How to modify the phase of a waveform in labview using an allpass filter?

[Jhon_2001]

Dear All,

I need to modify the phase of waveform that contains multiple frequencies. I found from a previous post (http://forums.ni.com/t5/LabVIEW/waveform-phase-modify/td-p/1907377) that someone suggested using an allpass filter; but did not offer additional information. Can someone provide some guidance on how to design and build the filter? Any other suggestion on how to accomplish this?

Thank you very much for your help.

[JaredRo]

Jhon_2001,

Do you have the Digital Filter Design Toolkit?  If so, you will be able to implement a lattice allpass filter using the DFD Build Filter from Lattice Coef VI.  

The website below has describes the filter structure graph for a lattice allpass filter:

Lattice Allpass Structures (Digital Filter Design Toolkit) - LabVIEW 2011 Digital Filter Design Toolkit Help

http://zone.ni.com/reference/en-XX/help/371325F-01/lvdfdtconcepts/lattice_allpass/

Here is a resource on reading filter structure graphs:

Understanding Filter Structure Graphs (Digital Filter Design Toolkit) - LabVIEW 2011 Digital Filter Design Toolkit Help

http://zone.ni.com/reference/en-XX/help/371325F-01/lvdfdtconcepts/dfd_structure_graph/

Hope this helps!

[Jhon_2001]

Jared,

Thank you very much for your reply.

I downloaded the demo version of the toolkit and took a look at the examples:Get Lattice Coeficients of Filters.vi and Create Filter from Lattice Coefficients.vi. I run the first to get coefficients for an allpass (selected from target structure) and I got an error. Is there something that I am missing? Does the toolkit have a tool where I can specify the filter response and get the coefficients?

Thank you in advance.

Jhon.

[JaredRo]

Jhon_2001,

What error did you get when you ran the VI?  Both shipping examples related to lattice filters ran without error on my machine.  The Classical Filter Design express VI will automatically generate a filter for you based on the filter response you'd like, but not for an allpass filter.  If we can get those two example VI's working, those are going to be your best bet.

[Jhon_2001]

Jared,

Thanks again.

This is the error that I get: "Digital Filter Design Toolkit:  The filter cannot be represented by the structure you specified". You are right the Classical Filter Design Express does not the capability to design allpass filters. 

The examples are working now; but they do not provide what I am looking for. I was hoping that I could specifiy the required response for the allpass, get the coefficients and filter my signal. Can you confirm that such a tool is not available in Labview?

Jhon.

[JaredRo]

Jhon_2001,

The tool that you're asking about doesn't exist in the Digital Filter Design Toolkit.  The only other VI in the toolkit that creates an allpass filter that we haven't mentioned thus far is the DFD Arbitrary Group Delay Design VI.  Here is the help file description of this VI:

DFD Arbitrary Group Delay Design VI - LabVIEW 2011 Digital Filter Design Toolkit Help
http://zone.ni.com/reference/en-XX/help/371325F-01/lvdigfiltdestk/dfd_arb_grp_dly/

There is also a shipping example that uses this VI called "Arbitrary Group Delay Filter Design.vi".

Good luck with your application.