Prev: FFT/IFFT on real signals
Next: Write to Geneva and plead the citizens of Geneva to block the entrances to LHC.
From: blackhermi on 22 Mar 2010 09:35
Hi

I need to find out the frequencies present in the variation of a physical
quantity from a discretely sampled data of finite length. Which window (if
any) should I use? I think doing an fftn in MATLAB uses a rectangular
function by default. Please correct me if I am wrong.

Also, is there a way to create a 2D version of a given window?

Thanks and Regards


From: Rune Allnor on 22 Mar 2010 09:43
On 22 Mar, 14:35, "blackhermi" <dheeraj.iitm(a)n_o_s_p_a_m.gmail.com>
wrote:
> Hi
>
> I need to find out the frequencies present in the variation of a physical
> quantity from a discretely sampled data of finite length. Which window (if
> any) should I use?

It depends entirely on the data and what you attempt to do.
If the SNR is large and the sinusoidals are well separated,
then don't use any. If the SNR is low and you need to
suppress side lobes, choose a window that can be tuned to
your particular needs (the Kaiser window is just about the
only one that fits that bill). Or avoid controversy by using
the most popular window that everybody else use. I don't have
any statistics, but the Hanning / Hann / von Hann window ought
to end up among the top 3 in the popularity polls.

Rune
From: Tim Wescott on 22 Mar 2010 10:14
Rune Allnor wrote:
> On 22 Mar, 14:35, "blackhermi" <dheeraj.iitm(a)n_o_s_p_a_m.gmail.com>
> wrote:
>> Hi
>>
>> I need to find out the frequencies present in the variation of a physical
>> quantity from a discretely sampled data of finite length. Which window (if
>> any) should I use?
>
> It depends entirely on the data and what you attempt to do.
> If the SNR is large and the sinusoidals are well separated,
> then don't use any. If the SNR is low and you need to
> suppress side lobes, choose a window that can be tuned to
> your particular needs (the Kaiser window is just about the
> only one that fits that bill). Or avoid controversy by using
> the most popular window that everybody else use. I don't have
> any statistics, but the Hanning / Hann / von Hann window ought
> to end up among the top 3 in the popularity polls.

Understanding the properties of the FFT would help, too -- that will let
you understand why you want to window the data, which in turn will help
you understand what window (if any) you want and why.

While I could stand in front of a room of fellow engineers and defend a
choice of window by "it's popular _and_ it doesn't matter much anyway",
defending it with "'cause it's popular" would quite deservedly get me
laughed out of the room.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
From: Rune Allnor on 22 Mar 2010 10:18
On 22 Mar, 15:14, Tim Wescott <t...(a)seemywebsite.now> wrote:
> Rune Allnor wrote:
> > On 22 Mar, 14:35, "blackhermi" <dheeraj.iitm(a)n_o_s_p_a_m.gmail.com>
> > wrote:
> >> Hi
>
> >> I need to find out the frequencies present in the variation of a physical
> >> quantity from a discretely sampled data of finite length. Which window (if
> >> any) should I use?
>
> > It depends entirely on the data and what you attempt to do.
> > If the SNR is large and the sinusoidals are well separated,
> > then don't use any. If the SNR is low and you need to
> > suppress side lobes, choose a window that can be tuned to
> > your particular needs (the Kaiser window is just about the
> > only one that fits that bill). Or avoid controversy by using
> > the most popular window that everybody else use. I don't have
> > any statistics, but the Hanning / Hann / von Hann window ought
> > to end up among the top 3 in the popularity polls.
>
> Understanding the properties of the FFT would help, too -- that will let
> you understand why you want to window the data, which in turn will help
> you understand what window (if any) you want and why.
>
> While I could stand in front of a room of fellow engineers and defend a
> choice of window by "it's popular _and_ it doesn't matter much anyway",
> defending it with "'cause it's popular" would quite deservedly get me
> laughed out of the room.

The alternative is to stand on the podium and defend why one uses
some obscure window:

"Why did you use the Gavrilovich-Cheng window? What properties
does that window offer that a regular window like, say, a Hann
window, does not?"

You'd better have a clear answer to that one, or avoid the
question being asked at all.

Rune
From: Vladimir Vassilevsky on 22 Mar 2010 11:46


Tim Wescott wrote:


> But using a technique just because all your friends use it* -- that's
> just crying out for some corrective action.

That's #1 point in the future book "How to quit being an idiot and start
living": Have a purpose. Don't do anything just because many other
people do that.

VLV
 |  Next  |  Last
Pages: 1 2
Prev: FFT/IFFT on real signals
Next: Write to Geneva and plead the citizens of Geneva to block the entrances to LHC.