Any thoughts on Impulse Response and Group Delay testing?

[Rick Turner]

The title just about says it.

As we know, inductors introduce considerable phase distortion into signals, and the phase lag is different at various frequencies. I'm interested in learning more about all of this. I see no references on line that are related to this other than what is out there about audio transformers and large power transformers.

My interest goes back to George Cardas' comments about my Litz wire pickups probably having better group delay characteristics based on my subjective observation of tighter "faster" low end response. I'd like to get beyond the subjective and into real measurements with this.

Time/phase is the 3rd dimension in audio...the depth behind ordinary magnitude measurements. Not all frequencies pass through audio systems at the same rate...that is a simple fact...and I think it greatly affects what we hear.

One idea is to compare the impulse response of a magnetic pickup vs. piezo, perhaps using a string pulled by a breaking loop magnet wire (a very repeatable string pluck method). Would Lissajous 'scope testing with the input signal to a send coil and the output of the pickup be sufficient to see phase shift at different frequencies? Does the signal send coil itself distort what one might see? Should the signal send coil be some rational fraction of the inductance of the pickup coil(s) under test?

Any thoughts?

I hope this is not too scientific... :-)

[bbsailor]

[QUOTE=Rick Turner;345094]The title just about says it.

As we know, inductors introduce considerable phase distortion into signals, and the phase lag is different at various frequencies. I'm interested in learning more about all of this. I see no references on line that are related to this other than what is out there about audio transformers and large power transformers.

My interest goes back to George Cardas' comments about my Litz wire pickups probably having better group delay characteristics based on my subjective observation of tighter "faster" low end response. I'd like to get beyond the subjective and into real measurements with this.

Time/phase is the 3rd dimension in audio...the depth behind ordinary magnitude measurements. Not all frequencies pass through audio systems at the same rate...that is a simple fact...and I think it greatly affects what we hear.

One idea is to compare the impulse response of a magnetic pi






HELP: I lost my internet connection in the middle of an edit. Can someone resstore my initial answer?

[salvarsan]

Sounds plausible.

Derivation of impulse response from MLS signals is 50 years old and there are plugins for audio workstations that will do it.

The practical method is signal induction using an 8 ohm drive coil.

[Rick Turner]

I wonder if using guitar strings themselves as the induction drive coil might be of interest...

[bbsailor]

I wonder if using guitar strings themselves as the induction drive coil might be of interest...

Rick,

See post #1 in this thread. http://music-electronics-forum.com/t14952-2/

And even more interesting, the strings do not need to be ferrous metal.

Joseph Rogowski

[Rick Turner]

OK, I dug up a Litz Tele neck pickup that I rewound many years ago. Here are the numbers:

DCR = 587.8 Ohms
L = 50.18 mH
Q = .536

So that brings us out past 80 K Hz if I'm getting the math right. Is this possible? Whew... Just checked the meter again, and those are the numbers. .05018/587.8 = .0000853691 = 85 K Hz Right?

This is with about 600 turns of 7x44 ga. Litz on a stock Fender Tele neck pickup core.

Fully understood that the strings need not be ferrous metal in a "string return pickup"...it's acting as a conductor in this case...it's a moving coil, so it would work with nylon cored metal wound strings. We first saw string return pickups in the early/mid 1970s on an electric violin; then Lane Poor went off in that direction along with Ron Armstrong, both of whom had worked for Alembic in the those days.

[Rick Turner]

And for a bizarre thought...wind a guitar (or other musical) string with magnet wire...and connect it up...wound guitar strings are just long coils; in fact they could be solenoids.

That's a public disclosure, by the way, whether it has any merit or not...

[bbsailor]

OK, I dug up a Litz Tele neck pickup that I rewound many years ago. Here are the numbers:

DCR = 587.8 Ohms
L = 50.18 mH
Q = .536

So that brings us out past 80 K Hz if I'm getting the math right. Is this possible? Whew... Just checked the meter again, and those are the numbers. .05018/587.8 = .0000853691 = 85 K Hz Right?

This is with about 600 turns of 7x44 ga. Litz on a stock Fender Tele neck pickup core.

Fully understood that the strings need not be ferrous metal in a "string return pickup"...it's acting as a conductor in this case...it's a moving coil, so it would work with nylon cored metal wound strings. We first saw string return pickups in the early/mid 1970s on an electric violin; then Lane Poor went off in that direction along with Ron Armstrong, both of whom had worked for Alembic in the those days.

I'll try to provide the answer that was deleted below during my edit.

