How can I set the coefficients in a Wavelet tree using an external variable?

I am trying to decompose a signal in a wavelet tree structure using the WPDEC function. For a specific bottom node on the tree, I want to then modify the coefficients and then reconstruct the tree. Is there a way I can modify the value and the number of coefficients in a wavelet tree?

Answer 1

[MathWorks Support Team]

It is not possible to change the number of coefficients in any node in the wavelet tree. However, it is possible to change the value of coefficients in a wavelet tree using the read and write methods of the wavelet object.

Provided below is an example on how this can be achieved:

load noisdopp;

x = noisdopp;

% decompose x at depth 3 with db1 wavelet packets.

wpt = wpdec(x,3,'db1');

% plot wavelet packet tree wpt.

plot(wpt);

[wpt,cA,cD] = wpsplt(wpt,[3 0]);

plot(wpt);

%extract coefficients

cfs = read(wpt,'cfs',[3 1]);

% may modify cfs or create new values.

% suppress the approximation coefficients

newcfs = zeros(size(cfs)); % has to be same size

% reconstruct the wavelet packet tree

wpt = write(wpt,'cfs',[3 1],newcfs);

plot(wpt);

The following documentation link in R2007b has additional examples on using read and write methods on wavelet tree objects:

web([docroot '/toolbox/wavelet/apxb_ob4.html#132777'])