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MathGroup Archive 2007

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Re: Re: Unwrap list of angles

  • To: mathgroup at smc.vnet.net
  • Subject: [mg74557] Re: [mg73291] Re: [mg73262] Unwrap list of angles
  • From: gardyloo <gardyloo at mail.wsu.edu>
  • Date: Mon, 26 Mar 2007 02:04:47 -0500 (EST)
  • References: <200702090718.CAA18382@smc.vnet.net> <200702100439.XAA07871@smc.vnet.net>

 Hi, all,

    This is in response to a relatively old (about two months' worth)
message concerning unwrapping phases. I realized a while ago that the
algorithm I sent to Ben (and to the list) was wrong -- it *usually*
works, but it does NOT ALWAYS return a phase whose sine matches the
original number fed to it. Therefore, if the original algorithm is used,
it can sometimes return incorrect values.

   Here's a more reliable algorithm; I haven't found any problems with
it yet:

(*********** UnwrapPhase follows  *************)

UnwrapPhase[data_?VectorQ, tol_:Pi, inc_:2 Pi] :=

  FixedPoint[

    # + 

    inc*FoldList[

                Plus, 0., 

                  Sign[

                      Chop[

                          ListCorrelate[{1, -1}, #],

                          tol]   (*close Chop*)

                      ]    (*close Sign*)

                 ]&,    (*close FoldList*)

    data]   (*close FixedPoint and overall function *)

UnwrapPhase[list:{{_,_}..} ]:=

  Transpose[

    {

      list[[All, 1]],

      UnwrapPhase[list[[All,-1]]]

      }

    ]

(*********** UnwrapPhase above  *************)



  I hope the previous thing I sent hasn't screwed anyone up!

                 C.O.




gardyloo wrote:
> Hi, Ben,
>
>    I ended up writing a version of this a while ago. Here's the 
> definition I use. The first is for a list of one dimension; it's used in 
> the second definition (since I usually deal with lists of two dimensions 
> where the second element in the ordered pairs is the phase.
>
> UnwrapPhase[list_ /; Length[Dimensions[list]] == 1] :=
>
>       Module[{newTest,
>
>           lC = 
>
>                Mod[
>
>                    ListCorrelate[{-1, 1}, list], Pi, -Pi/2]}, newTest[1] = list[[1]];
>
>     newTest[i_] := newTest[i] = newTest[i - 1] + lC[[i - 1]];
>
>     newTest[#] & /@ Range[Length[list]]]
>
> UnwrapPhase[list:{{_,_}..} ]:=
>
>   Transpose[
>
>     {
>
>       list[[All, 1]],
>
>       UnwrapPhase[list[[All,-1]]]
>
>       }
>
>     ]
>
>
>
>
>    Hope that helps!
>
>                  C.O.
>
> ben wrote:
>   
>> Dear all
>>
>> I am looking for a Mathematica-analogue of the unwrap function in another system:
>> I have a list of angles and want to add multiples of 2*pi to them, so
>> that there
>> are no 'jumps' between subsequent entries.
>> Is there a ready-made function?
>>
>> Bye
>> Ben
>>
>>
>>
>>   
>>     
>
>   


-- 
==========================================================
Curtis Osterhoudt
gardyloo at mail.remove_this.wsu.and_this.edu
PGP Key ID: 0x088E6D7A
Please avoid sending me Word or PowerPoint attachments
See http://www.gnu.org/philosophy/no-word-attachments.html
==========================================================



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