In its most basic form, signal phase is the way two or more frequencies react with each other. A guitar’s signal is much more complex than the examples we’ll be using to help demonstrate the effect, but they will help you to understand your guitar’s sound from the most basic level at the point of induction and onward. Understanding signal phase is also integral to recording music.
If you have a humbucker in your guitar, phase cancelling is already a major part of your sound. In fact, the humbucker is such a great demonstration of signal phasing and how it can be utilized, we’re going to focus on them entirely.
Starting Point: The Single Coil
Before we get to the humbucker, we’ll just take a look at how a single coil goes about inducting your guitar’s signal (in a very basic way – you can read more about it in the Custom Pickups article).
A passive single coil pickup is basically just a magnet with some copper wiring around it. Different varieties of magnets and wiring material can be used, but the function remains the same: a magnetic field is created around the strings, which will disturb the magnetic flux when plucked, which is inducted by the wires into an electrical signal.
The guitar’s signal is converted to electrical frequencies, but it’s not alone – the process of magnetic induction creates a low-frequency humming noise that we’re all familiar with (unless you’ve never played with a single-coil pickup before).
The more wire length wrapped around the magnet, the more noticeable the humming sound (an attribute that active pickups exploit to produce a cleaner sound). Unfortunately, the way to increase the signal strength on a passive pickup is to increase the winding. You can do things to reduce noise like potting and shielding, but that hum is just a part of the package.
The answer: Gibson’s humbucker
Luckily, some guy named Seth Lover at the Gibson guitar company came up with the humbucker pickup design to eliminate the hum without interfering with the guitar’s tone.
The variables: Polarity and Winding Direction
If you compare the images of the single coiled pickup and the humbucker, you’ll see the winding direction and the magnet’s polarity becomes much more important with a humbucker.
It doesn’t really matter with a single coil until you want to combine a second signal. A simple way to look at it is to consider that every sound wave has a positive value and a negative value. Reversing the magnet’s polarities and winding directions has the effect of reversing the sound waves produced.
When two equal and opposite frequencies are combined, they will cancel each other out – this is phase cancelling. When two signals of the same frequencies and polarities are combined, they will compliment each other by increasing amplitude (volume is increased).
Because both the polarity and the winding direction can reverse the signal, and the humming originates from the winding part of the process, the humbucker is able to separate the two signals and cancel the hum while increasing the volume of the guitar signal.
The chart below makes it easy to see how a humbucker’s parts are combined with purpose, and will also lead us into the next subject: phase switching.
The “Result” column is in reference to each coil setup when combined with #1 – so if one side of your humbucker with coil #1 and the other is #4, you’ll get the desired effect.
Combining 1 & 4, 2 & 3, or 2 & 5 will all phase cancel the hum and increase the guitar’s signal.
Out of Phase doesn’t always mean silence.
Looking at the chart above, let’s use combinations of 1 & 3 and 1 & 5 to demonstrate a completely out of phase signal. Both the signal and the hum are cancelled out in these instances, but if the two coils are located on different parts of the string, there will still be sound coming through.
How is this possible? Easy – the difference in sound between the two pickup coils is what’s leftover after the same frequencies are phase cancelled out. By nature, a string simply doesn’t vibrate exactly the same in two different places – compare the sound of your bridge and neck pickups.
Of course, a good portion of the sound is going to fall within the same frequencies and amplitudes – usually the bassier portion of the signal is cancelled out, leaving a twangy / shrill sort of tone that’s perfect for reggae and such.
Of course, the closer the locations of the two coils, the more of the signal will be cancelled out. Setting the bridge and neck pickups out of phase will give you more harmonic difference.
Setting the two coils within a humbucker configuration out-of-phase will leave you with very little sound leftover.
To add a further dimension of experimentation to the phasing of pickups, you can also wire the coils in series or parallel, in or out of phase – see the diagram below:
The wiring configurations above can be achieved easily with a 3PDT switch, which is how most phase-switching modifications are done.