Topic: Convolve CSD plots with minimum phase FIR filter? (Read 9121 times)

arnaud · « **Reply #10 on:** September 10, 2012, 04:22:26 AM »

To me, the most interesting part of xnor's experiment was to show that the headphone had pretty much minimum phase characteristics (because the equalizing filter based on the inverse of the
Magnitude response function was a minimum phase fir filter). This is something I had doubts about before considering the mix of direct / reverberant field in headphones.

I do agree that the reason why it looked so good is that this was purely convolution of an impulse response with its minimum phase inverse (an ideal inverse of a minimum phase headphone would produce a perfect wall in the csd).

In that sense marv, you should get excellent results until you start messing with robustness (you know better than me what happens when you reseat the headphone). The art comes in to having a robust response to correct.

This joins the discussion we had in the other thread about square wave response (http://www.changstar.com/index.php/topic,496.msg9399.html#msg9399 ). Actually, I could just replaced the DF / ID equlization targets with the inverse of the headphone response and could show the example result as I am using same process as xnor did to remove the influence of the measuring head on tyll's data.

ultrabike · « **Reply #11 on:** September 10, 2012, 07:31:00 AM »

I think that the headphone that xnor used was indeed minimum phase in most of the audio band. But I'm not certain all are.

Nevertheless, I have a feeling most headphones are "mostly" minimum phase by brief inspection of the impulse response... If you set t = 0 at the peak of the impulse response of most headphones, most of the energy lies at t > 0, i.e. energy is concentrated at the "front" of the impulse response. This means little group delay, which is an indication of minimum phase.

PelPix · « **Reply #12 on:** September 10, 2012, 08:24:02 AM »

Quote from: ultrabike on September 10, 2012, 07:31:00 AM

I think that the headphone that xnor used was indeed minimum phase in most of the audio band. But I'm not certain all are.

Nevertheless, I have a feeling most headphones are "mostly" minimum phase by brief inspection of the impulse response... If you set t = 0 at the peak of the impulse response of most headphones, most of the energy lies at t > 0, i.e. energy is concentrated at the "front" of the impulse response. This means little group delay, which is an indication of minimum phase.

Is it even possible for a sound emitter to be anything but minimum-phase? If you hit a drum, it doesn't start out quiet, go to a peak, and then go back to being quiet (and it CERTAINLY doesn't start ringing before you hit it

)

ultrabike · « **Reply #13 on:** September 10, 2012, 08:29:00 AM »

Quote from: PelPix on September 10, 2012, 08:24:02 AM

Quote from: ultrabike on September 10, 2012, 07:31:00 AM
I think that the headphone that xnor used was indeed minimum phase in most of the audio band. But I'm not certain all are.

Nevertheless, I have a feeling most headphones are "mostly" minimum phase by brief inspection of the impulse response... If you set t = 0 at the peak of the impulse response of most headphones, most of the energy lies at t > 0, i.e. energy is concentrated at the "front" of the impulse response. This means little group delay, which is an indication of minimum phase.

Is it even possible for a sound emitter to be anything but minimum-phase? If you hit a drum, it doesn't start out quiet, go to a peak, and then go back to being quiet (and it CERTAINLY doesn't start ringing before you hit it )

LOL! Check out the impulse response of the Ultrasone Edition 10 at IF...

PelPix · « **Reply #14 on:** September 10, 2012, 08:36:43 AM »

Quote from: ultrabike on September 10, 2012, 08:29:00 AM

Quote from: PelPix on September 10, 2012, 08:24:02 AM
Quote from: ultrabike on September 10, 2012, 07:31:00 AM
I think that the headphone that xnor used was indeed minimum phase in most of the audio band. But I'm not certain all are.

Nevertheless, I have a feeling most headphones are "mostly" minimum phase by brief inspection of the impulse response... If you set t = 0 at the peak of the impulse response of most headphones, most of the energy lies at t > 0, i.e. energy is concentrated at the "front" of the impulse response. This means little group delay, which is an indication of minimum phase.

