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Author Topic: current lags voltage question  (Read 4023 times)

locosoundman

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current lags voltage question
« on: October 02, 2004, 11:50:08 AM »

I know only the very basics, but I have been very curious about this topic.  

Since inductors and capacitors in a circuit introduce a difference in phase between voltage and current, and at different frequencies at that, how does this affect the sound that we actually hear?  Obviously, a different impedance at one frequency over another acts like an eq of sorts, but what about the difference in phase?  Also, since a transformer is similar to an inductor (in my mind), will it have the same effect?

Thanks,
Rob
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danlavry

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Re: current lags voltage question
« Reply #1 on: October 02, 2004, 06:54:06 PM »

“Since inductors and capacitors in a circuit introduce a difference in phase between voltage and current, and at different frequencies at that, how does this affect the sound that we actually hear? Obviously, a different impedance at one frequency over another acts like an eq of sorts, but what about the difference in phase? Also, since a transformer is similar to an inductor (in my mind), will it have the same effect?

Thanks,
Rob”


As you stated, inductors and capacitors in the signal path may act like EQ of sorts, changing amplitude response over the frequency range. And yes, short of a few specific circuits (such as the “all pass filter”) they also impact the phase response over the frequency range of interest.

Remember Fourier series? It tells you that you can take a periodic (repetitive) wave of any shape, say square wave, rectangular or what not, and view it as a sum of a fundamental frequency and harmonics at multiples of the frequencies. Say a square 1Khz wave is really a sum of a 1KHz sine wave, a 3Khz smaller sine wave, a 5Khz yet smaller sine wave and so on….

It would be easiest to understand if you can find a plot (graph) of Fourier series od say square wave. It is in a lot of textbooks.

Now lets leave the amplitudes alone – we are only taking phase. So say we leave the fundamental and the 3KHz harmonic alone but change the phase of the 5KHz sine wave. Change phase means sliding it horizontally by some amount. Now add it all together.

What does the wave look like? It is not the same wave shape! So phase alterations changes the waveform. One can take a periodic wave an make it look “unrecognizable” on a scope with phase alterations. And if it alters periodic waves, it will certainly alter non periodic waves like music…

So if you want to keep the wave shape intact, you want your system to be a linear phase system. Can you hear non linear phase? Yes you can.

Regarding transformers, yes the same effect. It is mostly a low frequency issue and a good transformer will have a lot of magnetizing inductance to cover the low end. The phase changes an octave below the low frequency pass band  (a transformer passing say 30Hz will have some phase shift at 15Hz).  Unfortunately so much inductance takes a lot of windings over iron core, which tends to be less than linear. There are few good audio transformers out there, and they cost a bundle.

BR
Dan Lavry

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volki

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Re: current lags voltage question
« Reply #2 on: October 04, 2004, 07:54:44 PM »

I hope I'm not imposing by this longish post...? =o)

Quote:

Can you hear non linear phase? Yes you can.

I've always wondered which aspect of non-linear phase (also known as group delay) people mean when they talk about audibility, under which circumstances they apply, and which orders of magnitude are considered to be the threshold level.

1) Signals that fall into the stationary category (meaning superimposed sine waves that don't change rapidly in amplitude over time) of not too high a level shouldn't be an issue, since the averaging principle of the ear applies here. Obviously transients are the signals where the effect gets audible most. If a transient is subject to group delay, a portion of it's frequency band will arrive somewhat before or after the rest. Now the question is, what is the threshold of audibility, or in other words, what is the smallest time delay our ears can resolve?

I have some older textbooks which I still regard as an authoritative standard, and they state that the minimun audible time delay is 2 ms. However, I suppose (but I'm not sure) under which conditions this is valid, if it gets larger for low frequencies, if other nearby frequencies of a delayed freq band are masked, etc. - anyone??

2) With stereo signals, you will surely get cancellation effects and shifting of stereo image to a greater or lesser extent if one channel has a relative group delay, even with stationary signals. but with mono signals, e.g. recording vocals through a microphone which introduces group delay, audibility should be restricted to the transient type.

3)If you consider the stationary case, but at high levels - approaching a device's headroom - you might get secondary audible effects due to non-linear distortion in the frequency domain. Example: Imagine two transients which closely follow each other in the original signal. If enough group delay is applied, a portion of the first transient is superimposed on the second, resulting in an increase of amplitude. In reference e.g. to a publication of Bill Whitlock's (of Jensen Xformers), these changes can be higher than 10dB in some cases, exceeding the device's headroom and resulting in freq distortion.

Regarding transformers: Actually, they are a bit more complex than a simple inductor, aren't they? They inherently also have all sorts of parasitic inductances and capacitances, series and shunt type. Different parts of these take effect at different frequencies, resulting in a band pass characteristic, and ringing at a certain resonant frequency. Which of course also means non-linear phase!! All this can be overcome more or less satisfactory by good construction and appropriate external loading, but it has to be considered.
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Volker Meitz

danlavry

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Re: current lags voltage question
« Reply #3 on: October 04, 2004, 09:16:49 PM »

“1) Signals that fall into the stationary category (meaning superimposed sine waves that don't change rapidly in amplitude over time) of not too high a level shouldn't be an issue, since the averaging principle of the ear applies here. Obviously transients are the signals where the effect gets audible most. If a transient is subject to group delay, a portion of it's frequency band will arrive somewhat before or after the rest. Now the question is, what is the threshold of audibility, or in other words, what is the smallest time delay our ears can resolve?”

