise it would appear to rotate clockwise or anticlockwise if the tuning was too high or two low). I think Neil Young used to have one on stage with a video camera pointed at it so he could check the tuning.

There are plenty of computer applets for printing out turntable strobe mats; I was thinking you could make one with extra circles like the old Conn unit. Instead on a neon you could modulate an LED with the output from your guitar.

The main advantage of this is that guitar signals are notoriously difficult for tuners to lock onto because the overtones are never quite the same frequency as the harmonics. The Conn approach not only avoids that problem, it actually will display the overtones as slightly rotating or counter-rotating "shadows" on the stationary bars.

So you'd basically get the turntable speed correct in the usual manner, and then use the signal-illuminated LED to check the tuning of the guitar.

Has anybody heard of such a device? I can't find anything about it on the Internet.

Peterson strobe tuners have taken over from Conn in the strobe tuning market and come in many different forms. I often use their tuning app on my iPhone for setting up guitars as the resolution on it is about 10 times better than most other tuners.

I need to get a better tuning pedal for my board. I've got the TC PolyTune 2 and TBH, its not very good. If you tune in the poly mode (strum all strings) you get a different result than if you tune in individual string mode. And if you try to just use individual strings, then it often slips into poly mode regardless, which isn't great if you've only got a few seconds between songs.

I have a pedal version of the D'addario Planet Waves strobe tuner. It's incredibly accurate, a little too accurate for a pedal board. Not quick and easy to use like a pedal tuner should be. Great for setups, but too finicky for much else.

I was thinking more of an inexpensive tool for initial setup and adjustment of guitars.
I'm pretty sure I can write the software to print out such a device; I just didn't want to re-invent the wheel if somebody had already done it!

Considering you would need a turntable and some sort of device to convert the guitar's frequency into a flashing LED, I'm not sure what you are suggesting would be inexpensive or practical. Not that I'm saying it's a bad idea, just would need to be streamlined pretty majorly to compete with what's already on the market.

For accuracy you can't rely on the mains frequency being exactly 50Hz (or 60Hz depending on where you live). There's normally a tolerance band around that. In the UK there's a statutory band of 49.5Hz-50.5Hz with operational ones set by the National Grid (the primary power provider) of 49.8Hz-50.2Hz. I think Australia did have a tighter nominal tolerance of 49.9Hz-50.1Hz but this has been relaxed. But even then, sudden changes in load (e.g. TV commercial time when a large number of people put kettles on) or a power station going off-line will cause bigger variations in frequency.

And it now seems that there's less concern about absolute frequency (which will always be fairly close to the nominal) and more about maintaining a tight phase angle between the different mains phases.

A 50.5Hz mains would give A=440Hz as A=444.4Hz, or 49.5Hz mains would give A=440Hz as A=435.6Hz. So you could be quite a long way out.

For accurate tuning of a disk-type strobe, you are better off using a speed controlled motor with a crystal clock plus disk speed feedback for extra stability. Conn strobe tuners used to use a mains-synchronised motor but switched to a crystal clock after they discovered the real-world variability of the mains frequency. If everyone used the same tuner, then it wouldn't matter quite so much, but if some were using battery ones, and others tuning forks, then any frequency drift would be quite noticeable.