The stroboscopic tuner is possibly one of the most underrated pieces of gear a musician can own. It doesn’t just tune—it tunes accurately, to a fraction of a cent, using optical feedback to show exactly how close you are to a target frequency. And it doesn’t just tune to the note. By design, a strobe tuner puts the idea of tuning offsets in front of you whether you asked for it or not.
As for why you’d want offsets? The reasons range from tracking varying concert pitches, to matching instruments that were never built for equal temperament, to making complex chords sound better on stringed instruments, such as the guitar. If you’ve never worked with micro-offsets before, this is the perfect time to get acquainted with the TU101 Strobe Tuner in AmpStamp 1.10.7.
The Long Road to Standardization
Today every tuner ships preset to A=440 Hz. That’s A4—the A above middle C. On guitar we’re tuning A2, which lands at 110 Hz, but it’s the same idea. The note-octave labels themselves—A4, C4, and so on—came from Scientific Pitch Notation (SPN), proposed in 1939 by acoustician Robert W. Young in the Journal of the Acoustical Society of America.
Getting to 440 Hz took some time, and the first push toward a standard had nothing to do with the music—it was about saving singers. Through the first half of the 1800s, orchestras drifted steadily sharp. Alexander Ellis put it down to two things: concert halls got bigger, and instrument makers competed on brilliance, building horns pitched a little higher than the competition’s so they’d cut through in a large room. Strings tuned up to match. In some opera houses A reached 450 Hz—more than a semitone above the 422 Hz Mozart would have known. Singers paid for it, performing music written for a much lower pitch with no ceiling in sight.
France drew the line. In 1859 the government fixed A at 435 Hz by law—the diapason normal—expressly to halt the climb. The number was a compromise: 450 Hz was wrecking voices, but 422 Hz would have sounded dull to audiences who had grown used to the brilliance. 435 Hz split the difference.
The British Compromise
Britain went its own way, and the reasoning is stranger than it looks.
The physics is real enough—a wind instrument’s pitch rises as a room warms, and by the 1880s that could be calculated. What the British got wrong was the premise. There was a common belief in Britain that the 1859 French commission hadn’t set an absolute frequency at all, but had specified 435 Hz as measured at 59°F, leaving the actual sounding pitch free to climb in a warm hall. That isn’t what the French had done. The Philharmonic Society ran the math anyway, working out what the same instrument would play at a normal room temperature of 68°F:
435 + [(68 − 59) ÷ 1000 × 435] = 438.915
Round it off and you get 439 Hz, which Britain adopted in 1896. A standard derived from a misunderstanding, landing conveniently higher than the French one—which is roughly where British orchestras wanted to be anyway.
The Radio and Electronic Era
By the 1930s, broadcasting needed a single reference. Radio could put an orchestra in London next to one in New York, and any mismatch in pitch was suddenly audible. In May 1939, an international conference in London settled on 440 Hz.
The British Broadcasting Corporation (BBC) then had to generate that tone accurately. Their reference came from a quartz crystal oscillating at one million Hertz, reduced to 1,000 Hz by electronic dividers, multiplied by eleven, then divided by twenty-five—which lands exactly at 440 Hz. You can’t get to 439 Hz that way, it’s a prime number so it won’t fall out of a chain of whole-number steps. That detail gets cited as the reason 440 Hz won out over 439 Hz, and it’s a good story. The generation problem was real—440 Hz falls out of the quartz chain and 439 Hz, being prime, doesn’t. What the record doesn’t show is that this is why the conference chose 440 Hz. That part is inference, repeated so often it now reads as fact. The International Organization for Standardization (ISO) would reaffirm 440 Hz in 1955 and again in 1975, where it survives as ISO 16.
Stretch Tuning and the Stiff String
That was a brief history of concert pitch. The next reason to reach for an offset comes from physics rather than a committee. An ideal string—the kind in a textbook—vibrates in perfect whole-number multiples of its fundamental. Play a note at 100 Hz and its partials sit at 200, 300, 400, and on up, dead on. Real strings don’t do this. A real string has stiffness, and stiffness makes every partial run progressively sharp of where the math says it should be. The nth partial isn’t at n times the fundamental; it’s sharp by a factor of roughly √(1+Bn²), where B is an inharmonicity constant set by the string’s diameter, length, tension, and stiffness. The higher the partial, the further sharp it drifts.
