[mathcad] Re: DSP method

Author: Quang-Viet Nguyen Posted: Tue, 17 Jan 2006 01:10:11 -0500

Hi Mark,

At 11:00 AM 1/16/2006 +0000, Mark Smith wrote:
>Hi again
>Thanks to all who pointed me in the right direction of cross correlation.
>I have a secondary question which hopefully someone might have an answer to.
>I have done the cross correlation and plot its value against the shift to
>find the maxima i.e. at what delay the 2 traces are most similar.
>Is it valid for me to fit a polynomial to the points around the maximum &
>solve for the maximum at a point intermediate to the sample points. In other
>words can I resolve a time delay less than the period between samples or
>must I increase the sample rate?

Since the data is sampled at discrete time-steps your time resolution is
Nyquist limited. So for example, if the data is sampled at 2 kHz, you
cannot resolve anything finer than 1 kHz - or 1 ms precision. So
increasing the sample rate to 2x greater than the highest frequency you
anticipate that you will need to resolve will work. Be careful to use the
correct low-pass filters to prevent aliasing the data.

Good Luck.

Viet Nguyen

Quang-Viet Nguyen, Ph.D.

NASA Glenn Research Center
21000 Brookpark Rd, MS 5-10
Cleveland, Ohio 44135

Voice: (216) 433-3574
Fax: (216) 433-5802
E-Mail: "Quang-Viet.Nguyen" or "Quang-Viet.Nguyen-1"

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