# [R] Frequency Spectrum fft plot dillema

From: Rthoughts <seb.brockel_at_northampton.ac.uk>
Date: Tue, 25 Nov 2008 06:34:09 -0800 (PST)

Hi,

I have more questions about the fft. The application in Excel is very limited.

In Excel I can adjust graphs and calibrate the x and y-axis. The input and process, however, is limited compared to R.

With a Dataset table where one column is the hour difference and the second are the values with hidden frequencies, how is it possible to plot a frequency spectrum with the x-axis calibrated to the hour difference where the left should be infinite frequency and half way to the right should be the midpoint.

Below is an example dataset with a 24 hour sinusoid that I would like to fft for assisting with my ability to interpret my output. How do I calibrate the plot so that the x-axis displays the cycles per that run and the y-axis is frequency? In the command line, (Dataset\$X10, inverse = FALSE, frequency = 24), what does the frequency line specify and and it is related to number of data points? By doing this am I generating frequencies rather than revealing them?

Secondly, how to do generate a spectrogram on R? I believe that this one is where the x-axis is the cycles per data point with the left being infinite cycles to 0.5 being one cylce per 2 data points and the y-axis is the amplitude.

Thank you very much in advance if you can help and share your experience/ knowledge. I am progressing with fourier transforms in my research but I am new to it so that doesn't help.

Thirdly, I came across in my reading that it is a good idea to generate a correlogram to determine if it is valid to do an fft on the dataset. How would I go about doing this and how is frequency/ lag determined?

```00/01 00:00 4.330
00/01 01:00 3.536
00/01 02:00 2.500
00/01 03:00 1.294
00/01 04:00 0.000
00/01 05:00 -1.294
00/01 06:00 -2.500
00/01 07:00 -3.536
00/01 08:00 -4.330
00/01 09:00 -4.830
00/01 10:00 -5.000
00/01 11:00 -4.830
00/01 12:00 -4.330
00/01 13:00 -3.536
00/01 14:00 -2.500
00/01 15:00 -1.294
00/01 16:00 0.000
00/01 17:00 1.294
00/01 18:00 2.500
00/01 19:00 3.536
00/01 20:00 4.330
00/01 21:00 4.830
00/01 22:00 5.000
00/01 23:00 4.830
01/01 00:00 4.330
01/01 01:00 3.536
01/01 02:00 2.500
01/01 03:00 1.294
01/01 04:00 0.000
01/01 05:00 -1.294
01/01 06:00 -2.500
01/01 07:00 -3.536
01/01 08:00 -4.330
01/01 09:00 -4.830
01/01 10:00 -5.000
01/01 11:00 -4.830
01/01 12:00 -4.330
01/01 13:00 -3.536
01/01 14:00 -2.500
01/01 15:00 -1.294
01/01 16:00 0.000
01/01 17:00 1.294
01/01 18:00 2.500
01/01 19:00 3.536
01/01 20:00 4.330
01/01 21:00 4.830
01/01 22:00 5.000
01/01 23:00 4.830
02/01 00:00 4.330
02/01 01:00 3.536
02/01 02:00 2.500
02/01 03:00 1.294
02/01 04:00 0.000
02/01 05:00 -1.294
02/01 06:00 -2.500
02/01 07:00 -3.536
02/01 08:00 -4.330
02/01 09:00 -4.830
02/01 10:00 -5.000
02/01 11:00 -4.830
02/01 12:00 -4.330
02/01 13:00 -3.536
02/01 14:00 -2.500
02/01 15:00 -1.294
02/01 16:00 0.000
02/01 17:00 1.294
02/01 18:00 2.500
02/01 19:00 3.536
02/01 20:00 4.330
02/01 21:00 4.830
02/01 22:00 5.000
02/01 23:00 4.830
03/01 00:00 4.330
03/01 01:00 3.536
03/01 02:00 2.500
03/01 03:00 1.294
03/01 04:00 0.000
03/01 05:00 -1.294
03/01 06:00 -2.500
03/01 07:00 -3.536
03/01 08:00 -4.330
03/01 09:00 -4.830
03/01 10:00 -5.000
03/01 11:00 -4.830
03/01 12:00 -4.330
03/01 13:00 -3.536
03/01 14:00 -2.500
03/01 15:00 -1.294
03/01 16:00 0.000
03/01 17:00 1.294
03/01 18:00 2.500
03/01 19:00 3.536
03/01 20:00 4.330
03/01 21:00 4.830
03/01 22:00 5.000
03/01 23:00 4.830
```
```--
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