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Just from practical experience it seems the former (two pitches on one distorted electric guitar) is nosier than the later (the two pitches on separate distorted guitars.) I don't know this is actually the case, but it seems so.

An example could be playing a passage in thirds around middle to high range. On one guitar those thirds seem to get very muddy and noisy. But if the parts are on separate guitars it seems less noisy.

Can someone elaborate on what happens in those two scenarios in terms of electric signals and acoustics? If there isn't really a difference, that would be helpful to know too.

Re. two separate guitars: I was thinking two separate speakers or two separate recorded tracks.

  • 1
    If those two separate guitars were playing through the same amp/speaker, it would probably sound about the same as the one guitar. – Tim Feb 1 at 4:41
  • @Tim, I added an clarifying note. – Michael Curtis Feb 3 at 16:19
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From https://en.m.wikipedia.org/wiki/Intermodulation:

Intermodulation (IM) or intermodulation distortion (IMD) is the amplitude modulation of signals containing two or more different frequencies, caused by nonlinearities or time variance in a system. The intermodulation between frequency components will form additional components at frequencies that are not just at harmonic frequencies (integer multiples) of either, like harmonic distortion, but also at the sum and difference frequencies of the original frequencies and at sums and differences of multiples of those frequencies.

Distortion can be seen as another word for (or the result of) the non-linearity referred to in that quote. When you make a change in the input signal, you get a disproportionate change in the output signal - as you'll know, that's how distortions work, and that's non-linearity.

The sum and difference frequencies generated from the sinusoidal components of a single note are mostly closely related to the original note's components. When you distort two notes separately, that's all you get - each note with its own sums and differences.

However, if you distort a signal containing two notes mixed, you also get a load of other frequencies generated that represent the sums and differences of the components of each other's sums and differences. Depending on the pitches and spectra of the two notes, these may well be less closely related to the frequencies of the components in the original notes, which can cause the sound to be 'rougher'.

This is one of the the main reasons for the use of power chords in rock - a distorted open fifth creates relatively few messy, 'unrelated' frequency components.

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topo Reinstate Monica's answer is much better, but I'll leave this here anyway.

From a signal processing perspective, you're looking at two different scenarios:

  • (1) : distort(guitar1 + guitar2)
  • (2) : distort(guitar1) + distort(guitar2)

There are two operations here: "+" and "distort". Out of these two, "+" is a linear, and "distort" is a non-linear operation. Distortion, by definition, cannot be undone: it means destruction of at least some aspects, some information in the original signal. When you play two different simultaneous notes into the same guitar amplifier, it's like scenario 1 - the signals are first summed and then the sum is distorted in the amp. But if you have a separate amp for each note, it's like scenario 2 - each signal is first distorted, and then the two distorted signals are summed linearly in the air.

For the linear summing operation, you can in theory get the original signal back with a reverse operation (subtraction).

  • Sum(AB) = A + B
  • A = Sum(AB) - B

But for the nonlinearity, the distortion, there isn't necessarily any way to reverse the operation and get a non-distorted clean signal back. For example, if the distortion is a simple clipper that saws off tops and bottoms of the sine waves - how do you undo? What if the input signal was like that to begin with and not nice round sine waves?

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  • I think this answer is spot on too... +1 from me! – topo Reinstate Monica Feb 1 at 12:48
  • @topo I don't really explain why this particular type of distortion makes the combined notes unclear, and what happens to the frequencies. I think that would be half of the required answer. – piiperi Reinstate Monica Feb 1 at 14:28
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Overall, your perception is correct.

There are a lot of different kinds of distortion, but at its most basic level, the effect of distortion is created by clipping the signal above a certain amplitude.

When playing into a distortion pedal or amp, decreasing the volume on your guitar will lower the amount of distortion (lower volume = lower amplitude). Similarly, playing only one note will have a lower volume than playing two notes. So, assuming all other factors are equal, the signal coming from the guitar playing two notes will have more volume (and amplitude) than the signal coming from the guitar playing a single note, and it will therefore be more distorted.

I have heard an anecdote about a hard rock band recording guitars for a record (I can't remember what bank), and they recorded the guitar parts in (at least) two layers: one playing power chords (just root-fifth-octave) and another playing just the thirds of the chords. The idea was that this allowed the third to be more clear and less distorted.

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