Since this has moved away from the point of the original query and to a technical issue, I'm starting a new thread. Please forgive me if I get a bit pedantic here. I'm still concerned that you mean something different by "transient time of arrival differences" than I do by ITDs.
I'll repeat my assertion that time of arrival differences only exist at the point of a listener hearing the sound. If there is a violin playing on stage, slightly to the right of center, the sound generated by the violin arrives at the point occupied by the listener all at the same time. The listener locates the violin largely by the fact that his ears are at two different places, and those places have different times of arrival. What we are concerned with is the sounds (ear signals) that the listener hears when he is listening to the reproduced sound. Stereo, and now I need to state explicitly that I mean ordinary intensity stereo as embodied in Blumlein recording, is a sort of coding and decoding system. The microphone array codes the direction of sound arrival by different intensities in the two channels. Those two channels are mapped to two spaced loudspeakers and it is the way in which those sounds are combined in the reproduction venue that results in the listener's perception of location. I believe that the "glockenspiel effect" that you describe arises because the localization cues experienced by the listener are different for ITDs than for ILDs. Because we primarily rely on ITDs at low frequencies and ILDs at high frequencies, if the reproduction system doesn't handle them in the same way then the listener experiences a disparity. This happens in both Blumlein stereo and in Ambisonics. It's just not possible to get the ILDs exactly correct, at least not with loudspeakers spaced by 60 degrees or 90 degrees. There simply aren't enough degrees of freedom, not by a couple of orders of magnitude. So the best that we can hope for is that the ILDs will be correct on average. That they won't be worse in some directions than in others. Think about what happens in Blumlein recording and reproduction. If the sound source is on the axis of of one of the microphones then it appears exclusively in that channel. It then appears exclusively in the associated loudspeaker. It sounds as though it were coming from that loudspeaker because it IS coming from that loudspeaker. It the sound source is located exactly between two microphones then the signal appears in both channels and is reproduced equally by both loudspeakers. The signals that arise at the listener's ears have substantial comb filtering due to the acoustic crosstalk. Crosstalk that wouldn't have been there if the listener were present in the recording venue but is present for the listener in the reproduction venue. The reproduced sound is more diffuse for the center image than for sounds combing from the directions of the two microphone capsules. The difference between Blumlein and Ambisonic reproduction of the sound (here I am referring primarily to first order) is that both the low- and high-frequency cues are much more even with respect to direction. But clearly there is a lot of room for improvement. I can demonstrate that all of this works the way that I described it by doing an experiment. We can go to the concert hall and make a series of recordings. Perhaps we might want to do three recordings. One with a dummy head (we'll use my head. It's available...), one with a Blumlein array, and one with a soundfield microphone. We will then analyze the recorded signals from the dummy head which will give us both ITDs and ILDs as experienced by the listener (me). We then reproduce the Blumlein and Soundfield microphone recordings using an array of four loudspeakers. That will gives us a 90 degree stereo pair which is, as I understand it, how you like to listen. We then record the acoustic signals going into my ears when I'm in the sweet spot and analyze those. The result that I expect is that the low-frequency ITDs will be correct and the high-frequency ILDs will not be correct. And they will be more confused for the Blumlein recording than for the Soundfield recording. Now I've actually done this for the case of Soundfield recording, although not as carefully or as thoroughly as I would like. I intend to do more such experiments. In particular I intend to explore what I call the plausibility of the recording. Several years ago I was asked by Svein Berge how to tell if a particular Soundfield recording was good or not. It took me a bit to realize that he was asking, not if it was aesthetically pleasing, but whether or not is was correct; that is, could those signals in the recorded file actually represent a real sound field. And as it turned out, in some of those cases they couldn't. Or at least they were highly implausible. In subsequent thinking about his question it occurs to me that the plausibility, not of the signals in the recording but of acoustic signals that enter the listener's ears, is an important indicator of whether the listener finds the reproduction to be realistic or not. If our ears receive a large number of cues that are wrong, or at least implausible, then the reproduction is unrealistic. Eric REG wrote: >>> Because one- point miking ignores transient time >>> of arrival differences as such , one of the basic cues of sonic perception >>> is suppressed explicitly EMB wrote >> That's not really true. I'm assuming that when you speak of time of arrival >> differences that you are referring to ITDs. The thing to remember here is >that >> ITDs are a function of our presence in the acoustic field, and as such aren't >> present in the recording environment and thus shouldn't be recorded. In a >> recording and reproduction scenario the ITDs happen in the reproduction of the >> recording, and as it happens ITDs are reproduced very well by Ambisonics, even >> first order Ambisonics. I showed this quite clearly (I hope) in AES preprint >> 8242. REG wrote: >I did say very explicitly transient time differences. >Maybe I am missing something but these are not detected >in Blumlein and I am not clear on why they would >be in first order. I don't think this works. >I could be wrong, however. But I think that >in Blumlein stereo anyway, everyone agrees it does >not. so that et what is called the "Glockenspiel effect" >arises where lower and higher frequencies are separated >in perceived position. >If this is wrong, I would surely be interested to know >why, and if it is what happens in BLumlein I do >not see why it would go away with first order Ambisonics _______________________________________________ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound
