Speed error, cents, and wow and flutter at 33 1/3, 45, and 78.26 RPM.
A strobe disc tells you whether the platter is fast or slow. It does not tell you how many cents the key of the record has moved. This guide does that conversion with the same formulas as the StrobeHz gyroscope studio, using a number you already measured.
Signed speed error
Speed error is the steady offset of average RPM from the target, in percent: (actual - target) / target * 100. A positive number means the platter is fast. A negative number means it is slow. This is not wow. Wow is the wander around that average.
Cents
Musical pitch follows the speed ratio, not a straight percentage. One percent fast is 17.23 cents sharp. Twice the speed is 1200 cents, one octave. A platter 0.8 percent fast sits near 14 cents sharp. A slow platter produces a negative number, flat of the target pitch. The calculator uses the RPM you type, so the cents figure matches that reading rather than a rounded story.
Wow and flutter
Speed error is the average. Wow and flutter are the movement around it. Wow is the slow part, once per revolution and below. Flutter is faster. Peak-to-peak wow and flutter in the StrobeHz engine is (fastest RPM - slowest RPM) / mean RPM * 100. With only two readings, the mean is their average. RMS is the square root of the mean squared fractional deviation across every sample in a series. Two readings can produce peak-to-peak. They cannot produce RMS. The iPhone app computes RMS from the Z-axis series at 60 Hz. This page leaves RMS blank on purpose.
Quartz lock, drift, and 78.26
StrobeHz calls the platter locked when the absolute speed error is within 0.15 percent. From there to 0.50 percent it is drift. Past 0.50 percent it asks for a recalibration: belt, idler, or the pitch control. 78.26 RPM, not 78.00, is the 60 Hz strobe standard the studio uses for shellac. 33 1/3 RPM is exactly 100/3.
Calculator
Pick the target the record was cut for, then type the average RPM you measured. Add a slowest and a fastest reading only if you actually saw both. The page will not invent a flutter number from a single average.
Questions about speed, cents, and wow
Why 78.26 RPM and not 78?
StrobeHz uses 78.26 RPM, the 60 Hz stroboscopic standard for shellac, so a strobe disc cut for 60 Hz mains locks at that speed. 78.00 would sit slightly slow against that disc. 33 1/3 is 100/3, and 45 is 45.00.
Why will this page not show RMS wow and flutter from two numbers?
Peak-to-peak is (fastest minus slowest) divided by the mean, times 100. RMS needs the whole series of samples, because it is the root of the mean squared deviation from that series. Inventing an RMS from two endpoints would look precise and be false. The iPhone app has the gyroscope series. This page does not.
Does a positive cent value mean the record is sharp?
Yes. Cents use 1200 * log2(actual / target). Actual faster than target is positive, which is sharp. Actual slower than target is negative, which is flat. One percent fast is 17.23 cents.