Topic: Different Stuff (Read 8063 times)

Marvey · « **Reply #10 on:** August 11, 2012, 04:49:29 AM »

Yup - changes in phase can really result in some screwy square waves. This is why square waves (or step responses) are more suited for crossover design where filters will have a serious influence on said square wave.

I did make a bad assumption: while each successive harmonic does have less influence, the decibel scale is logarithmic, so the influence of the higher harmonics is much more significant than I thought.

For our viewing and masturbatory pleasure: HE400 square wave @ 30Hz with spectrum plot attached.

ultrabike · « **Reply #11 on:** August 11, 2012, 05:04:52 AM »

O M G. Look at the curves on that square wave!

ihasmario · « **Reply #12 on:** August 11, 2012, 05:24:47 AM »

10/10 post ultrabike. Could you link me to some literature to read up on phase, as it's not something I have looked at. Could you explain how the Hamming creates the ideal square wave pictured? Because I don't understand that part at the moment (Obviously I need to learn more).

I'm liking how this thread is shaping up. Thanks for the post, purrin!

I've heard all manner of absurd claims with square wave, including "showing you how fast the headphone is"

By the way, returning to the tile subject; how do we read inverted impulse responses? I've always wondered. Is it read exactly the same way? For example, if you had a dipole speaker, would the impulse response be inverted if you stood behind the speaker? Or is it simply an issue of polarity?

This begs the question further: Does it matter for dipole speakers (for example, a lot of stax phones seem to have an inverted impulse response)

ultrabike · « **Reply #13 on:** August 11, 2012, 08:32:58 AM »

Quote from: ihasmario on August 11, 2012, 05:24:47 AM

Could you explain how the Hamming creates the ideal square wave pictured? Because I don't understand that part at the moment (Obviously I need to learn more).

You and I both

. As far as I know, the requirement for a nice looking SW response are: filter/headphone/linear-system FR flat magnitude and linear phase across the SW frequencies. There are many FIR filters that satisfy those requirements (Like that Hamming FIR), but that is not what headphones necessarily are. Like Purrin said, your ears, FR, CSD, and Distortion plots will likely better serve you...

Marvey · « **Reply #14 on:** August 11, 2012, 04:51:26 PM »

Quote from: ihasmario on August 11, 2012, 05:24:47 AM

By the way, returning to the tile subject; how do we read inverted impulse responses? I've always wondered. Is it read exactly the same way? For example, if you had a dipole speaker, would the impulse response be inverted if you stood behind the speaker? Or is it simply an issue of polarity?

This begs the question further: Does it matter for dipole speakers (for example, a lot of stax phones seem to have an inverted impulse response)

We just flip the inverted response over - it's just polarity - switching the +/- on the wires will do. It doesn't matter. Some amps will invert polarity.

ihasmario · « **Reply #15 on:** August 16, 2012, 12:53:26 PM »

Quote from: purrin on August 11, 2012, 04:49:29 AM

I did make a bad assumption: while each successive harmonic does have less influence, the decibel scale is logarithmic, so the influence of the higher harmonics is much more significant than I thought.

Care to comment/demonstrate how much when you have the chance? Ty

Marvey · « **Reply #16 on:** August 21, 2012, 05:33:07 PM »

The full spectrum of 30Hz SW shows that even past the 9th harmonic, the higher frequencies still have a good amount of influence on the shape of the square wave. (I thought the higher harmonics would just die, but I mistakenly did not account for the log nature of our hearing.)

Analysis of several (more than one) square waves to determine FR is not necessary wrong since as I mentioned before, a SW can be obtained from FR (but not the other way around). A much better way to get a grasp of a headphone's tonal characteristics (or frequency response) would be by looking at the FR graph instead of a set of two or three square waves. Don't ya think?

P.S. One minor issue which I forgot to mention is that if the FR of a headphone has a narrow bump at an even harmonic (which does not extend to the odd harmonics) of a square wave, the square wave won't catch it. This is very unlikely though.

arnaud · « **Reply #17 on:** August 21, 2012, 10:09:56 PM »

I've been meaning for some time to extract IR, SW, FR, CSD from idealized headphones to illustrate effect of roll off at both ends of the spectrum, presence of resonances and what not. The main objective was to highlight that SW don't bring much to the table and probably best left to where they come from (checking bandwidth / phase of electronic components).

I agree that SW on a headphone measurement takes some expertise to interpret because I believe it is an uncompensated result. Tyll's ID correction is magnitude as function of frequency only, you can't apply it to a time domain response (IR, step or SW) without creating an artificial filter under some assumption, minimum phase for example even though I can't imagine an HRTF is minimum phase. Anyhow phase itself takes an interesting meaning when the equalization curve is not 1 direction (FF) but an average of several (DF or Tyll's ID).

Personally, I can't interpret a SW although I can imagine how some basic headphone traights will make it deviate from the ideal (which isn't a prefect swuare as discussed above). In particular:
- HF roll off should be visible as rounded off SW (provided the SW fundamental is high enough, say 1kHz)
- Low Q resonances with cause large overshoot and ringing at every corners of the SW (provided the SW harmonic content is such that it meshes reasonably well with the headphone ringing frequency(ies)
- Large phase variation across the frequency range of the SW must indeed cause interesting changes, the possibilities are endless here

As you can see, there are too many provisions in the usage of the SW data to make it reliable. It's a crap shoot at best and one is better off looking at FR, CSD, IR.

Im(not so humble)o...

arnaud · « **Reply #18 on:** August 21, 2012, 10:20:04 PM »

Quote from: ultrabike on August 11, 2012, 04:36:56 AM

What is not shown at IF is the FR phase of the headphone which can mishappen a square wave. I had an exchange with xnor at HF that got me motivated to write some code:

http://www.head-fi.org/t/612665/how-far-can-eq-really-go-towards-truly-equalizing-headphones/75#post_8455637

At the bottom of the code there are a set of plots related to 3 different filters: A 5th order Butterworth IIR, a 5th order Chevyshev, and a windowed-based (Hamming) linear phase FIR. Shown are the FR magnitude response of the 3 filters which are flat from 200Hz and above. Only the linear phase filter lets the 300Hz square wave through virtually untouched. The rest look rolled off.

The square wave high frequencies are not absent, they are just linearly re-combined with it's low frequencies with a modified phase due to the filter FR phase.
.

Ultrabike, nice, this is part of what I meant to do, but never did (what a lazy b. i am

)

Tyll Hertsens · « **Reply #19 on:** August 24, 2012, 02:25:50 AM »

Hey dudes! Nice yabber. Gonna have to look around a bit.

Author Topic: Different Stuff (Read 8063 times)

Marvey

Re: Different Stuff

ultrabike

Re: Different Stuff

ihasmario

Re: Different Stuff

ultrabike

Re: Different Stuff

Marvey

Re: Different Stuff

ihasmario

Re: Different Stuff

Marvey

Re: Different Stuff

arnaud

Re: Different Stuff

arnaud

Re: Different Stuff

Tyll Hertsens

Re: Different Stuff

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Author Topic: Different Stuff (Read 8063 times)