I tend to agree that the issue is more important at higher levels (faster slew rates) and for transients. A few comments regarding my “history” of linear phase:
I recall the days where people thought phase is not important. But during the 80’s, when I was designing the first audio converters for Analog Solutions (division of Silicon General) and later the first Apogee DA and AD, I already cared about linear phase very much. The Apogee anti aliasing and anti imaging filters I used (before the development of upsampling and oversampling concepts) were designed and manufactured by Christoff Hielderbeg (Switzerland) and with near 10 poles in each filter, the phase lag was huge. I believe a good ear could tell phase mismatch of tens of degrees.

Now, you view it from a hearing standpoint, and also from room acoustic and reflection standpoint, and that is fine and correct. But I can add to it a very technical angle that happens way before the sound gets to the ear. And it does not have to be a transient either.
Take a 1KHz square wave, with say 20KHz bandlimit. Check the rise time. Now let me re arrange the phase from liner to “anything I wish”. I did just that and the waveform looked like a “mushed up” triangle wave with 1/3 the slew rate!!!

What am I getting at? The circuitry ahead may react differently to different waveshapes.        

"Regarding transformers: Actually, they are a bit more complex than a simple inductor, aren't they? They inherently also have all sorts of parasitic inductances and capacitances, series and shunt type. Different parts of these take effect at different frequencies, resulting in a band pass characteristic, and ringing at a certain resonant frequency. Which of course also means non-linear phase!! All this can be overcome more or less satisfactory by good construction and appropriate external loading, but it has to be considered."

I am glad you said it. Of course they are different than inductors. And they are a good example for a circuit element reacting differently to a signal with non linear phase. The magnetizing of the core depends on the wave shape (area under the curve – volt second).
That square wave with non linear phase has a different “volt second” integral. There are many other examples of phase impacting the circuit behavior (such as the very critical sample hold circuits, speakers and more).

BR
Dan Lavry
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volki

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Re: current lags voltage question
« Reply #4 on: October 06, 2004, 01:55:17 PM »

Quote:

 I believe a good ear could tell phase mismatch of tens of degrees.

This would be largely frequency-dependant, no? But maybe psychoacoustics are too off-topic for this thread...? I would really like to see any more recent publishings about the subject. Have browsed the AES library, but couldn't find too much. Maybe someone can give me a hint?

Quote:

Take a 1KHz square wave, with say 20KHz bandlimit. Check the rise time. Now let me re arrange the phase from liner to “anything I wish”. I did just that and the waveform looked like a “mushed up” triangle wave with 1/3 the slew rate!!!

To be clear, we're still talking about a signal to be yet inserted into the circuitry under test?

Quote:

What am I getting at? The circuitry ahead may react differently to different waveshapes.

I figure your statement means a generalization of what i wrote under 3) last time?  
So you're talking about circuitries with different slew rates? Producing different amounts of complex distortion, such as Dynamic Intermodulation, etc.? Which are sublte, and the correlation of hearing and measurement of which obviously isn't very easy. - I haven't had a chance to get into technical detail on the subject of AD converters yet, but I suppose the phenomenon would also apply for these?

 
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Volker Meitz

danlavry

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Re: current lags voltage question
« Reply #5 on: October 06, 2004, 03:31:20 PM »

I figure your statement means a generalization of what i wrote under 3) last time?
So you're talking about circuitries with different slew rates? Producing different amounts of complex distortion, such as Dynamic Intermodulation, etc.? Which are sublte, and the correlation of hearing and measurement of which obviously isn't very easy. - I haven't had a chance to get into technical detail on the subject of AD converters yet, but I suppose the phenomenon would also apply for these?


Yes, I am not getting into the hearing side of the issue, as indicated by the first thread defining what this forum is about.

Regarding the technical: Yes, a periodic signal with the same harmonic content, but different phase between harmonics yield a different time domain picture (different wave shape).
Some circuits may be impacted by it more than others. The transformer is a good example. The SH (sample hold) of an AD is a good example. I stated a couple of examples:

Slew rate change will probably not be a big deal with most OPamps to day, but with a SH, you are often on the hairy edge. Any available margin in slew rate is gladly traded off for lower feedthrough and lower droop of the hold cap...

Think of it from a power amplifier or transformer stand point:
You change the phase and in fact you change the power itself. The power is found by squaring the waveform... Take a square wave and the power is at "some maximum". Now much with the phase, and pretty soon the power is a lot lower, while the harmonics are still all present. That can impact a circuit, such as a power amp.

Regarding audibility of linear phase, this is the wrong forum for that. I accept it as a fact. I heard it myself.

BR
Dan Lavry

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