Double Bass phase question

[Mike Carr]

We all know that combining two pickups, for instance an Underwood + Realist can result in an out of phase sound. But can adding a third pickup in the chain return the signal to "in phase"?
My current road rig is an old Walter Woods Low Power 2 channel head. This one does not have a phase switch like some of them from that era do. Each channel has 2 inputs, high and low. My bass has both a Wilson and a Realist installed and either of these sounds fine on their own. But together, when blended through both channels or into both inputs of either channel the result is that "out of phase, thin and hollow sound". I happen to keep an Underwood and a Stat-B in my bag for spares, just in case (I'm in China) of an emergancy. I'm wondering if adding either one of these would restore the resulting signal to an "in phase" state. With two channels and a total of 4 inputs my Woods could offer a lot of flexability, and the Stat-B's pre-amp has it's own volume control as well. So, I guess the question is, does two signals out of phase, + a third signal ='s "in phase?" Please, no suggestions about buying a blender or mixer, I've got a perfectly fine Raven Labs PBM II back in the States, and I kick myself every day for not bringing it but I'm trying to keep the amount of stuff I travel with to a minimum.

 

[drurb]

We all know that combining two pickups, for instance an Underwood + Realist can result in an out of phase sound. But can adding a third pickup in the chain return the signal to "in phase"?
My current road rig is an old Walter Woods Low Power 2 channel head. This one does not have a phase switch like some of them from that era do. Each channel has 2 inputs, high and low. My bass has both a Wilson and a Realist installed and either of these sounds fine on their own. But together, when blended through both channels or into both inputs of either channel the result is that "out of phase, thin and hollow sound". I happen to keep an Underwood and a Stat-B in my bag for spares, just in case (I'm in China) of an emergancy. I'm wondering if adding either one of these would restore the resulting signal to an "in phase" state. With two channels and a total of 4 inputs my Woods could offer a lot of flexability, and the Stat-B's pre-amp has it's own volume control as well. So, I guess the question is, does two signals out of phase, + a third signal ='s "in phase?" Please, no suggestions about buying a blender or mixer, I've got a perfectly fine Raven Labs PBM II back in the States, and I kick myself every day for not bringing it but I'm trying to keep the amount of stuff I travel with to a minimum.

The short answer is "no." First, let's consider the simple case of pickups being only perfectly in or out of phase (i.e., 0 or 180 degree phase shift). Furthermore, we'll make the simplifying assumption that each pickup has the same amplitude, 1.0.

Suppose that at the frequencies of interest, a pair of pickups are out of phase with respect to each other. That means we have:

(+1) + (-1) = 0;

Now, adding a third pickup will produce a total of +1 or -1, depending upon its phase. That is, you will have either

(+1) + (-1) + (+1) = 1

or

(+1) + (-1) + (-1) = -1

In either case, you would have done as well with a single pickup.

Now, if you had started with two pickups in phase, you would have had:

(+1) + (+1) = 2

or

(-1) + (-1) = -2

In either case, if the pickups were in phase with respect to each other, you would have doubled the amplitude, which would yield a 6-dB increase.

In reality, two pickups are not likely to be perfectly in or out of phase with each other. Furthermore, their phase relations will likely vary as a function of frequency and their amplitudes will not be equal. Combining pickups or, for that matter, any pair of transducers (e.g., piezo and microphone) is a hit-or-miss affair.

 

[Mike Carr]

I knew I could count on DRURB for a reply! So does the sum of the 3 combined pickups = 1 signal consisting of the th 3 pickups. operating properly and for better or worse, sounding like they should? I'm just trying to get a sense of the techno-crap before I start off running wires and knob twiddling. I've set a fine standard for the requirements of my gig. I'm happy enough using a single channel of the Woods with the Wilson or The Realist, just trying to explore some options.

 

[BEEF]

No.

When combining any 'similar' signals (like different pickups/mics from the same source) you change the sound in other ways than just amplitude (volume).

If you have two lines of the same signal and one is 'out of phase' with the other (reversed), you actually CANCEL the signal. The closer to 'exacly the same' the closer to a complete NUL of sound.

When the two signals are a little different (like mic/pickup or two different pickups), you generally cancel out a bunch of different frequencies in the sound (and boost others). It's generally heard as 'losing the low end'.

Even when 'in phase', two signals will combine and push up/down certain frequencies. It will act as an 'EQ' when combined. This is why changing the levels of the two and tweaking gives a broad range of sound.

Adding a third source REALLY complicates the signal. It is not just 'adding' signal. Less is more in this case.

