Automatic Speed Matching Using Tape Hum

Many tape recordings captured prominent hum at either 50 or 60 Hz, and we can use this to bring digital transfers to their original speed.

Note:

• This method assumes the hum frequency is constant throughout, so it will not correct any wow or flutter.
• This method can be ambiguous if your source is removed from the master by several analog generations, as successive hums will be captured in each copy and may interfere with each other. You may want to investigate the low end in pyrespeeder with huge FFT sizes.

How to use

1. Open an audio file. The program places crosshairs on the detected peaks in the spectrum.
2. Adjust the target hum base frequency (usually 50 or 60 Hz) and the number of hum harmonics you want to analyze. The crosshairs will move if the new settings require it. Higher harmonics have better frequency resolution, but also more interfering proper signal. Your last crosshair should land on the tallest and narrowest hum peak, which is usually the base frequency or its first harmonic.
3. If your hum peaks are not very tall, the expected speed change is small and wrong peaks are selected, lower the tolerance percentage threshold. The program will then look for peaks that are closer to the target frequencies.
4. You may want to try looking at only the left or right channel using the channel dropdown, as the hum is sometimes more prominent in one channel. Use the setting that yields the most prominent hum peaks.
5. Whenever you change settings, the speed change percentage value (of the rightmost peak / crosshair) is copied to your clipboard so you can paste it into audacity's speed change dialog. Alternatively, you can create a speed corrected file directly from the program if you click the "Resample" button.