Subject: Re: Get lost, Mr. Cochlea!! --- The Brain (Jont Allen ) From: Ramdas Kumaresan <kumar(at)ELE.URI.EDU> Date: Mon, 26 Feb 2001 10:18:27 -0500Dear Jont: Jont Allen wrote in http://sound.media.mit.edu/dpwe-bin/mhmessage.cgi/AUDITORY/postings/2001/135 >The ear IS similar to a floating point converter. The ear does not have an infinite >dynamic range or signal to noise ratio. This limited dynamic range >shows up as masking. Do you disagree? I don't know, but masking of a weak signal due to an intense signal in its neighbourhood, is it entirely due to what happens in the periphery? (We know about asymmetry, spreading and shifting of excitation to higher frequencies.) What if the periphery still accurately (to the extent it is allowable by timing jitter etc) represents the weak and intense signal combo and the higher centers ignore the weak component, say, because there is much more precise phase locking to the intense signal. I am not too hot on the trail in masking. Is it established that the information loss (masking) is entirely due to the periphery? Jont Allen wrote: >The auditory nerve signal is not about zero crossing. Even zero crossing >are not exact, and would have jitter. But masking is NOT timing jitter. We thought the classical theory of a neuron firing says that if the membrane potential exceeds a threshold then it fires. If so, then it IS some form of zero or level crossing detector. It is a question of how the cochlear mechanics transforms the signal and presents it to the neuron/haircell. Zero-crossings, as descriptors of a signal, have acquired an undeserved bad reputation. As we have pointed out in our original post, the zero-crossings of a STIMULUS SIGNAL, themselves are NOT of much use. But there are ways to carry reliably in zero-crossings (of other related signals) information about the temporal envelope and phase of a stimulus signal, thereby implicitly, but completely representing a signal. This is our Main point. Those familiar with speech signal processing know about what is called Line-Spectrum-Frequencies (LSFs) originally proposed by Fumitada Itakura, which represent the spectral envelope of a signal. These LSFs are used reliably and successfully in speech coding, recognition etc. These are indeed 'zero-crossings' that represent the spectral envelope, except that these zero-crossings occur along the frequency axis, instead of time axis. Thus, there is already evidence albeit in the other (frequency) domain that these the zero-crossings are reliable. On a lighter note, I asked Yadong Wang (my grad student), two years ago, to take a look at zero-crossings after reading your 1985 paper in which you seemed to be saying that the auditory nerve signal IS based on zero-crossings. (Jont B.Allen, "Cochlear Modeling", IEEE Acoustics, Speech and Signal Processing Magazine,January 1985, p.3-28.) Refer to Figure 25 and Figure 26 in this paper. Quoting from captions of Figure 25: "Based on the model of the haircell, we assume here that the information is carried by the zero-crossings of the multitudinous narrow band signals. This is because the hair cell cilia appear to act as a switch, given moderate and high level signals, transforming the signals to peak-clipped signal. In an infinitely peak-clipped signal the the information is coded by the zero-crossings..." It is heart breaking to see that you would abandon zero-crossings and us midstream. Rmadas Kumaresan Yadong Wang xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Subject: Re: Get lost, Mr. Cochlea!! --- The Brain From: Jont Allen <jba(at)RESEARCH.ATT.COM> Date: Tue, 27 Feb 2001 00:01:19 -0500 Yadong, This is all very cute, and I dont want to be accused of not having a sense of humor, (clearly you do, and it is refreshing), but there is a thing called masking. Information is lost in the early auditory stages, due to neural coding. The auditory nerve signal is not about zero crossing. Even zero crossing are not exact, and would have jitter. But masking is NOT timing jitter. The ear IS similar to a floating point converter. The ear does not have an infinite dynamic range or signal to noise ratio. This limited dynamic range shows up as masking. Do you disagree? Jont