How can I get 2 signal differences in log scale?

Can anybody help me, please!! I have a problem, hope you can help me! I have 2 signals in logarithmic scale, their power levels are different. The plot is here: https://www.dropbox.com/s/17dkngq0a4dqzgc/plot.bmp

I need to subtract this 2 signals and get their power level differences in dB, for example, for 1K level difference is appx. 30 dB. All results I need to plot in log scale as well. I used 10^(db/20) to get them into power scale, but do not know how to get them back to dB scale. So, which command to choose to subtract them in log-scale??

Thank you in advance!!

Answer 1

[Matt J]

Wouldn't it just be the inverse of 10^(db/20), i.e.,

 db=20*log10(power)

[Lucky]

thank you for your answer, but I tried this, the argument should be a number log10(20) for example, but I have a signal curve, log10(signal) doesn't work for me.. should I calculate to each frequency dB values by this formula??

[Matt J]

Works fine for me

>> signal=1:5; db=20*log10(signal)

db =

           0    6.0206    9.5424   12.0412   13.9794

Answer 2

[Iain]

You can get the ratio between the power levels simply by taking dB1 - dB2. This is probably more sensible unless the numbers are very similar.

 p1 = 10^(10*dB); % etc
 dp = p1 - p2
 ddB = 10*log10(dp); %

Thats to do it through power. For voltage or current:

 i1 = 10^(20*dB1); 
 di = i1 - i2
 ddB = 10*log10(di);

[Lucky]

thank you, now its more clear, but the next question is, how to extract values (dBs from X axes) from my signal figures? As I understand, these formulas work only for real values?