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Crossfeed frequency response

post #1 of 8
Thread Starter 

I was testing some crossfeeds (headfit, isone, xfeed) and noticed that there is some minor frequency response changes.

Are these frequency changes intentional?

 

Using a program that measures the frequency response of VST plugins (http://www.savioursofsoul.de/Christian/programs/measurement-programs/),

I noticed that most crossfeeds (above metioned) have a dip around 2-3khz and small dips in the higher range.

 

Should I leave these frequency shifts alone? Or is it more accurate to use a eq and invert the effect?

post #2 of 8

With crossfeed frequency response is a little more complicated because instead of having two independent frequency responses (one for each channel), you have four. Combining the direct and crossfeed frequency responses is IMO the wrong thing to do because they don't get feed the same signal (unless playing mono.) However, one should expect wonky results by doing so:

 

http://www.stereophile.com/content/headroom-total-bithead-headphone-amplifier-measurements (figure 1)

 

I think I was able to confirm some of those measurements (with my Total BitHead) through the humble mic input of my laptop (crossfeed engaged with both channels driven in-phase, and no crossfeed.)

post #3 of 8

Proper cross feed systems depend on HRTF, which includes an element of time delay, usually carefully chosen group delay pinch-hitting for real time delay.  If you were to combine a delayed and non-delayed signal together the result would indeed be wonky response.  If you then add the deliberate response contour that comes along with an HRTF based system, it will look pretty strange.  But, the actual response  from a point source in space arriving at your eardrum will also be pretty wonky because the same thing is happening out there in life, in fact even more complicated because of many other reflections, unless you live in an anechoic chamber.  But it all gets sorted out by using two ears and the brain to process that all into directional cues, wonky (I do love that word) response and all.  All those cross-feed things in headphone amps are trying to do is simulate the natural cross feed at the hears of two speakers in space in front of you at probably +/- 30 degrees or so, which gets the headphone image someone out of the head and in front of the listeners.  That's when they work right, of course.  

 

The response you see is a result of the processing.  

post #4 of 8
LOL! I actually like and use crossfeed occasionally depending on the recording. When I said I felt combining the direct and crossfeed channels was the wrong thing to do, I meant measurement-wise. I feel each of the 4 channels should be characterized individually. Sorry if my post was a bit misleading.
Edited by ultrabike - 2/21/13 at 12:12am
post #5 of 8
Quote:
Originally Posted by jaddie View Post

Proper cross feed systems depend on HRTF, which includes an element of time delay, usually carefully chosen group delay pinch-hitting for real time delay.  If you were to combine a delayed and non-delayed signal together the result would indeed be wonky response.  If you then add the deliberate response contour that comes along with an HRTF based system, it will look pretty strange.  But, the actual response  from a point source in space arriving at your eardrum will also be pretty wonky because the same thing is happening out there in life, in fact even more complicated because of many other reflections, unless you live in an anechoic chamber.  But it all gets sorted out by using two ears and the brain to process that all into directional cues, wonky (I do love that word) response and all.  All those cross-feed things in headphone amps are trying to do is simulate the natural cross feed at the hears of two speakers in space in front of you at probably +/- 30 degrees or so, which gets the headphone image someone out of the head and in front of the listeners.  That's when they work right, of course.  

 

The response you see is a result of the processing.  


I think you should be contributing more to head-fi !! 

post #6 of 8

I do what I have time for...

post #7 of 8
Quote:
Originally Posted by jaddie View Post

I do what I have time for...


Thats a reasonable response...but your posts are informative, and head-fi could do well with some professional opinion....thats IMO.

post #8 of 8

I've developed xfeed for fb2k and jaddie basically wrote what it's all about.

 

The most basic crossfeed takes the left channel signal, attenuates it (interaural level difference), filters it*, delays it (interaural time delay) and mixes it into the right channel and vice versa. Mixing in a delayed version is basically a comb filter. The first notch happens to fall between 2 - 3 kHz.

*) simple crossfeeds use a lowpass filter that attenuate high frequencies which is why the dips get smaller with increasing frequency

 

 

This also happens if you're listening with stereo speakers, to some extent at least.


Edited by xnor - 2/21/13 at 5:13pm
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