This is called inharmonicity, and on a piano it’s impossible to ignore. If you tune the octaves so the fundamentals sit at a mathematically perfect 2:1, the shared upper partials won’t line up—they beat against each other, and the octave sounds sour and narrow. So piano technicians do something that looks wrong on paper: they stretch the tuning. Treble notes get tuned progressively sharp, bass notes progressively flat, until the partials agree instead of the fundamentals. At the extremes of a concert grand the stretch can reach 30 cents or more. Plot that deviation across all 88 keys and you get the Railsback curve, first measured in 1938 by O.L. Railsback. It’s the fingerprint of a piano tuned to sound right rather than to measure right—and it’s why a strobe tuner built for piano, like Peterson’s AutoStrobe 490ST, ships with dedicated stretched-tuning tables instead of flat equal temperament.
Guitars have inharmonicity too. It’s smaller, but it’s there—strongest in the wound low strings—and it’s part of why a guitar that reads perfectly “in tune” on the meter can still sound slightly off. Which brings us to the instrument this app was built around.
Tuning the Guitar
Equal temperament is itself a compromise. To let you play in any key without retuning, equal temperament spreads a small amount of “wrong” across every interval except the octave. The major third takes the worst of it—an equal-tempered third sits about 14 cents sharp of a pure, just third. Your ear hears those extra cents as a faint restlessness in the chord. On a fretted instrument you can’t fix that note by note the way a violinist does with finger placement; the frets lock every interval in place. So guitarists have always fudged it by ear. The classic case is the open G or open E chord where the low string sounds a hair sharp against everything above it—players learn to tune that string a couple cents flat so the whole chord settles.
Strobe tuners turned that folk knowledge into presets. Peterson’s Sweetened Tunings apply a specific, tiny offset to each string—a few tenths of a cent here, a couple cents there—so the open chords guitarists actually play ring cleaner than dead-flat equal temperament allows. Other systems chase the same goal through hardware: the Buzz Feiten Tuning System shifts the nut position and adds per-string offsets, and True Temperament replaces straight frets with curved ones. All of them are doing what a strobe tuner does—trading mathematical “correctness” for chords that sound in tune. And this is exactly where a strobe tuner earns its keep. A needle or a row of LEDs can get you to the note. Only a strobe display—reading down to a fraction of a cent—lets you tell the difference between “the meter says zero” and “the chord actually rings,” and then hold an intentional offset there on purpose.
The TU101 Strobe Tuner in AmpStamp gives you that resolution for free. You can pull the reference off 440 Hz when a track calls for it, hold small per-string offsets to enhance your open chords, or match an instrument that was never built for equal temperament—and watch the strobe lock in real time instead of chasing a swinging needle. It’s the same tool piano technicians and pedal-steel players have leaned on for decades, now built into AmpStamp.
Hear It for Yourself: A=435Hz
Reading about the diapason normal is one thing. Hearing it is another. Dropping your reference from 440 Hz to 435 Hz takes the whole guitar down about 20 cents, a fifth of a semitone—too small to read as a different key, but enough to change how the instrument sounds in your hands. Strings come down slightly in tension. The top end loses a little glare. Whether that reads as “warm” or just “flat” depends on your ears. We’ve dropped an A=435Hz preset below. Load it into the TU101 Strobe Tuner, tune up as you normally would. The reference is the only thing that’s moved—your intervals, fingerings, and relationship to the fretboard are all untouched.
A word of warning: tuned to 435 Hz, you’re 20 cents off from everyone else on the planet so anything you play against a backing track, a click with pitched material, or another player at 440 Hz is going to fight you. Worth remembering that for most of the 1800s, this was just a Tuesday.
Notes
- Cavanagh, Lynn. A Brief History of the Establishment of International Standard Pitch a=440 Hertz. 1999, https://www.wam.hr/sadrzaj/us/Cavanagh_440Hz.pdf.
- Hamm, Chelsey, and Bryn Hughes. “American Standard Pitch Notation (ASPN).” Open Music Theory, https://viva.pressbooks.pub/openmusictheory/chapter/aspn/.
- Shah, Sneha, and Vesa Välimäki. “Automatic Tuning of High Piano Tones.” Applied Sciences, vol. 10, no. 6, 2020, article 1983, https://doi.org/10.3390/app10061983.
- International Organization for Standardization. “ISO 16:1975 Acoustics—Standard Tuning Frequency (Standard Musical Pitch).” ISO, https://www.iso.org/standard/3601.html.
- “Peterson Sweetened Tunings.” Peterson Strobe Tuners, https://www.petersontuners.com/sweeteners/. Accessed 24 July 2026.