Is it even possible for a sound emitter to be anything but minimum-phase? If you hit a drum, it doesn't start out quiet, go to a peak, and then go back to being quiet (and it CERTAINLY doesn't start ringing before you hit it )

LOL! Check out the impulse response of the Ultrasone Edition 10 at IF...

Ultrasones don't count; they defy our pitiful laws time and space to be terrible in every possible reality that this universe contains.

The Nickelback of the headphone world...

ultrabike · « **Reply #15 on:** September 10, 2012, 09:09:03 AM »

LOL! real world systems are causal but don't necessarily react immediately to an stimulus.

Bellow is a linear phase (non-minimum phase) impulse response, and it's minimum phase approximation (through rceps,) to give a feel as to what is meant by "energy in the front."

It is not a very good approximation BTW, if we go from 200 to 2000 samples, the approximation improves (I don't think the FR minimum phase equivalent of an arbitrary system is guaranteed to exist...)

Anyhow, the point is just to show how to roughly get a feel for what is minimum phase and what is not by looking at the impulse response, that's all...

anetode · « **Reply #16 on:** September 10, 2012, 09:46:22 AM »

I've always wondered whether the pre-ringing as a result of a DAC filter has any audible effect at the transducer. I can see it getting... confused

arnaud · « **Reply #17 on:** September 10, 2012, 10:06:10 AM »

Unfortunately, these concepts can get a little complicated. It turns out a typical room response is not actually minimum phase. See the introduction here for example (in particular the reference to the work from Lyon about room phase response) : http://www.acoust.rise.waseda.ac.jp/~takahashi/AES28-000046.pdf

Simply said: when you're close to the source, with mostly direct field / little room reflections, you essentially see the "propagating phase" component. Once the reverberant field blends in (you can view it as a bunch of reflections from the walls, it turns into sharp peaks and notches in the magnitude response, at each acoustic resonance / node). The resulting phase is a bit of a mess and does not typically have minimum phase characteristics. The intuitive notion is that not all the speaker energy arrives at the same time. It is causal but it isn't minimum phase.

Now comes headphones and my lack of feel for it: I am still suspecting we could find examples of non minimum phase headphones where the reflected field is in equal or similar proportion to the direct field. Ultrasone is a good design candidate with a baffle plate in front of the driver. From looking at the latest Denon marvel (D7100), it seems ot could be a candidate as well.

ujamerstand · « **Reply #18 on:** September 10, 2012, 02:03:50 PM »

STAX sr-sigma?

Joe Bloggs · « **Reply #19 on:** September 10, 2012, 04:42:49 PM »

Quote from: wiinippongamer on September 10, 2012, 03:14:12 AM

I'm pretty sure there was no sound being produced/recorded from the headphones in the second graph. That said ringing/resonance does go hand in hand with FR peaks/dips.

Just a note about this (and similar posts): the waterfall plot in the second graph is essentially a vertical cliff from the first pixel. You can see the red line near the top on the left, it merges with the top and "disappears" in the middle of the graph.

Author Topic: Convolve CSD plots with minimum phase FIR filter? (Read 9121 times)

arnaud

Re: Convolve CSD plots with minimum phase FIR filter?

ultrabike

Re: Convolve CSD plots with minimum phase FIR filter?

PelPix

Re: Convolve CSD plots with minimum phase FIR filter?

ultrabike

Re: Convolve CSD plots with minimum phase FIR filter?

PelPix

Re: Convolve CSD plots with minimum phase FIR filter?

ultrabike

Re: Convolve CSD plots with minimum phase FIR filter?

anetode

Re: Convolve CSD plots with minimum phase FIR filter?

arnaud

Re: Convolve CSD plots with minimum phase FIR filter?

ujamerstand

Re: Convolve CSD plots with minimum phase FIR filter?

Joe Bloggs

Re: Convolve CSD plots with minimum phase FIR filter?

(adsbygoogle = window.adsbygoogle || []).push({});

Author Topic: Convolve CSD plots with minimum phase FIR filter? (Read 9121 times)

ujamerstand