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A secret "known by signal processing experts" or nonsense?
adamdea
post Jan 10 2013, 17:16
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http://www.wickeddigital.com.au/index.php/...eally-necessary
Sets out what is described as a White Paper by Marco Manunta of M2Tech. It contains the following passage (my emphasis)

"But there is something more. Itís known by signal processing experts, and absolutely not popularized amongst music lovers, that converting an analog signal into a discrete-time one (as it happens when converting from analog to digital) destroys the phase information in the two top octaves of the resulting spectrum. In a CD-standard digital recording, all phase information are lost from 5.5kHz up to 22kHz,"

Can any signal processing experts comment on whether this is true or not, and if so why. As a mere layman I am not able to understand why it even might seem be true.
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adamdea
post Jan 10 2013, 23:08
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Thanks everyone, I thought it seemed like nonsense but I wanted to check unless it referred to some little known effect. I can understand that analog brickwall filters could mess with phase, but am relieved that there does not appear to be an insurmountable problem.

I may take this up with m2tech.
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Arnold B. Kruege...
post Jan 11 2013, 16:58
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QUOTE (adamdea @ Jan 10 2013, 17:08) *
Thanks everyone, I thought it seemed like nonsense but I wanted to check unless it referred to some little known effect. I can understand that analog brickwall filters could mess with phase, but am relieved that there does not appear to be an insurmountable problem.

I may take this up with m2tech.


Above some frequency in what we call midrange, the ears lose their ability to discern phase. This is because our ears are built like spectrum analyzers, but only what we call the real portion or amplitude is conveyed to the brain. If you wish to obtain phase information from a spectrum analyzer you need two independent kinds of (quadrature) data for each frequency band. Our ears only pass one kind of information to the brain for each frequency band above medium frequencies.

One consequence of this loss of phase information in our ear/brain interface is that massive amounts of phase shift (e.g. 1,000 degrees or more) can be applied to critical high resolution audio signals, with no discernible change in perception. The only caveat is that the phase shift applied to both channels must be essentially the same or else the phase shift will turn into response changes that will be audible. Above 5 Khz this situation dominates with total supremacy.

So even the old CD players with analog filters did have fairly well matched channels, and while what they did to phase was not numerically pretty, there wasn't any serious effect.

Another ugly thing that old CD players sometimes did is share the same DAC between the 2 channels so that their outputs were 1/2 sample time apart. If you electrically summed the two channels this led to a minor frequency response roll-off that on a really good day might be mildly audible.

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Ethan Winer
post Jan 11 2013, 21:27
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QUOTE (Arnold B. Krueger @ Jan 11 2013, 10:58) *
massive amounts of phase shift (e.g. 1,000 degrees or more) can be applied to critical high resolution audio signals, with no discernible change in perception. The only caveat is that the phase shift applied to both channels must be essentially the same or else the phase shift will turn into response changes that will be audible.


That's what I would have said if you didn't beat me to it. The only other time phase shift is audible, even when the channels are matched, is if the amount of shift is currently changing. My AES Audio Myths Workshop video demonstrates this. You can jump ahead to that part of the video at 47:45.

--Ethan


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jmvalin
post Jan 11 2013, 22:26
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QUOTE (Ethan Winer @ Jan 11 2013, 15:27) *
That's what I would have said if you didn't beat me to it. The only other time phase shift is audible, even when the channels are matched, is if the amount of shift is currently changing. My AES Audio Myths Workshop video demonstrates this. You can jump ahead to that part of the video at 47:45.


Actually, I did a bit of research on that topic, for the purpose of decorrelating channels to help stereo acoustic echo cancellation. What I found was that you can make phase shift inaudible even when channels aren't matched and while varying the amount of phase shift. What's important is only matching the channels below about 2 kHz, where the ear is sensitive to interaural phase difference (IPD). Beyond 2 kHz, you can get away with unmatched phase differences (which was the useful part of my research). As for changing the phase, the main difficulty is to do that without causing glitches or spectral distortion. With the help of some overlap-add, I was able to vary the phase shift without problem. Interestingly, when I tried varying the phase shift below 2 kHz, I could induce motion sickness just from listening to music with headphones.

