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Hi-Rez Highway: Re: The reality of figure of 8 microphones: by regreene

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Re: The reality of figure of 8 microphones:

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I have limited time at this very moment, so I cannot respond to all of Mr. Geijsen's interesting remarks. I shall return to this later, if I may.
But I do want to clarify one point. When I was talking about the moving away from the central axis with the same pink noise signal in each of two speakers, I was trying to give people a way to hear time-delay comb filtering, not commenting on stereo playback itself. This was not very clearly expressed perhaps. The point is that when one listens to the same source with a time delay, as when picking up a live (single) source with spaced out microphones, comb filters are introduced when the microphone signals are combined subsequently.
It is of course the case that stereo playback itself introduces time delays when one moves off the central plane.

Let me suggest a test. Imagine walking across a stage carrying a pink noise source and that this source is going to be recorded.
If you record with a Blumlein pair, there is no time delay and all that changes is the relative intensity of the signal in the two channels. If you record with two (say) spaced omnis, then each microphone will pick up the same signal with slightly changing intensity but very substantial time delays.

Now imagine playback. In the Blumlein, the two speakers are playing each an uncolored signal with no time differential. The ear/brain is receiving no time information but is locatiing the source by intensity stereo. As it happens, up to around 1500 Hz, the signals combine around the head to give almost exactly the phase and frequency response that would have happened in reality. In the higher frequencies, reconstruction is not perfect on account of head shadowing effects, but it is quite good.

Now imagine the spaced omni playback. When the signals arrive at the listening position, there are substantial time shifts. This moves the tonal character around.

You do not have to take my word for this. Buy J. Boyk's(Performance Recordings) test CD. This is a straight forward test. Recordings were made of click sounds from a collection of identical speakers spaced along a line. It is analogous to my walking across the stage.

Blumlein plays back to locate the clicks correctly and ALL THE CLICKS HAVE ESSENTIALLY THE SAME TIMBRE. But the spaced omnis not only locate the clicks in the wrong places, they also makes teh clicks from different locations sound quite different in timbre.

Of course, three mikes(with three channel playback) would do better. And even more would do even better. Indeed according to Huygens' Principle from physics if one had infinitely many mikes and channels and playback speakers one would end up with... Blumlein actually, for all practical purposes.

I know it is complicated to compute what happens with more than two mikes(mixed to two channel playback), although it can be done.

But one really cannot get the comb filters out once they are in.

Moreover, the test is very convincing. Everyone interested in this subject really ought to get this CD and listen.


As to time difference stereo: yes of course time differences shift images. But these time differences in the spaced omni technique are FAR larger than the ones which occur naturally. In fact if one is anything but very far away, they are VERY FAR larger. The differences presented could not occur with a natural sound source ,exceeding as they do the max interaural time-of-difference from a natural source in any location at all(eg that directly to the side).

It is not believable, and in my experience is not true, that such a thing produces anything like convincing and precise imaging.

Of course Blumlein erases the interaural time differences altogether. This might be expected to cause trouble(and it is one argument for ORTF--but not for spaced omnis which are over the top exaggerated in this regard). But surprisingly it seems not to. I have to go now but I discuss this point in my article on Boyk's test CD on my website www.regonaudio.com

To save me typing time I would be most grateful if everyone interested in this discussion would read that article before we continue.
Thank you,
Robert E. Greene



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