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I'd like to put different effects on my low strings and high strings (or low notes and high notes), e.g. an octavizer and delay on the bass notes and vibrato and reverb on the treble. Is there a simple way to achieve this?

Could a low/high pass filtering solution work, or are the higher harmonics of the low notes too important? The only other solutions I can think of are either going through MIDI, which would change the sound too much, or using a custom guitar with two sets of pickups, which would be way too much effort.

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    Interesting idea. Most pups are wired so that all strings are accomodated in one unit. It would be expensive to have some 'custom wired', although if split, they could still be switcheed to work as standard. Or, mounting two pups so that each was under three strings - odd looking though! – Tim Jun 3 at 11:01
  • Never tried anything like this myself, so I can't expand this into an answer, but possibly the Boss GP-10 could work – Shevliaskovic Jun 3 at 11:13
  • What low pass & high pass filtering will do is process the upper harmonics lf the low strings along with the high strings. That said, people have done exactly that with guitar parts in the past. – Todd Wilcox Jun 3 at 11:14
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    I seem to recall somewhere seeing a bass with 2-element pickups spaced both laterally and longitudinally such that each one only sensed two strings. – Carl Witthoft Jun 3 at 13:33
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    @CarlWitthoft They exist for 6-string guitar too, e.g. the "Z-coil pickups" from G&L. I assume you could rewire them, but you'd lose the noise-cancelling effect. You can buy them from G&L for $80, but they obviously aren't a simple drop-in replacement for other pickups without some DIY. – Your Uncle Bob Jun 3 at 14:31
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Could a low/high pass filtering solution work, or are the higher harmonics of the low notes too important?

It can have an interesting effect - in fact I have experimented a lot with this on bass guitar - but you are correct that the harmonics of the lower notes would be above the fundamentals of the higher notes, and so you wouldn't really get full separation of notes. Even with very low or high notes, parts of the 'noisy' initial transients will appear on both sides of the crossover filter output.

On the plus side, this technique should be easy to experiment with using a conventionally-recorded part. You can simply clone your track and pass differently-filtered versions of it through different effects. You might even be able to chop-up copies of the track in the time dimension too, to create a poor man's version of Todd Wilcox's suggestion.

One way you might get even more separation could be to load your recording into melodyne and see if you could use that to separate things out. You might even be able to achieve some effects that can't be achieved by treating things per string - e.g. you might be able to isolate runs and riffs that go across strings.

or using a custom guitar with two sets of pickups, which would be way too much effort.

This is likely to be a much more inspiring playing experience, IMO. You don't necessarily need two sets of pickups - If you google for hexaphonic pickups, polyphonic pickups, or divided pickups, you may find something that would fit in an existing guitar with only a rewiring job needed.

  • Hmm. What if you take the low-passed signal, add harmonics to it, and then use that to phase cancel the harmonics from the high frequency output signal? I wish I had the hw knowledge to just try it, sigh. ... But this much might be ok to peel off the lowest note, then a stack of these could collect several low notes and split off the highs. – luser droog Jun 3 at 17:48
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    @luserdroog sneaky idea! But there would be a huge amount of work to do to generate harmonics accurately enough to do the phase cancellation - it's probably an easier job to just isolate the notes from the mix (I've just added a link to melodyne) – topo Reinstate Monica Jun 3 at 19:08
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I’ve done this before with a completely normal guitar but you won’t like how I did it:

I simply learned to play the guitar part all over again, but I learned how to play the part only the bottom three strings and then only the top three strings and then I recorded two separate tracks so I could process them differently. Obviously this method can’t be used live. Note that Matt Bellamy (Muse) did something like this on the recording of “Time Is Running Out”.

Roland 13-pin pickup systems can do this live, but note that what’s really happening in this case is you would be processing COSM modeled synthesized guitar sounds and not the actual magnetic pickup. There are also some rare “hexaphonic” pickups that do this, I think Line 6 guitars can do this. But those are all in the category of “MIDI” solutions.

I believe the Moog guitar might have done this out of the box with the magnetic pickups, but it was very expensive when in production and is no longer available, so buying one second hand would cost a mint.

  • You could probably do what you describe with a looper - then it could be played 'live'. – Tim Jun 3 at 11:54
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It's possible (and fairly easy) to selectively mute strings on a humbucker pickup by placing a piece of soft steel between the pole pieces serving the strings in questions. If e.g. one mutes the upper four strings of the bridge pickup (which is what I often do), then the bridge pickup will just capture the bottom two strings. While I just do this to get some extra brightness on the bass strings, it would be simple to wire in an extra jack and have a bass-strings-only signal which could be processed separately from an all-strings mix. If there is adequate physical clearance, it might be possible to use a similar approach on single-coil pickups, but I would expect that for best results one would want to use "U-shaped" pieces of steel which reached around the sides of the pickup so as to provide a magnetic path most of the way from one end of the pole piece to the other.

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    For many single coils it is also possible to just remove the individual magnets for the unwanted strings. – leftaroundabout Jun 3 at 21:21
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Filters are a good start for isolating low vs high parts of a signal, but as a observed in comments and other answers, the upper harmonics of the low notes will be in the high portion of the output.

But, we can refine this method by using phase cancellation as well. Start by taking the frequency spectrum with wide buckets to find the low note (this may be a cascade of FFTs to catch the low note). Next, we generate an envelope by adding harmonics of this note according to the expected spectra for the type of instrument (plucked string, bowed string, etc.). Then invert the signal and mix it with the input to phase cancel all of these frequencies. We can also use the envelope to control multiple band pass filters to extract the low note and (most of) its harmonic content.

A refinement would be to do the last two steps in reverse: use the generated envelope to band pass the signal to create a more accurate envelope for the phase cancellation.

In theory, this should give you two usable output signals: the low note and its harmonics, and the remaining high notes. By stacking several of these gizmos we could isolate the lowest two or three notes from a chord.

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At that point, it might be easier to just split that into two parts and apply the effects separately. It could be interesting to split the signal, run it through two banded EQs configured as low-pass and high-pass filters heavily attenuated and set at the same cutoff frequency, and see what the output sounds like, but as mentioned elsewhere, the low-end channel of your signal is going to sound really odd without the harmonics that you're filtering out.

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