See details in:
J.-M. Valin, Perceptually-Motivated Nonlinear Channel Decorrelation For Stereo Acoustic Echo Cancellation, Proc. Joint Workshop on Hands≠free Speech Communication and Microphone Arrays (HSCMA), 2008.
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Ethan Winer
post Jan 12 2013, 19:32
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QUOTE (jmvalin @ Jan 11 2013, 16:26) *
Actually, I did a bit of research on that topic, for the purpose of decorrelating channels to help stereo acoustic echo cancellation. What I found was that you can make phase shift inaudible even when channels aren't matched and while varying the amount of phase shift. What's important is only matching the channels below about 2 kHz, where the ear is sensitive to interaural phase difference (IPD).

Interesting, you've done more types of testing than I have. Also, when I said "the phase shift is changing" I should have been clearer. The type of phase shift effects I've tested vary the center frequency of the phase shift, not the amount of shift at a fixed frequency. So by "changing" I mean sweeping the frequency. Though I imagine changing the amount of shift at a fixed frequency would also be audible at midrange frequencies.

--Ethan


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Woodinville
post Jan 12 2013, 20:21
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QUOTE (Ethan Winer @ Jan 12 2013, 10:32) *
QUOTE (jmvalin @ Jan 11 2013, 16:26) *
Actually, I did a bit of research on that topic, for the purpose of decorrelating channels to help stereo acoustic echo cancellation. What I found was that you can make phase shift inaudible even when channels aren't matched and while varying the amount of phase shift. What's important is only matching the channels below about 2 kHz, where the ear is sensitive to interaural phase difference (IPD).

Interesting, you've done more types of testing than I have. Also, when I said "the phase shift is changing" I should have been clearer. The type of phase shift effects I've tested vary the center frequency of the phase shift, not the amount of shift at a fixed frequency. So by "changing" I mean sweeping the frequency. Though I imagine changing the amount of shift at a fixed frequency would also be audible at midrange frequencies.

--Ethan


So, try delaying a 10kHz gaussian impulse with a 4khz bandwidth by half a sample in one channel. Tell me you don't hear that in headphones, eh?


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Ethan Winer
post Jan 13 2013, 20:03
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QUOTE (Woodinville @ Jan 12 2013, 14:21) *
So, try delaying a 10kHz gaussian impulse with a 4khz bandwidth by half a sample in one channel. Tell me you don't hear that in headphones, eh?

If only I understood what that meant, or how to delay something by half a sample! biggrin.gif

I agree that left-right phase differences are audible. I haven't tested this enough to be more specific, but I'm glad you and others have.

--Ethan


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jmvalin
post Jan 13 2013, 20:17
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QUOTE (Ethan Winer @ Jan 13 2013, 14:03) *
QUOTE (Woodinville @ Jan 12 2013, 14:21) *
So, try delaying a 10kHz gaussian impulse with a 4khz bandwidth by half a sample in one channel. Tell me you don't hear that in headphones, eh?

If only I understood what that meant, or how to delay something by half a sample! biggrin.gif

I agree that left-right phase differences are audible. I haven't tested this enough to be more specific, but I'm glad you and others have.

