Need Matlab help please ...

[Holaa]

Hi Guys,

Can anybody help me to change my y-axis in matlab to (db) logarithmic..
part of my code looks like:


c=sqrt(abs(data,2)).^2 + abs(data,3)).^2);
b=data,1);
tmp=folderContents(i,;
plot(b,c,'red');

BUT: I do not want to choose some commands like semilogy, loglog ...etc
Tanks in advance for your help.......

 

[nitishn5]

Why don't you convert c to dB scale and then plot like this...

d = 20*log(c);
plot(b, d, 'red');

 

[Holaa]

Why don't you convert c to dB scale and then plot like this...

d = 20*log(c);
plot(b, d, 'red');

Thanks for your answer..
That is what I did exactly
but got wrong plots :-(

 

[albbg]

Are you sure that "c" is always positive that is strictly > 0 (coming from a sqrt will not be negative for sure but it could be zero) ?
Sometimes also numbers very very close to zero (even if positive) could cause problems since generating huge numbers after the log function

 

[Holaa]

Are you sure that "c" is always positive that is strictly > 0 (coming from a sqrt will not be negative for sure but it could be zero) ?
Sometimes also numbers very very close to zero (even if positive) could cause problems since generating huge numbers after the log function

yes, all of them are positive and near to this number 0.9923, 0.9933 .... etc
here is example::

0.9334
0.9331
0.9329
0.9326
0.9323
0.9321
0.9318
0.9315
0.9313
0.9310
0.9307
.
.
etc

 

[albbg]

OK, what do you mean exactly with "wrong plots". Did you get any error message ?
You can try to plot a small subset of data (f.i. the first 20 values), so you can also display each value and have a look if something strange appear.
Just an additional thing: as far as I remember log(x) is the logarithm in base "e"; to convert in dB you need instead the logarithm in base "10" so you have to use log10(x)

 

[Holaa]

OK, what do you mean exactly with "wrong plots". Did you get any error message ?
You can try to plot a small subset of data (f.i. the first 20 values), so you can also display each value and have a look if something strange appear.
Just an additional thing: as far as I remember log(x) is the logarithm in base "e"; to convert in dB you need instead the logarithm in base "10" so you have to use log10(x)

Thanks alot for your answers

I mean with ""wrong plots" that I get something not expected and illogical. even that, I tried log10(x) but still same problem :--(

 

[albbg]

Sorry, but I don't understand what you mean. Illogical with respect to what ?
Could you post more informations ? F.i. the plot you did and the expected one. Also if possible 10 values of the data vector leading to a "wrong" graph.