To crossfeed or not to crossfeed? That is the question...

Discussion in 'Sound Science' started by jasonb, Oct 21, 2010.
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  1. pinnahertz
    An exaggeration, but I can see how you might feel that way.
    90 degree phase shift networks have been used in audio matrix systems for 50 years (4-2-4 quad, Dolby Stereo, etc.) I'm sure you can figure out how to do it.
    Well, this is interesting! I went back to look for examples where you have declared yourself right and anyone else who doesn't agree is an idiot (or words to that effect), claimed that 98% of all recorded stereo music benefits from cross-feed, and if anyone doesn't agree they're spatially deaf (or worse). It looks like the thread has been "cleaned up" just a tad, large groups of posts are now gone, others edited. But I do have all the originals in email notifications. I guess citing all those many occurrences would not benefit the thread and would just get deleted again. I can forward all of them to you privately if you like.
    I'm not the one proclaiming superiority of concept, intelligence or hearing ability.
    Oh look! There's an example now!
    ...and you don't see what I mean from the above? I highlighted a few things to help you out. The red one, in particular, keeps coming up again and again. People's preferences make them ignorant, spatially or otherwise.
    You've sensed pretty much everything incorrectly about me, so why not just add that one to the growing list?
    And THAT from the same one who denies that choices in stereo perspective in headphones could possibly be artistic intent!

    Back to the deleted posts (about a month's worth) and edits for a second, I really have no issue with that, but it does indicate something. It has become quite clear that what little educational benefit this thread might have once had has been obliterated by intense propaganda promoting a polarized, but scientifically unproven viewpoint. I recall a time in the not too distant past on this forum when threads would have been locked for less. If education is at all important, perhaps a return to the actual scientific method in the sound science forum should be considered rather than foot stamping, and the synthesis of terminology, statistics, and pseudofacts.
     
    Last edited: Jan 3, 2018
  2. 71 dB
    Yes, audio matrix systems use 90° phase shift, but it's not "trivial." The networks are an approximation of 90° phase shift within a "narrow" frequency band. The question is whether this can give better results than my vivid mono algorithm.

    I think this method could work:

    (1) Break the original signal into narrow band partial signals (say 10 octave bands).
    (2) Allpass flter each partial signal so that the 90° phase shift hits the octave band middle frequency.
    (3) Construct original phase shifted signal summing all these partial signals together.

    Since the partial signals overlap a bit and in every partial signal the frequencies below middle frequency are phase shifted less than 90° and frequencies above the middle frequency more than 90°, summing these overlapping octave band will give pretty "flat" ~90° phase shift over the whole audio band.
     
  3. WoodyLuvr
    A quick question: your DIY crossfeed adapter is specifically made for the HD598? What year is your HD598? Any modifications? Cheers.
     
  4. pinnahertz
    No, that's incorrect. They are relatively trivial, were realized with basic analog circuitry, and were pretty dead-on across the audio band.
    Not necessary!
    Getting warm....
    You're overthinking it. Think about doing that in 1968. And think not global spectral phase shift, but relative inter-channel phase.
     
  5. 71 dB
    Not made specifically really for HD598. Works with any headphone I believe. I bought my HD 598 in 2011, serial number 0500001929. Earpads and headband padding renewed a few months ago. That's it.
     
    WoodyLuvr likes this.
  6. 71 dB
    I found this example of an 90° phase shifter:

    Hilbert transformer.gif
    Source: http://www.microwave.gr/content/view/48/67/

    This works in audio band 50-5000 Hz with a theoretical phase error of ±0.0607° (accurate as hell, but real life tolerances make the error much bigger).
     
  7. 71 dB
    Okay, I wrote today a nyquist-plugin that simulates that 90° phase shifter above. The phase shift looks very accurate, but there's high frequency attenuation happening so that 20 kHz is down about 4 dB. All I can think of causing this is accuracy problems with low pass filter. The nyquist code looks like this:

    ;nyquist plug-in
    ;version 2
    ;type process
    ;name "Hilbert Transformer"
    ;action "Hilberting..."
    ;info "90 degrees phase shifter.\nWritten Jan. 4, 2018.

