pdf64 wrote: ↑Mon Mar 05, 2018 12:53 pm
Really, when comparing 2 signals and their sources, their relative phase and polarity are completely different, separate characteristics.
That's why it may be better to use the terminology signal 'polarity' or 'inversion', rather than signal 'phase'.
As noted, phase applies to specific frequencies/distances/times, whereas polarity (theoretically) applies equally across the bandwidth.
Mixing truly 'equal but out of phase' real signals (ie not single frequency) will result in a comb filter response, whereas mixing equal signals of opposing polarity will result in silence.
Unfortunately we're stuck with the terminology, I think that Langford Smith et al just assumed that anyone who had to deal with these things would have had the education background to understand the difference when they used the words interchangeably in terminology such as 'phase inverters'.
Op amp terminology eliminates this confusion, eg inverting and non-inverting inputs.
Ha, i didn't want to even mention comb filtering
Actually if you mix two real signals and flip one out of phase then it will silence if the two are identical, it doesn't have a to be a single frequency. Comb filtering arises when the various frequencies are arriving at your ear at different times and thereby causing the filtering due to the cancellations or peaks caused by the time delays.
it's interesting that our brain use all this phase, comb filtering to work out spatial awareness.
pdf64 wrote: ↑Mon Mar 05, 2018 12:53 pm
Really, when comparing 2 signals and their sources, their relative phase and polarity are completely different, separate characteristics.
That's why it may be better to use the terminology signal 'polarity' or 'inversion', rather than signal 'phase'.
As noted, phase applies to specific frequencies/distances/times, whereas polarity (theoretically) applies equally across the bandwidth.
Mixing truly 'equal but out of phase' real signals (ie not single frequency) will result in a comb filter response, whereas mixing equal signals of opposing polarity will result in silence.
Unfortunately we're stuck with the terminology, I think that Langford Smith et al just assumed that anyone who had to deal with these things would have had the education background to understand the difference when they used the words interchangeably in terminology such as 'phase inverters'.
Op amp terminology eliminates this confusion, eg inverting and non-inverting inputs.
Ha, i didn't want to even mention comb filtering
Actually if you mix two real signals and flip one out of phase then it will silence if the two are identical, it doesn't have a to be a single frequency. Comb filtering arises when the various frequencies are arriving at your ear at different times and thereby causing the filtering due to the cancellations or peaks caused by the time delays.
it's interesting that our brain use all this phase, comb filtering to work out spatial awareness.
I agree the terminology we use can be squirely.
Marcus
Actually actually, pdf64 was correct. He was explaining the difference between two different things:
a) Two identical signals that are out of phase with each other cause comb filtering.
b) Two nearly identical signals, one of which has had its polarity inverted, will cancel each other out entirely.
In the purest, most technical sense, phase is used to quantify a relative shift in time, as seen in a). In contrast to that, in b), inversion is what happens when the positive half of a signal is made negative and vice verse. The word "phase" is abused pretty heavily in the audio world though, and most people are used to it and don't raise much of a fuss very often.
yes, I know the difference.
I was making the point that it doesn't have to be a single frequency signal to phase cancel fully to zero.
Actually two identical signal 180 deg out of phase with each other wont comb filter they'll cancel.
two nearly identical signals that have their phase flipped will not cancel full they'll leave the residue of the sum of the difference
As audio engineers we do this to check things all the time if someone say's mixes/tracks/summing bus in a DAW etc etc are different, if they phase cancel to zero then they must be exactly the same.
norburybrook wrote: ↑Mon Mar 05, 2018 5:47 pm
yes, I know the difference.
I was making the point that it doesn't have to be a single frequency signal to phase cancel fully to zero.
Actually two identical signal 180 deg out of phase with each other wont comb filter they'll cancel.
two nearly identical signals that have their phase flipped will not cancel full they'll leave the residue of the sum of the difference
As audio engineers we do this to check things all the time if someone say's mixes/tracks/summing bus in a DAW etc etc are different, if they phase cancel to zero then they must be exactly the same.
M
You say you know the difference but your exmples are a mixture of kind of correct and kind of confused.
