From: Michal Wallace (sabren_at_manifestation.com)
Date: 2002-06-15 23:33:54
On Sat, 15 Jun 2002, Jim Peters wrote:
> Jim Peters wrote:
> > Bear in mind that the lowest band in your FFT will be centred on
> > 0Hz, then 4Hz, then 8Hz, 16Hz, 24Hz, and so on up to 128Hz.
>
> Oops -- my mistake. It goes 0, 4, 8, 12, 16, 20, ... 128;
> basically (0..32)*4Hz. The 4Hz comes from the
> window-length: 256/64 == 4Hz.
Hey Jim,
Thanks for this, too. I think I'm starting to understand the
properties of the FFT results a little better.
Let me see if I have this straight:
1. We want to pick up waves in the 0-Beta range, where
beta is around 30. Some people may even want "super beta",
so we may want to go up as high as 40-50?
2. Since Nyquist's rule says we need to sample at least
twice as fast as the highest frequency we want to detect,
the hardware folks have chosen a sample rate of 128Hz
or 256Hz. (I'm not sure about this, I thought I'd read
it somewhere... Can anyone tell me the actual rates?)
3. The FFT returns an array of the same length as the
input, but we discard half of it because it deals with
imaginary numbers. Each slot in the array (or "bin", I've
seen it called) represents a set of frequencies
max-detectible-frequency/window-length wide.
Is that true?
So, if I want each bar to represent 0hz, 1hz, ... 32hz, and
we have 256 samples per second, then then I need to have all
256 samples, which gives me 128 bins, each 1Hz wide. Then I
can just show the first 32. Is that right?
If so... Do I just update the screen once a second? Or
should I show the last second's worth of data every 1/4th of
a second, even though 75% of the data is the same each time?
Thanks,
- Michal http://www.sabren.net/ sabren_at_manifestation.com
------------------------------------------------------------
Switch to Cornerhost! http://www.cornerhost.com/
High Powered Hosting - With a Human Touch. :)
------------------------------------------------------------
This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:43 BST