Re: FFT: Finding signal frequency at wrong place

[Judd Storrs]

On Sun, Dec 5, 2010 at 4:04 AM, Isak Delberth Davids
<address@hidden> wrote:
> Thanks Judd, was the line you suggested intentionally
>
> t = t + ampl*sin(2*pi*freq_test1*T) +  ampl*sin(2*pi*freq_test2*T);
>
> and was not meant to be
>
> t = T + ampl*sin(2*pi*freq_test1*T) +  ampl*sin(2*pi*freq_test2*T);
>

I meant the first one in the sense of adding two additional
frequencies to the FFT of t because I wasn't sure what t represented.
I don't think either of these is the correct way to add new peaks to
your data.

> ** yes, 't' is the time at which a gamma-ray photon is detected in seconds
> after a reference time t_0 = 0 secs by a ground-based array of telescope
> dishes (simultaneously) --- so these are photon arrival time-stamps,

I'm not familiar with how you analyze this sort of data. Your data
aren't of this form:

S(t) = S(T_0 + i*delta_T)

where S(t) would be the measurements (i.e. the 't' vector) and delta_T
is the sampling rate. This is what the FFT expects. Your data is more
like a series of dirac delta functions that occur at {t_i}.

S(t) = sum_i delta(t-t_i)

I don't know how to analyze this sort of data, but I don't think the
way we're discussing adding frequencies is correct.


--judd