Two signals that are 180 degrees out of phase and cancel completely is something that happens really easily with signals generates by test equipment, but real world audio signals tend to be more complicated and diverse and cause comb filtering when two of them are out of phase. Note that the degree of phase shift is defined by the frequency of the signal; a real world audio signal can only be 180 degrees out of phase with itself at one specific frequency. The audio signal at that one specific frequency cancels completely and the frequencies close to it cancel proportionally to how close they are to the 180 degree out of phase frequency.
If you have two real world audio signals and one is an inverted copy of the other and they're in phase, they will cancel perfectly.
Phase is a shift in time. Inversion is a swap on the amplitude axis.
you said two identical signals out of phase will cause comb filtering. that will only happen if the starting or end point is different in the time domain. When working with audio ,especially digitally that's not usually the case unless you're talking about two separate microphones and that has already been discussed in regard to phase issues.
In my experience as an audio engineer two identical signals out of phase will cause complete cancellation.
PDF64 said only single frequency signals out of phase would cancel, I don't agree, as I said I use this to compare two master audio mixes for example if someone say's they're different. They will null if they're the same and we're talking full frequency complex mixes, there is no shift in time here.
p.s. all professional audio equipment has a 'phase' switch , this is in reality a polarity switch, however once you get to 180 phase then it is in fact polarity swap.
There does seem to be a misunderstanding of terms leading to confusion. I think that some are thinking of frequency dependent phase shift, like what you get running a signal through a RC network while others are thinking of something more along the lines of two speakers who are driven by the same signal but whose polarities are reversed or 180 out of phase. The first would lead to frequency dependent phase shift, which I'm not sure we perceive, while the the second would cancel if you disregard where you are listening from relative to the location of the two speakers.
It's possible that the what the OP wanted to know is how the phase of the amp effects the feedback/sustain from the acoustic coupling with the guitar when playing too loud? Both of those amps feedback pretty well though even if they are out of phase.
norburybrook wrote: ↑Mon Mar 05, 2018 8:56 pm...two identical signals out of phase will cause complete cancellation...
I think that will happen only if that signal is a single frequency; with real world signals, there will be a comb filter effect. Complete cancellation of a real signal would require it to be mixed equally with the inverse of itself.
norburybrook wrote: ↑Mon Mar 05, 2018 8:56 pm...
PDF64 said only single frequency signals out of phase would cancel, I don't agree, as I said I use this to compare two master audio mixes for example if someone say's they're different. They will null if they're the same and we're talking full frequency complex mixes, there is no shift in time here.
p.s. all professional audio equipment has a 'phase' switch , this is in reality a polarity switch, however once you get to 180 phase then it is in fact polarity swap...
So what you have actually done is to flip signal polarity, rather than apply a full bandwidth 180 degree phase shift (which I think is an invalid concept).
norburybrook wrote: ↑Mon Mar 05, 2018 8:56 pm...once you get to 180 phase then it is in fact polarity swap...
as long as both signals have no time domain factor then they will null if they're identical if one is out of phase with the other, it doesn't matter how simple or complex the signal is as they will both have exactly the same phase/frequency response if you think about it.
It's easy to try, as I say It's a technique used frequently in the audio world to tell if two signals are exactly the same.
Just place two identical signals of any kind on two separate tracks in some DAW software-(download Reaper for this purpose if you don't have any DAW software as it's free to use for a month), flip the phase on one track and you'll hear...silence.
Comb filtering only comes into the equation when there's a time difference and this is usually caused by reflections in the room delaying certain frequencies before they hit your ears or when using multiple microphones on a single source, incorrect placement will mean the signal will be arriving at the microphones at slightly different times.
norburybrook wrote: ↑Mon Mar 05, 2018 8:56 pm...once you get to 180 phase then it is in fact polarity swap...
Again, only at one frequency.
Maybe in the context of a gain stage, but you can get 180 phase shift digitally by shifting the signal.
Even in an analog gain stage you could minimize the effect of frequency dependent shift by choosing the RC values to make the knee outside the range of frequencies of interest. How close does a phase inverter get to being able to cancel out when the outputs are summed together?
Littlewyan wrote: ↑Sun Mar 04, 2018 12:34 pmAnother thing to consider is phase shift, something else I don't know much about. I heard that every time the signal goes through a capacitor it phase shifts slightly. Is that true?
