Path: utzoo!attcan!uunet!husc6!mailrus!cornell!batcomputer!sandell From: sandell@batcomputer.tn.cornell.edu (Gregory Sandell) Newsgroups: comp.sys.next Subject: Re: Fwd: LSC problem Message-ID: <6843@batcomputer.tn.cornell.edu> Date: 16 Nov 88 19:05:00 GMT References: Reply-To: sandell@tcgould.tn.cornell.edu (Gregory Sandell) Organization: Cornell Theory Center, Cornell University, Ithaca NY Lines: 39 In article wb1j+@andrew.cmu.edu (William M. Bumgarner) writes: >---- >From: fry%zariski@harvard.harvard.edu (David Fry) > >I was wondering if anyone has thought of using the NeXT >computer for an anti-noise demonstration. I think it would be >a nice little trick. >For those who don't know, systems are being developed that >can do a Fourier analysis of repeated sounds to find their >component sound waves, and then emit a sound composed of the >the mirror image of these waves, resulting in destructive >interference. Your ear then hears nothing. This could be >used in an airplane, for instance, so the passengers would >hear no engine noise. > >it seems that it would be pretty simple for the NeXT to do >this in real time. > >Maybe the people at NeXT have already done this? > >David Fry fry@huma1.harvard.EDU Well, since you're a mathematician you may have an idea of how complicated a task this is. Most people in DSP work I have talked to about this idea says, "yeah, sure...show me it happening." Noise by definition is aperiodic, so your FFT is going to have a hell of a time finding some REGULAR waveform to destructively interfere. If that can't be done, then were talking about analyzing the signal in real time, and for each sample inputted, outputting a destructive signal for that sample. So they say that the Motorolla could squeeze in 600 instructions between every sample at 44.1kHz. Maybe the calculation could be done...but you also have to be recording samples and running both D/A and A/D at the same time. Could it really be done? Greg Sandell mostly aperiodic it will be VERY difficult for a fourier analysis to find any kind of regularity in the