All inductors operate according to the laws of physics in that current rises according to the coil's time constant (TC). The TC is calculated by dividing the coil inductance in Henries by the coil resistance plus any series resistance. A typical 2H pickup that is 6,000 ohms has a TC of 2/6000 or .0003333 seconds. This means that it takes .0003333 seconds for the coil current to rise to 63 percent of it's maximum and in another .0003333 seconds or 2 TCs to 85 percent of maximum and in another .0003333 seconds to about 97 percent of maximum. One TC, in this case, is 1/.0003333 3000 Hz. It is interesting that the TC of 3,000 Hz is near the resonance of a typical pickup of this type.

The initial string attack carries the greater part of the perceived group delay characteristics and may account for a faster and tighter low end response by keeping the group delay more tightly alined so that while the fundamental is providing it's initial impression, other harmonics are staying more aligned. In three TCs .0009999 (or 1 milli second) seconds pass and that is the area below which most notes on the guitar occur. The tonal flavor of a string occurs after the initial attack with the various harmonics not only being present but also being aligned or non-aligned with the fundamental.

I have been using various size wires in my low impedance pickups where the string loop is about 5" to 6" long of AWG 12 to AWG 6 feeding a current transformer single turn primary (CSE187L, 500 turn secondary). I suspect in addition to the tonal changes I hear with thicker wire, I am also hearing the different group delay characteristics when I use many insulated strands of smaller wire (like Litz wire) to equal a single strand of thicker wire. Trying the keep the primary string loop in the 500 micro-ohm range provides an output impedance in the 220 ohm range. AWG 11 is 105 microohms per inch so 5.5 inches of AWG 11 x 250,000 (500 squared) equals 144 ohms plus the impedance of the transformer primary being is series to form a current loop. The transformer primary adds about another 80 ohms so the total output impedance is about 225 ohms. This is a good range to directly feed a mic input transformer or a mic mixer via an XLR connector using balanced line.

I wish I could give you some references but I too am plowing new ground.

Joseph Rogowski

Additional thought: your 50 mh coil has a TC frequency near 11.7Khz 1/ .0008536. Most of the musical spectrum of the guitar/bass including the most upper harmonics are well below this frequency number and that may account for the tighter sound.

[bbsailor]

And for a bizarre thought...wind a guitar (or other musical) string with magnet wire...and connect it up...wound guitar strings are just long coils; in fact they could be solenoids.

That's a public disclosure, by the way, whether it has any merit or not...

Rick,

Try this. Obtain an 8 ohm to 20K (or higher) miniature transformer. Alligator clip the 8 ohm side across one test string behind the nut and bridge. Make sure that other strings are not shorting out the test string. Attach the transformer secondary to the amp input. Hand hold a magnet and bring it near the string as you pick it. You will have an output near 100mv. A longer magnetic field along the string will increase the output. With a trussrod ground return, you could electrically balance each string by using a separate transformer for each string. The strings are acting like the ribbon in a ribbon microphone. It's a moving conductor in a magnetic field.

Joseph Rogowski

[Rick Turner]

Totally understood, Joe. I've read a lot on ribbon mics and had a great meeting with another Rick...cofounder of Royer microphones...at NAMM last January. One trick to try is a balanced magnetic field for a string return pickup that would allow much higher flux density without creating false string harmonics. neoD's between the strings in a NS, SN, NS, SN, etc. configuration...with seven magnets...got to cover the outside of E and E...would be interesting.

You have to be careful with NS, NS or any magnet configuration that tends to flip magnetic domains in the string...Barkhausen distortion...another time domain issue. Those little domains don't want to flip from N to S all at the same time in a ferrous material. Another inherent issue with transformers...

[bbsailor]

Totally understood, Joe. I've read a lot on ribbon mics and had a great meeting with another Rick...cofounder of Royer microphones...at NAMM last January. One trick to try is a balanced magnetic field for a string return pickup that would allow much higher flux density without creating false string harmonics. neoD's between the strings in a NS, SN, NS, SN, etc. configuration...with seven magnets...got to cover the outside of E and E...would be interesting.

You have to be careful with NS, NS or any magnet configuration that tends to flip magnetic domains in the string...Barkhausen distortion...another time domain issue. Those little domains don't want to flip from N to S all at the same time in a ferrous material. Another inherent issue with transformers...

Rick,

If you attach the transformer like I mentioned, raise the amp gain and move the magnet along the string, you will hear a crackling sound of the electrons being displaced or the Barkhausen Effect. Yes, 7 neoD magnets placed between the strings will work but I have used 6" long plastic flexible magnets under the strings from the neck heel to the bridge to capture a more acoustic quality from magnetizing a longer string length and more harmoic locations. By changing the length of the magnets you can tune and adjust the individual string output to obtain a more balanced output.

Joseph Rogowski

[Rick Turner]

I wish there was more documentation of what Lane Poor had been doing in this realm in the 1980s. He was adjusting magnet length under strings for voicing, too. I know he did a presentation for the folks at C. F. Martin, but they thought what Lane presented just looked like a bunch of popsicle sticks under the strings and rejected the whole thing.

