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Re: [ccp4bb] Why Do Phases Dominate? |
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CCP4bb navigationCCP4bb <-- 1999 <-- November 1999 <-- 30 November 1999Subject: Re: Why Do Phases Dominate? From: Eleanor Dodson ccp4 {- at -} YSBL {- dot -} YORK {- dot -} AC {- dot -} UK Date: 2010-03-22 It is quite instructibe to draw the 2-d vector representing the amplitude and then the error vector when you assume certain things... Change the magnitude by 50% and see the error vector, then change the phase by a random shift - say 90 degrees and draw the error vector. In general it is much more severe for case 2 than case 1.. eleanor Edward A. Berry wrote: > This bias is exacerbated by the convention that phases go from 0 to 360* > while amplitudes go from zero to Plus. > Thus the phase decides where to put it, and whether to add or take away, > while the amplitude only decides how much. > > If phase was 0 to 180* and amplitude was Minus to Plus, then > amplitude would decide whether to add or take away as well as how much. > > > > Lijun Liu wrote: >>> Does anybody have a good way to understand this? >> ========= >> There are a lot of good ways to understand this. The amplitudes >> determines how much >> to put, while the phases tell you where to/how to correctly put. For >> example, treating San >> Francisco as a cell, the heights of buildings and lines of streets >> determine the landscape. >> Moving all buildings along some streets separately will change more the >> landscape than >> just changing some buildings' height along the street. Another example, >> taken at different >> lighting/darkness conditions, the photos from the same face could be >> easily recognized >> and compared. However, with the same light condition, when the position >> of nose, eyes, >> mouth, etc., are dislocated from their original positions, the face will >> be very different. >> >>> One possible answer is "it is the nature of the Fourier Synthesis to >>> emphasize phases." (Which is a pretty unsatisfying answer). But, could >>> there >>> be an alternative summation which emphasizes amplitudes? If so, that >>> might >>> be handy in our field, where we measure amplitudes... >> ========== >> It does have. For example, Patterson function. >> >> Lijun >> >>> >>> Regards, >>> >>> Jacob Keller >>> >>> ******************************************* >>> Jacob Pearson Keller >>> Northwestern University >>> Medical Scientist Training Program >>> Dallos Laboratory >>> F. Searle 1-240 >>> 2240 Campus Drive >>> Evanston IL 60208 >>> lab: 847.491.2438 >>> cel: 773.608.9185 >>> email: j-keller2@northwestern.edu >>> ******************************************* >> >> Lijun Liu >> Cardiovascular Research Institute >> University of California, San Francisco >> 1700 4th Street, Box 2532 >> San Francisco, CA 94158 >> Phone: (415)514-2836 >> >> >> CCP4bb navigationCCP4bb <-- 1999 <-- November 1999 <-- 30 November 1999 |
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