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From: Kamran Iranpour on 2 Apr 2010 09:40
Hi,
I have two signals, one sampled at 1ms (ricker), the other anything
greater, 4 or 5 or 10ms. I need to do a convolution, that is a
multiplication in freq. domain. My question is, given that the second
signal is a series of spikes, do I need to use some sort of
interpolation (e.g. sinc) or is it enough to add samples (zeros)
between the spikes, depending on the ratio of the signals sampling
rate? When I test with matlab, the difference is not much, make me
want to drop the whole interpolation and just add samples (I have to
do that for thousands of traces).
I thought of cutting off the the frequency contents of the first
signal (ricker) to the nyquist of the second one, but that won't do,
it completely distorts my ricker (no surprise there).


From: John on 2 Apr 2010 10:08
On Apr 2, 9:40 am, Kamran Iranpour <kamran.iranp...(a)gmail.com> wrote:
> Hi,
> I have two signals, one sampled at 1ms (ricker), the other anything
> greater, 4 or 5 or 10ms. I need to do a convolution, that is a
> multiplication in freq. domain. My question is, given that the second
> signal is a series of spikes, do I need to use some sort of
> interpolation (e.g. sinc) or is it enough to add samples (zeros)
> between the spikes, depending on the ratio of the signals sampling
> rate? When I test with matlab, the difference is not much, make me
> want to drop the whole interpolation and just add samples (I have to
> do that for thousands of traces).
> I thought of cutting off the the frequency contents of the first
> signal (ricker) to the nyquist of the second one, but that won't do,
> it completely distorts my ricker (no surprise there).

Am I the only one who has no idea what a 'ricker' is?

John
From: Kamran Iranpour on 2 Apr 2010 10:16
On Apr 2, 4:08 pm, John <sampson...(a)gmail.com> wrote:
> On Apr 2, 9:40 am, Kamran Iranpour <kamran.iranp...(a)gmail.com> wrote:
>
> > Hi,
> > I have two signals, one sampled at 1ms (ricker), the other anything
> > greater, 4 or 5 or 10ms. I need to do a convolution, that is a
> > multiplication in freq. domain. My question is, given that the second
> > signal is a series of spikes, do I need to use some sort of
> > interpolation (e.g. sinc) or is it enough to add samples (zeros)
> > between the spikes, depending on the ratio of the signals sampling
> > rate? When I test with matlab, the difference is not much, make me
> > want to drop the whole interpolation and just add samples (I have to
> > do that for thousands of traces).
> > I thought of cutting off the the frequency contents of the first
> > signal (ricker) to the nyquist of the second one, but that won't do,
> > it completely distorts my ricker (no surprise there).
>
> Am I the only one who has no idea what a 'ricker' is?
>
> John

a mexican hat, in Geophysics it is called a ricker.

Kamran
From: Jerry Avins on 2 Apr 2010 10:42
On 4/2/2010 10:16 AM, Kamran Iranpour wrote:
> On Apr 2, 4:08 pm, John<sampson...(a)gmail.com> wrote:
>> On Apr 2, 9:40 am, Kamran Iranpour<kamran.iranp...(a)gmail.com> wrote:
>>
>>> Hi,
>>> I have two signals, one sampled at 1ms (ricker), the other anything
>>> greater, 4 or 5 or 10ms. I need to do a convolution, that is a
>>> multiplication in freq. domain. My question is, given that the second
>>> signal is a series of spikes, do I need to use some sort of
>>> interpolation (e.g. sinc) or is it enough to add samples (zeros)
>>> between the spikes, depending on the ratio of the signals sampling
>>> rate? When I test with matlab, the difference is not much, make me
>>> want to drop the whole interpolation and just add samples (I have to
>>> do that for thousands of traces).
>>> I thought of cutting off the the frequency contents of the first
>>> signal (ricker) to the nyquist of the second one, but that won't do,
>>> it completely distorts my ricker (no surprise there).
>>
>> Am I the only one who has no idea what a 'ricker' is?
>>
>> John
>
> a mexican hat, in Geophysics it is called a ricker.

I'm always glad to learn new words.

The Mexican hat that I know is a 3-D plot of a radially symmetric sinc,
and is easily generated by formula once the amplitude of the central
peak and the spacing of the nulls are known. Why not simply calculate as
many points as you need? There is evidently more to this than I've
assumed above.

Jerry
--
"It does me no injury for my neighbor to say there are 20 gods, or no
God. It neither picks my pocket nor breaks my leg."
Thomas Jefferson to the Virginia House of Delegates in 1776.
���������������������������������������������������������������������
From: Kamran Iranpour on 2 Apr 2010 11:05
On Apr 2, 4:42 pm, Jerry Avins <j...(a)ieee.org> wrote:
> On 4/2/2010 10:16 AM, Kamran Iranpour wrote:
>
>
>
> > On Apr 2, 4:08 pm, John<sampson...(a)gmail.com>  wrote:
> >> On Apr 2, 9:40 am, Kamran Iranpour<kamran.iranp...(a)gmail.com>  wrote:
>
> >>> Hi,
> >>> I have two signals, one sampled at 1ms (ricker), the other anything
> >>> greater, 4 or 5 or 10ms. I need to do a convolution, that is a
> >>> multiplication in freq. domain. My question is, given that the second
> >>> signal is a series of spikes, do I need to use some sort of
> >>> interpolation (e.g. sinc) or is it enough to add samples (zeros)
> >>> between the spikes, depending on the ratio of the signals sampling
> >>> rate? When I test with matlab, the difference is not much, make me
> >>> want to drop the whole interpolation and just add samples (I have to
> >>> do that for thousands of traces).
> >>> I thought of cutting off the the frequency contents of the first
> >>> signal (ricker) to the nyquist of the second one, but that won't do,
> >>> it completely distorts my ricker (no surprise there).
>
> >> Am I the only one who has no idea what a 'ricker' is?
>
> >> John
>
> > a mexican hat, in Geophysics it is called a ricker.
>
> I'm always glad to learn new words.
>
> The Mexican hat that I know is a 3-D plot of a radially symmetric sinc,
> and is easily generated by formula once the amplitude of the central
> peak and the spacing of the nulls are known. Why not simply calculate as
> many points as you need? There is evidently more to this than I've
> assumed above.
>
> Jerry
> --
> "It does me no injury for my neighbor to say there are 20 gods, or no
> God. It neither picks my pocket nor breaks my leg."
>           Thomas Jefferson to the Virginia House of Delegates in 1776.
> ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Sorry if I was not clear. My mexican hat is sampled at 1000Hz, my
other signal(spiky signal) is sampled at for example, 200Hz.
Obviously, if I am to convolve the two (by freq. multiplication), I
have to cut (zeroed) all the frequency components of the mexican hat
above 100Hz, to make sure I am multiplying the correct frequency
components of the two (downsample the mexican hat in time). I don't
want to do that. Firstly, because my mexican hat is not much of a
mexican hat any longer and secondly, because I have been told
(ordered) to avoid that.
What I thought instead, is that I upsample my second signal (the spiky
one) to 1000 Hz. The question is, is it enough just to put 5 zeros in
between two existing samples in the spiky signal or do I have to put 5
zeros in between and then interpolate (with some proper filter, the
way matlab's 'resample' does)?

Kamran
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