Yes and it's frequency dependent. Have a look here:https://www.electronics-tutorials.ws/fi ... ter_3.html and scroll down to the Bode and phase shift plots. You'll recognize that this network appears after each gain stage.
I think it may be worth noting that for the main body of the filter's passband, any phase shift is likely to be negligible; it's only around, and below (HPF) or above (LPF), the corner frequency that phase shift might be seen to become significant.
And given the complex culmulative effects of frequency (and so phase) shaping within most any amp, it becomes difficult if not impossible to know what the total effect may be.
norburybrook wrote: ↑Tue Mar 06, 2018 1:34 pm
as long as both signals have no time domain factor then they will null if they're identical if one is out of phase with the other, it doesn't matter how simple or complex the signal is as they will both have exactly the same phase/frequency response if you think about it.
It's easy to try, as I say It's a technique used frequently in the audio world to tell if two signals are exactly the same.
Just place two identical signals of any kind on two separate tracks in some DAW software-(download Reaper for this purpose if you don't have any DAW software as it's free to use for a month), flip the phase on one track and you'll hear...silence...
But in this context (pro audio equipment, analogue or digital), my understanding is that 'flipping the phase' actually means inverting signal polarity, the 'phase' switch just inserts an inverting unity gain stage into the signal path; nothing actually to do with the phase of the signal. To actually affect phase, just nudge one of the tracks in time one way or the other, the result (EDIT when the signals are mixed equally together) being a comb filter.
Maybe in the context of a gain stage, but you can get 180 phase shift digitally by shifting the signal.
Oh heck, this topic is enough of a head f**k as it is, without bringing in the possibilities that DSP has introduced.
Last edited by pdf64 on Tue Mar 06, 2018 3:19 pm, edited 1 time in total.
norburybrook wrote: ↑Tue Mar 06, 2018 1:34 pm
as long as both signals have no time domain factor then they will null if they're identical if one is out of phase with the other, it doesn't matter how simple or complex the signal is as they will both have exactly the same phase/frequency response if you think about it.
It's easy to try, as I say It's a technique used frequently in the audio world to tell if two signals are exactly the same.
Just place two identical signals of any kind on two separate tracks in some DAW software-(download Reaper for this purpose if you don't have any DAW software as it's free to use for a month), flip the phase on one track and you'll hear...silence...
But in this context (pro audio equipment, analogue or digital), my understanding is that 'flipping the phase' actually means inverting signal polarity, the 'phase' switch just inserts an inverting unity gain stage into the signal path; nothing actually to do with the phase of the signal. To actually affect phase, just nudge one of the tracks in time one way or the other, the result being a comb filter.
Maybe in the context of a gain stage, but you can get 180 phase shift digitally by shifting the signal.
Oh heck, this topic is enough of a head f**k as it is, without bringing in the possibilities that DSP has introduced.
Wait, I think I'm getting what you have been saying. Thinking of a signal as composed of many frequencies, and phase shift being a delaying effect, it alters each of the components of the signal. They won't all be multiples of the fundamental and won't be 180 degress out when the fundamental is. I keep thinking of the signal as a single sine.
cxx wrote: ↑Tue Mar 06, 2018 2:20 pm...Wait, I think I'm getting what you have been saying. Thinking of a signal as composed of many frequencies, and phase shift being a delaying effect, it alters each of the components of the signal. They won't all be multiples of the fundamental and won't be 180 degress out when the fundamental is. I keep thinking of the signal as a single sine.
Yes, that's my perspective on things.
And even for a more complex wave eg square, where the fundamental frequency has been shifted 180 degrees, when a component of the signal is an integer multiple of the fundamental, the phase shift will be 180 + (n x 360) degrees (ie those components would be 'out of phase' but with a phase shift much bigger than 180 degrees).
I think it's the conflation of 'signal' with 'sine wave' that led to experts / textbooks of the golden age using 'out of phase' and 'polarity inversion' interchangeably, and so to the confusion that inevitably results when that conflated terminology became used by those outside of the original intended audience, eg those who haven't studied the textbooks (no offence intended ).