You typically use two sources only when you want a certain aspect of each in the final sound. (ie. the lows and body of one source with the attack and highs of the other)

Hope this helps.


p.s. Also, there is a difference between POLARITY and PHASE that should be explored. We're throwing around the word PHASE when it's actually the POLARITY of a source thats causing PHASE to be reversed. A signal can be out of PHASE due to time (one signal is there just before/after the other), but reversed POLARITY is when one signal is going up (think of a sin wave) when the other is going down.


(shure is tough to type about this stuff)

 

[ToneRanger]

Have you tried flipping one of the pickups upside down? That might affect the phase relationship in a useful way.

 

[Mike Carr]

The Wilson and the Realist don't really flip over, they are each both more or less permanently mounted. My notion of adding a third was to attempt phase (polararity?) correction when running both, which is not my usual practice with the Woods.I went thru a period several years ago when I used a Realist/Underwood combo with the Raven Labs Blender, which offers phase switches for each of it's two channels and had good results with whatever amps the venues provided. But I always missed the little extra something that my Woods brings to the tone. I've got a lot of gear, besides the Woods I've got a Clarus, an SWR Bass 350, an AI Contra EX, a Razers Edge 10, Goliath and Goliath Jr. cabs, I've even got a mint 1963 Ampeg SB-12 Fliptop. But for the road right now it's a matter of keeping it simple, the Woods and a Countryman DI is about as much gear as I'd care to bring these days, plus the bass of course. To tell the truth, either the Wilson or the Realist alone sounds great thru the Woods and the clubs house bass box, an SWR 15.

 

[drurb]

p.s. Also, there is a difference between POLARITY and PHASE that should be explored. We're throwing around the word PHASE when it's actually the POLARITY of a source thats causing PHASE to be reversed. A signal can be out of PHASE due to time (one signal is there just before/after the other), but reversed POLARITY is when one signal is going up (think of a sin wave) when the other is going down.

Polarity can be thought of as a special case of phase. Two signals that are of opposite polarity are two signals that have all of their spectral components 180 degrees out of phase with each other. This holds for simple or complex waveforms, not only sine waves. When one waveform is time-delayed with respect to the other, it can never be truly "out-of-phase" unless the signal is composed of a single component (a sine wave) or a series of odd harmonic components such that the time delay causes all components to be shifted by 180 degrees. This assumes the usual definition of "out-of-phase" as being 180 degrees out of phase.

 

[BEEF]

I was just pointing out that all of the 'phase' buttons on amps and such are actually reversing the polarity.

Two signals can be the same polarity and be 'out of phase' by being time shifted. This is the case when a mic and pickup (DI) are used together. One signal arrives just after the other. Only time alignment completely 'fixes' the phase problem. 'Sometimes' fliping the polarity of one of the signal makes it 'better' (or makes a fine sound of it's own), but it's not completely fixing the problem.

If you look at a wave (ANY wave, not just a sin wave), it goes up first then down. An 'opposite polarity' goes down first then up. This is commonly called 'out of phase', but there is a distinct difference between 'out of phase' and 'out of polarity'. The difference is wether your signals are different in time vs. different in polarity.

Chorus is an example of shifting the 'time' before and after an original signal to achieve that sweeping effect. You actually hear the 'comb filtering' effect of the phase change between the two signals. It's very similar to sweeping an eq up and down.

Phase pedals (ironically) play both signals at the same time but gradually 'reverse' the polarity back and forth to achieve it's effect. You can usually hear when the 'phaser' is hitting the exact middle (180 degrees out) as a nul, or soft spot in the sound.

The two occurances often get lumped into one category of 'phase' problems, and understanding both problems and how they work will help everyone achieve the sound they're after.


Another fine example of the difference is the typical surround sound set-up for watching movies.

A speaker that is wired incorrectly will have it's polarity reversed. It has to be fixed by flipping the polarity (once again, that little button marked 'phase' on an amp). It's done by re-connecting the wires correctly.

If there are one or more speaker 'further away' than others. Thee closer speakers are 'time delayed' to match up to the others. This avoids phase problems. Things like this are done in studio recording when a DI and Microphone are combined or when two microphones of different distances are combined. Sometimes you WANT to fix this, sometimes you have the time delay phase problem on purpose.

I'm sorry to go on and on, but I run a recording studio in my home and Phase/Polarity issues are part of everyday life. Once you have more than one signal, you MUST know how to handle these problems or you'd be chasing your tail through the whole process.