The two basic types of distortion are harmonic and intermodulation, and both are almost always present together.
Harmonic distortion adds new frequencies that are musically related to the source. Ignoring its own inherent overtones, if an electric bass plays an A note whose fundamental frequency is 110 Hz, harmonic distortion will add new frequencies at 220 Hz, 330 Hz, 440 Hz, and subsequent multiples of 110 Hz. Some audio devices add more even harmonics than odd, or vice versa, but the basic concept is the same.
In layman’s terms, harmonic distortion adds a thick or buzzy quality to music, depending on which specific frequencies are added. The notes created by most musical instruments include harmonics, so a device whose distortion adds more harmonics merely changes the instrument’s character by some amount.
Electric guitar players use harmonic distortion—often lots of it—to turn a guitar’s inherent plink-plink sound into a singing tone that has a lot of power and sustains.
Intermodulation distortion (IMD) requires two or more frequencies to be present, and it’s far more damaging audibly than harmonic distortion because it creates new sum and difference frequencies that aren’t always related musically to the original frequencies.
For example, if you play a two-note A major chord containing an A at 440 Hz and a C# at 277 Hz through a device that adds IM distortion, new frequencies are created at the sum and difference frequencies:
Sum: 440 Hz + 277 Hz = 717 Hz
Difference: 440 Hz + 277 Hz = 163 Hz
717 Hz is about halfway between an F and F# note, and 163 Hz is slightly below an E note. Neither of these are related musically to A or C#, nor are they even standard note pitches. Therefore, even in relatively small amounts, intermodulation distortion adds a dissonant quality that can be unpleasant to hear. Again, both harmonic and intermodulation distortion are caused by the same nonlinearity and thus are almost always present together. What’s more, when IM distortion is added to notes that already contain harmonics, which is typical for all musical instruments, sum and difference frequencies related to all of the harmonics are created, as well as for the fundamental frequencies.
Another type of distortion is called aliasing, and it’s unique to digital audio. Like IM distortion, aliasing creates new sum and difference frequencies not harmonically related to the original frequencies, so it can be unpleasant and irritating to hear if it’s loud enough. Fortunately, in all modern digital gear, aliasing is so low in level that it’s rarely if ever audible. Aliasing artifacts are sometimes called “birdies” because difference frequencies that fall in the 5 10 KHz range change pitch in step with the music, which sounds a little like birds chirping. An audio file letting you hear what aliasing sounds like is in Chapter 3.
Transient intermodulation distortion (TIM) is a specific type of distortion that appears only in the presence of transients—sounds that increase quickly in volume such as snare drums, wood blocks, claves, or other percussive instruments. This type of distortion may not show up in a standard distortion test using static sine waves, but it’s revealed easily on an oscilloscope connected to the device’s output when using an impulse-type test signal such as a pulse wave.
TIM will also show up as a residual in a null test when passing transient material. Negative feedback is applied in amplifiers to reduce distortion by sending a portion of the output back to the input with the polarity reversed. TIM occurs when stray circuit capacitance delays the feedback, preventing it from getting back to the input quickly enough to counter a very rapid change in input level. In that case the output can distort briefly. However, modern amplifier designs include a low-pass filter at the input to limit transients to the audible range, which effectively solves this problem.