Boss TU-12 Chromatic Tuner Digital Processing

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The TU-12 Chromatic Tuner, on the market since 1983, features digital processing, needle and LED display, a built-in microphone and a tone display with 8 LEDs.

The LED display functions are.
Pitch Selection:  A=440, 441, 442, 443, 444 or 445 Hz. 
In the Chromatic scale mode, all the segments of the LED display are in use: C, D, E, F, G, A, B, and a Sharp indicator. Guitar Scale: With the power switch in the 'Guitar' position, the first four LEDs represent E, A, D, G (for Bass guitars) and the fifth and sixth B and E (for 6-string guitars). The meter covers a range of plus or minus 50 cents, or 10 Hz. 
Construction and Operation: The tuner is based on a quartz crystal oscillator running at 3.579545 MHz. Internally, all of the components being mounted on one double sided PCB. The digital processing is done inside a large 72-pin flat pack LSI chip, which uses the crystal as a reference.
(Source: muzines)