Lane disappeared for quite a while, and it seems that he was suffering for a long time from undiagnosed Lime disease...not good. He'd thought it was lead poisoning from too much exposure to solder, but that may not have been right. He was our main pickup winder and electronics tech for a couple of years there in the mid 1970s. And he was really good.

[Joe Gwinn]

The title just about says it.

As we know, inductors introduce considerable phase distortion into signals, and the phase lag is different at various frequencies. I'm interested in learning more about all of this. I see no references on line that are related to this other than what is out there about audio transformers and large power transformers.

Back a few years ago, I did some impulse-response tests on a random collection of pickups. The setup was to use a signal generator to deliver ten microsecond pulses to a drive coil near the pickup. The drive coil had a DC resistance of 50 ohms (to match the generator) and relatively low inductance. (Don't recall the details, but I can look it up if needed.)

I started at a lot of impulse responses, played with putting sheets of copper, aluminum, steel, stainless steel (type 303 or 304). Can't say that the response functions were that helpful.

My interest goes back to George Cardas' comments about my Litz wire pickups probably having better group delay characteristics based on my subjective observation of tighter "faster" low end response. I'd like to get beyond the subjective and into real measurements with this.

I've always doubted that Litz wire has much effect, because the copper diameter of #42 wires is far less than the skin depth of copper at audio frequencies. There must have been something else going on, like a difference in physical dimensions of the winding (and how much of the 3D magnetic field is intercepted) and/or different self-capacitance and/or self-inductance.

Time/phase is the 3rd dimension in audio...the depth behind ordinary magnitude measurements. Not all frequencies pass through audio systems at the same rate...that is a simple fact...and I think it greatly affects what we hear.

I do believe that linear phase is required to properly reproduce the attack transients. There have been a number of threads on this issue.

One idea is to compare the impulse response of a magnetic pickup vs. piezo, perhaps using a string pulled by a breaking loop magnet wire (a very repeatable string pluck method).

The breaking wire method is an oldie but goodie, originally was a bit of cotton thread that was burnt to release the string.

I would expect that the impulse response of a piezo pickup will be far more concentrated in time than that of almost any electromagnetic pickup, and will basically match that of what it's glued or clamped to.

Would Lissajous 'scope testing with the input signal to a send coil and the output of the pickup be sufficient to see phase shift at different frequencies?

The Lissajous figure of the pulse (X-axis) in comparison to the response (Y-axis) will be uninterpretable, just a spaghetti-bowl of squiggly squirming lines.

If one wants to see phase versus frequency, digitize the impulse response function, and take the Fast Fourier Transform. With impulse drive, all frequencies are in phase at the peak of the drive pulse.

Does the signal send coil itself distort what one might see? Should the signal send coil be some rational fraction of the inductance of the pickup coil(s) under test?

A properly damped send coil of many fewer turns of wire does not distort the pulse. The purpose of the damping (implemented as series resistance here) is to prevent ringing in the drive coil.

The drive coil and the pickup coil need not have any specific ratio of inductances. All that's needed is that the drive coil's self-resonant frequency be a factor larger than that of the pickup coil. Reducing the turns count reduces both inductance and self-capacitance.

[Rick Turner]

Joe,I think your math is off by an order of magnitude. I get 0.00008517 There's another zero in there, that should put the time constant out at 117 Khz, unless I'm doing this all wrong.

Also, understood re the drive coil. And yes, I'm headed into FFT measurements.

What I hear with piezos is extremely coherent and fast response. That's one of the reasons we delved into Mama Bear...to slow things down...selectively.

I shall set up the exploding wire thing again. It wasn't useful for our Mama Bear impulse tests, we wound up deriving those. I think we were getting too much air and not enough top nor torque action on the tops.

One of the things I tried to do with the Litz pickups was to make the coils have the same physical size as the original coils. I need to go back to those experiments and document things better.

[bbsailor]

Joe,I think your math is off by an order of magnitude. I get 0.00008517 There's another zero in there, that should put the time constant out at 117 Khz, unless I'm doing this all wrong.

Also, understood re the drive coil. And yes, I'm headed into FFT measurements.

What I hear with piezos is extremely coherent and fast response. That's one of the reasons we delved into Mama Bear...to slow things down...selectively.

I shall set up the exploding wire thing again. It wasn't useful for our Mama Bear impulse tests, we wound up deriving those. I think we were getting too much air and not enough top nor torque action on the tops.

One of the things I tried to do with the Litz pickups was to make the coils have the same physical size as the original coils. I need to go back to those experiments and document things better.

Rick,

Sorry, I left off one zero, but .00008517 (four zeros) divided into one is still 11.75Khz.

Joe R.