Interesting Gear Page Thread on Phase Inversion

Zander's picture

So there's a really interesting thread that was started on The Gear Page that we happened upon a few days ago and it really got us thinking - not just about guitarists, but about bass players, keyboardists, and singers too. Yes, yes, we know that drummers get no love - but drummers don't seem to use, or even need a lot of effects in their repertoire and so, perhaps, this isn't as germane.

The thread is a list of effect pedals that "invert phase" - along with a discussion of what's going on, how phase inversion is perceived, by the inversion on a show or a recording... It takes a bit of time to get through, but is very informative.

Now, from the lay person standpoint, from the fan standpoint, and from a "beginning musician" standpoint, phase inversion is a little bit of an esoteric subject. We touched on it a bit when in the article that discusses Steve Albini on Recording Drums. Steve was talking about placing microphones on both sides of a drum (top and bottom) and how they cancel out, resulting in a practically silent drum recording...

(Please feel free to check out the article - we're not going to rehash the physics of this particular situation. However, the nature of the physics apply, in a very real sense, to the situation described in this article... Read on!)

There isn't an actual explanation of the sonic physics happening in the Gear Page thread, so if you're interested about the physics, please continue reading. Otherwise here is the GearPage thread:

As it turns out, in many cases, when a musical signal is being amplified, the act of amplification inverts the sound wave. The Wiki here at Thermionic has a brief discussion of why a music signal gets inverted in tube amplifiers, but this is only half of the story.

Most pedal effects, and many rack effects modify sound as a result of the solid-state circuits that are under the hood. Transistors can fulfill a few different functions in an electronic circuit, but what they're used for mostly in musical equipment is for amplification. Whether those transistors are inside an integrated circuit or whether the problem of inversion is caused by a single transistor (like in an Electro-Harmonix LPB-1), if a signal is being amplified, it will get inverted. Yes, there are some transistors that will amplify without inverting, However, most audio electronics courses will teach engineers to invert their signals, because while the inverting condition limits the gain of the circuit, the stability of an inverting circuit itself is much higher than a non-inverting one. This means that in most audio amplifier design the solid-state componentry tends to be designed to invert phase.

The number of transistors, or stages of amplification, will tend to inform you of whether an effect will invert phase or not. Following the rule-of-thumb, effects with an even number of amplification stages (transistors) will not invert phase, while those with an odd number will. Effects where the designer has failed to take into account phase inversion are usually those that invert phase.

Now that we've established that most audio amplifiers invert a signal (and in the better-designed circuits, inverted again), we're prepared to get into the meat of the situation - why is phase inversion a bad thing?

Let's take another look at the diagram that was created for the Albini article:

So when you have two audio sine waves that are exactly the opposite of each other AND that are played in close enough proximity (so that each sound wave affects the other), the positives and the negatives in the sound waves get summed together. Just like the diagram, +5 + -5 = 0. As mentioned in the Albini article - this is the same principle around which Bose, and other companies make their "silent" headphones.

Why is this issue being raised again? Well, in the thread, the discussion that takes place is a discussion of mostly of stereo, or two-channel output music systems. When a bassist, or a guitarist, or a keyboardist, or a singer (or a drummer!) is splitting his signal into left and right channels, it's oftentimes the case that the performer will also apply an effect to either the left, or the right, but not both channels. The difference in these two channels is usually called "Dry" (no change) and "Wet" (with effects).

When such a setup is implemented, and an effect inverts phase on only one of the channels, the result can, at worst, cause a large decrease in volume for a player. If it's not manifested as such, then there can be a bizarre out-of-phase or "sea shell" sound that can be perceived.

So final word of advice is that if you're playing through a dual- or multi-channel set up for a rehearsal or a show, make sure that your channels share a common phase. If you do have any effects that are inverting phase, there are a few solutions:

  • make sure to get a buffer/signal inverter that will take your inverted signal on the "wet" side and re-invert it so that both phases are back to being "in common", or
  • remove the offending, inverting, effect from the channel, or
  • invert phase on the dry channel so the signals are again in common.

Now that you've made it to the bottom - Here's the link for the thread again...