Re: Some questions about function FFTA and FFTP

["Dr.Sudheer Joseph" <sjo@xxxxxxxxxxxxx>]

Dear DUhui,
               There is a slight correction in my previous mail Box size is
the grid interval rather than total time.  Niquest frequency is 1/2* del_T
not 1/2*N* Del_T
1/2* Ndel_T is the fundamental frequency
Suorry for the mistake in
first mail
Sincerely,
Sudheer
****************************************************************
Dr.Sudheer Joseph
Scientist- 'B'
Indian National Centre for Ocean Information Services
Plot No# 3, Nandagiri Hills Layout, Jubilee Hills,
Hyderabad, India, Pin:500 033.
Ph: +91- 040-23553542/43/ (O); +91-040 23044600 (R) ;
Fax:+91-040-3551096
E-mail:sjo@rediffmail.com;callsjo@yahoo.com
*****************************************************************
"The ultimate measure of a man is
not where he stands in moments of
comfort and convenience, but where
he stands at times of challenge and
controversy."
                        Martin Luther King, Jr.
************************************************************

----- Original Message -----
From: "du hui" <bjduhui@hotmail.com>
To: <oar.pmel.ferret_users@noaa.gov>
Sent: Wednesday, May 26, 2004 2:34 PM
Subject: Some questions about function FFTA and FFTP


> Dear ferret users:
>  I am using FFTA to do some research. I just want to use different units
of
> T axis.
> I have read the user's guide. But I can not understand them well.
>   In the ferret guides , there is the following description about FFTA
>
> "The units of the returned time axis are "cycles/Dt"  where Dt is the time
> unit of the input axis.  The Nyquist frequency is yquist =
1./(2.*boxsize),
> and the frequency axis runs from freq1 = yquist/ float(nfreq) to freqn =
> yquist "
>
> question 1.
>  the returned time axis should be day because the time unit of my input
> axis is day. However the actual returned time axis is hour.
> question 2
> what does Nyquist frequence mean? What is the boxsize?
>
> actually I am not very clear to the process of converting the time axis
> into what I need. For example, from cycle/day to cycle/month
> ok ,that is the problme
> Thanks a lot
>
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