MATLAB >> Question re: 'clipping' data

by Bill Blum » Sat, 17 Jul 2004 07:13:18 GMT

Okay, I have a question for the group:

I've got a A/D converter pulling data from one of our instruments, and
we're reading it in over the serial port.

The data is all 8-bit unsigned int, range 0-255 inclusive.... I'm
dealing with vectors that contain between 1 and 2 million samples total.

*However*, due to an issue in the A/D converter, we're getting repeated
'spikes', of sorts, about every 2000 samples.

If we subtract 127 from each element of the data vector (to recenter it
around zero), the meaningful part of the signal is generally confined
between +/- 50.

My question is this: is there an elegant, vectorized way of setting
any element outside that range to zero? Right now, we're doing the
brute-force forloop method, but we'd like to expand to 5 million data
points in the vector, and any time we save would be a benefit.


Thanks for your time, and replies to the group-- email address goes nowhere.

- Bill Blum, Wright State Univ.

MATLAB >> Question re: 'clipping' data

by allnor » Sat, 17 Jul 2004 18:05:11 GMT



I can think of two ways:

1) The memory-preserving way, which is to write a mex function in C or
FORTRAN that runs through the vector and replaces every instance of
0 and 255 with 127 (before subtracting 127 from the data). It would
take some time to code, but is probably the fastest method to run.

2) The matlab way:

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
maxidx=find(x==255); % x is the data column vector
x(maxidx)=127*ones(length(maxidx),1);
minidx=find(x==0);
x(minidx)=127*ones(length(minidx),1);
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

This snippet sets only the saturated samples to zero. It should be
straightforward to adjust the FIND statements to your needs.

Rune

MATLAB >> Question re: 'clipping' data

by Bill Blum » Sat, 17 Jul 2004 21:49:27 GMT


No mex compiler available, unfortunately.....



Thanks!

That should help me clip the saturated points for now, until I get an
adaptive interpolator working to handle the outlying points.

( Most of my previous programming experience is in Java, I'm pretty much
learning MATLAB as I go )


- Bill Blum

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