Bass FAQ's

[mrlizard]

Originally Posted by GaryM

Because reversing the polarity results in a 180 degree shift in phase of the signal. I guess most people (like me) tend to think more in terms of the phase of signals when mixing, than in terms of the polarity of circuits.

its not really a shift in phase though, phase is time related, if you invert a waveform, it hasn't been shifted in time. If i think about something causing phase shift when i'm mixing i think of something creating a delay.

[GaryM]

From Phase (waves) - Wikipedia, the free encyclopedia :

"A shift in time is equivalent to a phase shift, for an infinitely long sinusoid. Conversely, a change in the initial phase is tantamount to a shift in time."

Inverting a waveform is the equivalent of a 180 degree change in initial phase, but you're right in that a 180 degree change in phase could also be caused by the same waveform being delayed for the time it takes half a wave to pass. I'll be clearer next time I talk about changes in phase and changes in initial phase

[mrlizard]

I know what pushing a phase button on a desk does, but if you have two microphones on a source and you invert the waveform 180 degrees its gonna sound drastically different to shifting the phase. i just don't know why people don't avoid the confusion and call it polarity.

[Hometown Zero]

because it is phase (i think) as it is to do with AC waves.

I supose you could say out "out of time" or "out of synch" aswell... or are tehy different too?

[GaryM]

Originally Posted by mrlizard

if you have two microphones on a source and you invert the waveform 180 degrees its gonna sound drastically different to shifting the phase.

OK, I'll try to illustrate this.

Imagine the bottom two waves shown are the signals from two microphones.



In the left-hand example the two waves are in-phase and combine to create the wave at the top (the same sound twice as loud), and in the right-hand example the two waves are out-of-phase and combine to create silence.

In the right-hand example, the top wave of the pair at the bottom could be gotten by reversing the polarity one one of the microphones, in effect inverting the waveform. But imagine instead of turning the waveform upside-down, that you slid the original wave sideways, until the peaks on one wave lines up with the troughs on the other. You would end up with the same result as inverting the wave, except this time you would have shifted the wave by 180 degrees. This is why a 180 degree phase shift is regarded as the same as inverting the polarity of a wave.

[mrlizard]

Originally Posted by GaryM

OK, I'll try to illustrate this.

Imagine the bottom two waves shown are the signals from two microphones.



In the left-hand example the two waves are in-phase and combine to create the wave at the top (the same sound twice as loud), and in the right-hand example the two waves are out-of-phase and combine to create silence.

In the right-hand example, the top wave of the pair at the bottom could be gotten by reversing the polarity one one of the microphones, in effect inverting the waveform. But imagine instead of turning the waveform upside-down, that you slid the original wave sideways, until the peaks on one wave lines up with the troughs on the other. You would end up with the same result as inverting the wave, except this time you would have shifted the wave by 180 degrees. This is why a 180 degree phase shift is regarded as the same as inverting the polarity of a wave.

YES, i understood this already, yes...

my point "180 degrees out of phase" and "polarity reversed"
are not equivalent. if you have two microphones on a source reverse the polarity on one, it is going to sound drastically different from delaying the signal until it is 180 degrees out of phase, perhaps this will work with a constant frequency and amplitude. but if we're talking about a guitar amp, this doesn't work. if you gradually increase the gap between the two you get a sort of comb filtering effect, but the two will never cancel out in the same way reversing the polarity will, because 180 degrees out of phase at 50hz is different to 180 degrees at 1k.

I've just made sure i'm not mad and tried it on some guitars that i recorded today. I'm right, it sounds totally different. Yes you do get a certain amount of cancellation but its not the same, they sound different, very different.

But yes, i understand that if you had a constant 50hz sine wave and you put another one 180 degrees out of phase with it, they would cancel each other out.

[mrlizard]

Originally Posted by Hometown Zero

I supose you could say out "out of time" or "out of synch" aswell... or are tehy different too?

because it is phase (i think) as it is to do with AC waves.

no, you don't get an out of time button on a mixing desk, although if it cancelled out sloppy playing it would be nice.

AC or DC, polarity is just the way round the positive and negative are . with an ac signal swap them over and the troughs become the peaks.
Thats exactly what the phase button on a desk does, you get the exact same effect from swapping pins 2+3 on a mic cable, its got nothing to do with phase other than it can cure some phase problems.

anyway this thread is supposed to be about bass's, sorry for going off topic.