    (setf sigl (aref s 0))
    (setf sigr (aref s 1))

    ;; Left Channel All Pass Filters

    (setf sigl (sim (lp sigl 51) (mult -0.5 sigl)))
    (setf sigl (sim (lp sigl 201) (mult -0.5 sigl)))
    (setf sigl (sim (lp sigl 677) (mult -0.5 sigl)))
    (setf sigl (sim (lp sigl 2354) (mult -0.5 sigl)))
    (setf sigl (mult 32 (sim (lp sigl 16442) (mult -0.5 sigl))))

    ;; Right Channel All Pass Filters

    (setf sigr (sim (lp sigr 15) (mult -0.5 sigr)))
    (setf sigr (sim (lp sigr 106) (mult -0.5 sigr)))
    (setf sigr (sim (lp sigr 369) (mult -0.5 sigr)))
    (setf sigr (sim (lp sigr 1246) (mult -0.5 sigr)))
    (setf sigr (mult 32 (sim (lp sigr 4881) (mult -0.5 sigr))))


    (if (arrayp s)
    (vector (abs-env
    sigl)
    (abs-env
    sigr)))
     
  8. adamlr
    [QUOTE="jasonb, post: 7007848, member: 159987"

    "A delayed, lowpass-filtered version of the opposite channel is added to the current channel. The delay is achieved bs2b-style using a single high shelve filter giving about 0.5 ms delay. After that, the signal is mixed without phase delay with 12 dB attenuation. In addition, there is a small reverb based on Haas stereo widening effect of 30 ms ping-pong buffers."

    I have also used one of the crossfeed plugins that are available for winamp with good results as well.
    [/QUOTE]

    you blew my mind with this. sounds like its a common deal but ive never heard of it.
    you listen to your music through an effect that, in addition to playing the standard L R, also send them into each-other, with a lower level, tiny delay, a filter and reverb?
     
  9. 71 dB
    Think about what happens when you listen to loudspeakers.

    Your left ear hears the (direct) sound from left speaker.
    Your right ear hears the (direct) sound from right speaker.

    But it doesn't end here.

    Your left ear hear the sound from right speaker, delayed because of additional distance of about 10 cm or 4 inches and filtered because going "round" the head.
    Your right ear hear the sound from left speaker, delayed because of additional distance of about 10 cm or 4 inches and filtered because going "round" the head.
    Your ears also receive early reflections from surfaces, your upper body, furnitures etc.
    Your ears also hear the reverberation in the room.


    Nobody thinks there's something funny about these things and most recordings are mixes in studios with speaker for this kind of situation (the acoustic environment in a studio is much "better" and more controlled than a typical living room is, but anyway…)

    When you listen to headphones:

    Your left ear hears the left channel.
    Your right ear hears the right channel.

    That's pretty much it. Open headphones leak some sound and there is very minor acoustic crosstalk happening, but unless you do something to the signal entering your headphones, none of the violet stuff is happening. That's why some people including me find headphone listening without crossfeed unnatural, annoying, spatially broken and tiring.
     
    adamlr likes this.
  10. Erik Garci
    Biometrics will be used by Creative's Super X-Fi for headphones. It will be demonstrated at CES next week.

    https://us.creative.com/sxfi/
     
    jgazal likes this.
  11. RRod
    But aren't the mics already adding in acoustic crosstalk? If I record using binaural mics in my ears, for instance, I would want no contralateral content out of the playback speakers. It would seem that the argument is that more typical micing and mixing schemes are judged based on normal speaker playback.
     
    jgazal likes this.
  12. 71 dB
    More or less yes. Depents on how the recording is mixed. You can record instruments with separate mono mics and hard pan them for example. Binaural recordings by definition have correct amount of channel separation (correct spatial information) and should of course be listened to without crossfeed (that's why crossfeeders have off/bypass switch). The things is, binaural recordings are VERY rare. I think I have two CDs (out of my ~1500 discs) having binaural sound. Most recordings are recorded and mixed so that ILD and ITD content is too much for headphones without crossfeed. Stereo mics that are more than about 10 inches away from each other produce too much ITD and the directivity + set up of mics easily produce too much ILD.
     
  13. jgazal
    Besides spot close microphones on each instrument that only captures mono direct field and are downstream mixed, what commonly used stereo spaced pair of microphones arrangements are more than 10 inches away?

    I have been told that usually ILD is not encoded:

    I see what you mean when choosing different spacing with A-B stereo pairs:

    From your ~1500 discs how many were recorded with stereo A-B pairs spaced more than 17 cm?

    And how do you know exactly how much ITD you need for each type of recording or mixing?
     
    Last edited: Jan 5, 2018
  14. 71 dB
    AB pair can be close to each other, but also a few meters (10 feet!) away.

    Maybe 500? The "needed" amount is 0-640 µs.
     
  15. jgazal
    I see it.

    So do you digitally analize the ILD and ITD parameters in the recording using some software the allows to do that and then you are able to know how the recording was made and how much ILD and ITD it has?

    And then do you adjust your algorithm accordingly?

    Do you mind telling me what sofware I can use to discover ILD and ITD from a given recording?

    I beleive the Realiser with a crossfeed free PRIR would allow to spatially perceive those ITD differences, but it would be nice to have a sotware that allows to numerically/quantitatively confirm such perceptions...
     
    Last edited: Jan 5, 2018
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