--Ethan


Actually, I've no idea what a "10kHz gaussian impulse with a 4khz bandwidth" would be, but delaying something by half a sample (or any fractional number of samples) is very easy to do, e.g. using a sinc filter. One thing I should have been more precise about in my earlier comment... you can get away with modifying the phase as long as it's above ~2 kHz and as long as the group delay (derivative of the phase response wrt frequency) is small enough. But as I said, as long as you're careful with how you do it, it's amazing how much you can get away with when playing with the phase. The real difficulty when the phase is involved is really how messing it up can cause other artefacts. e.g. changes in the actual power spectrum.
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Posts in this topic
- adamdea   A secret "known by signal processing experts" or nonsense?   Jan 10 2013, 17:16
- - greynol   It's complete nonsense. But hey, it's the ...   Jan 10 2013, 17:31
- - saratoga   QUOTE (adamdea @ Jan 10 2013, 11:16) Can ...   Jan 10 2013, 17:33
- - DonP   QUOTE (adamdea @ Jan 10 2013, 11:16) http...   Jan 10 2013, 17:44
- - jmvalin   QUOTE (adamdea @ Jan 10 2013, 11:16) ...   Jan 10 2013, 18:19
- - Kohlrabi   Their website is filled with various other nonsens...   Jan 10 2013, 18:33
- - eahm   Thank you adamdea, now we all need a 5.6MHz/1mbit ...   Jan 10 2013, 19:01
- - mzil   QUOTE (adamdea @ Jan 10 2013, 12:16) http...   Jan 10 2013, 19:54
|- - db1989   QUOTE (mzil @ Jan 10 2013, 18:54) However...   Jan 10 2013, 21:09
- - adamdea   Thanks everyone, I thought it seemed like nonsense...   Jan 10 2013, 23:08
|- - Arnold B. Krueger   QUOTE (adamdea @ Jan 10 2013, 17:08) Than...   Jan 11 2013, 16:58
|- - dhromed   QUOTE (Arnold B. Krueger @ Jan 11 2013, 16...   Jan 11 2013, 17:12
||- - saratoga   QUOTE (dhromed @ Jan 11 2013, 11:12) QUOT...   Jan 11 2013, 17:22
|- - mzil   QUOTE (Arnold B. Krueger @ Jan 11 2013, 11...   Jan 11 2013, 17:58
||- - Arnold B. Krueger   QUOTE (mzil @ Jan 11 2013, 11:58) QUOTE (...   Jan 11 2013, 21:31
||- - mzil   QUOTE (Arnold B. Krueger @ Jan 11 2013, 16...   Jan 11 2013, 23:33
||- - Kees de Visser   You're being mean with old men (like me)   Jan 12 2013, 10:25
|- - Ethan Winer   QUOTE (Arnold B. Krueger @ Jan 11 2013, 10...   Jan 11 2013, 21:27
||- - jmvalin   QUOTE (Ethan Winer @ Jan 11 2013, 15:27) ...   Jan 11 2013, 22:26
||- - Ethan Winer   QUOTE (jmvalin @ Jan 11 2013, 16:26) Actu...   Jan 12 2013, 19:32
||- - Woodinville   QUOTE (Ethan Winer @ Jan 12 2013, 10:32) ...   Jan 12 2013, 20:21
|||- - Ethan Winer   QUOTE (Woodinville @ Jan 12 2013, 14:21) ...   Jan 13 2013, 20:03
|||- - jmvalin   QUOTE (Ethan Winer @ Jan 13 2013, 14:03) ...   Jan 13 2013, 20:17
||- - jmvalin   QUOTE (Ethan Winer @ Jan 12 2013, 13:32) ...   Jan 13 2013, 08:46
|- - Woodinville   QUOTE (Arnold B. Krueger @ Jan 11 2013, 07...   Jan 12 2013, 20:17
- - Arnold B. Krueger   QUOTE (adamdea @ Jan 10 2013, 11:16) http...   Jan 11 2013, 03:58
- - greynol   Detection of phase differences between channels do...   Jan 11 2013, 20:32
|- - mzil   QUOTE (greynol @ Jan 11 2013, 15:32) Dete...   Jan 11 2013, 21:31
- - greynol   @Ethan: I hope this isn't, "I tried it on...   Jan 11 2013, 21:32
|- - Ethan Winer   QUOTE (greynol @ Jan 11 2013, 15:32) I ho...   Jan 11 2013, 22:00
- - greynol   I did not take the OP as meaning phase relation be...   Jan 11 2013, 21:39
- - Woodinville   QUOTE (adamdea @ Jan 10 2013, 08:16) http...   Jan 12 2013, 20:14
- - greynol   Delay by a half sample: up-convert to a higher sam...   Jan 13 2013, 20:09
- - Woodinville   QUOTE (greynol @ Jan 13 2013, 11:09) Dela...   Jan 14 2013, 03:16
- - Ethan Winer   Oh, you math guys... Though I do understand up-sa...   Jan 15 2013, 21:42


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