How does command STEM work for complex numbers?

Ques: How does command STEM work for complex numbers?

Description: I have taken a signal, and using the matlab command i found its fft. The fft results will have same no. of points as it is in the signal. Most of the fft points will be complex no. When i do the plot of the fft signal, it will display on the graph in discrete format.

I tried this:

a=[0,0,0,0,1,1,1,1,1,1,1,1,1,1,1,1,0,0,0,0]
f=fft(a);
disp(f);
figure, stem(a);
figure, stem(f)
imtool(f,'InitialMagnification','fit');

Query: When i see the elements of 'f' its something like this

    12.0000          
      -6.0047 + 0.9511i
      -1.8090 + 0.5878i
       1.1536 - 0.5878i
       1.3090 - 0.9511i
            0          
      -0.6910 + 0.9511i
      -0.2995 + 0.5878i
       0.1910 - 0.5878i
       0.1506 - 0.9511i
            0          
       0.1506 + 0.9511i
       0.1910 + 0.5878i
      -0.2995 - 0.5878i
      -0.6910 - 0.9511i
            0          
       1.3090 + 0.9511i
       1.1536 + 0.5878i
      -1.8090 - 0.5878i
      -6.0047 - 0.9511i

and when i plot 'f' using command stem(f), it gives a plot of all these values.

What i noticed is that, the real part is not considered while plotting, but its only the imaginary part which is considered for plotting, and that too its reversed, it means the positive values are shown below x axis and negative values are shown above x axis.

Any clarification? I hope that i'm doing the right thing, and looking into the right values.

[Matt J]

I recommend modifying your Question's title. There is nothing here that is tied to FFTs. Your question is really about how the stem() command processes complex-valued data.

[Chandra Shaker Balure]

@Matt J - Thank you, this is my first ever question posted on matlab central. I will surely give the relevant title for the question asked.

Answer 1

[Matt J]

It is interesting that the real part is ignored by STEM. That is undocumented behavior, as far as I can see. I do not see the flipping of the sign that you mention. Negative imaginary values for me appear below the x-axis, as they should.

I hope that i'm doing the right thing, and looking into the right values.

It's not clear what behavior you expected from STEM for complex-valued input, since none is documented.

Answer 2

[Jos (10584)]

STEM uses PLOT to make its lines. The documentation of PLOT is clear on complex numbers:

PLOT(Y) plots the columns of Y versus their index. If Y is complex, PLOT(Y) is equivalent to PLOT(real(Y),imag(Y)). In all other uses of PLOT, the imaginary part is ignored.

Try to use STEM with an X argument to plot the real part of Y, or get the part of Y you are interested in using stem(real(Y)) (or stem(imag(Y))).