File:Circular convolution example.svg

% Options
  frame_background_gray = true;

  if frame_background_gray
   graphics_toolkit("qt")         % has "insert text" option
%  graphics_toolkit("fltk")       % has cursor coordinate readout
   frame_background = .94*[1 1 1];
   d = 2;                         % amount to add to text sizes
  else
   graphics_toolkit("gnuplot")    % background will be white regardless of value below
   frame_background = .94*[1 1 1];
   d=0;
  endif

% (https://octave.org/doc/v4.2.1/Graphics-Object-Properties.html#Graphics-Object-Properties)
% Speed things up when using Gnuplot
  set(0, "DefaultFigureColor",frame_background)
  set(0, "DefaultAxesFontsize",10+d)   % size of numeric tick labels
  set(0, "DefaultTextFontsize",12+d)
  set(0, "DefaultAxesXtick",[])
  set(0, "DefaultAxesYtick",[])
  set(0, "DefaultLineLinewidth",1)
 
xmax = 3000;
%=======================================================
hfig = figure("position",[100 100 488 512], "color",frame_background);

x1 = .02;               % left margin
x2 = .02;               % right margin
y1 = .08;               % bottom margin for annotation
y2 = .08;               % top margin for title
dy = .04;               % vertical space between rows

width = 1-x1-x2;
height= (1-y1-y2-5*dy)/6; % space allocated for each of 6 rows

x_origin = x1;
y_origin = 1;           % start at top of graph area
%=======================================================
y_origin = y_origin -y2 -height;        % position of top row
% subplot() undoes all the "color" attempts above.  (gnuplot bug)
subplot("position",[x_origin y_origin width height])
L = 100;
f = ones(1,L)/L;
plot(-100:200-1, [zeros(1,100) f*L zeros(1,100)], "linewidth",2, "color","magenta")
xlim([-100 xmax]); ylim([0 2])
title("Circular convolution example", "fontsize",16)
text(100, 1.6, "h[n]")
%text(xmax/2, 0.4, '\leftarrow n \rightarrow')
 text(2500, 0.330, '\leftarrow n \rightarrow')

y_origin = y_origin -dy -height;
subplot("position",[x_origin y_origin width height])
a = [zeros(1,20) ones(1,L) zeros(1,300) 0.5*ones(1,100) zeros(1,1000-L-20-400)];
b = [zeros(1,1000-L-20) ones(1,L) zeros(1,20)];
a1 = [zeros(1,1000) a zeros(1,1000)];
b1 = [zeros(1,1000) b zeros(1,1000)];
plot(1:length(a1), a1, "color","blue", 1:length(a1), b1, "color","red")
xlim([0 xmax]); ylim([0 2])
text(200, 1.6, "X[n]")

y_origin = y_origin -dy -height;
subplot("position",[x_origin y_origin width height])
a1 = conv(a1,f);
b1 = conv(b1,f);
plot(1:length(a1), a1+b1, "color","green", "linewidth",2)
xlim([0 xmax]); ylim([0 2*max(a1)])
 text(200, 1.6, "X[n] * h[n]")
%text(200, 1.6, "X[n] ∗ h[n]", "interpreter","none") % requires PERL post-processor

y_origin = y_origin -dy -height;
subplot("position",[x_origin y_origin width height])
a = [a a a];
b = [b b b];
L = 1:length(a);
plot(L, a, "color","blue", L, b, "color","red")
xlim([0 xmax]); ylim([0 2.5])
set(gca,"xtick", [1000 2000]);
%set(gca,"xticklabel",["N" "2N"])
set(gca,"xticklabel",[]); text(981,-.5, "N"); text(1955,-.5, "2N")
text(200, 2.0, 'X_N[n]')

y_origin = y_origin -dy -height;
subplot("position",[x_origin y_origin width height])
a1 = conv(a,f);
b1 = conv(b,f);
b1(1:90) = b1(3000+[1:90]);
L = 1:length(a1);
plot(L,a1,"color","blue", L,b1, "color","red")
xlim([0 xmax]); ylim([0 2*max(a1)])
text(200, 1.6, 'components of X_N[n] * h[n]') % can't use "interpreter","none" here

y_origin = y_origin -dy -height;
subplot("position",[x_origin y_origin width height])
c = a1+b1;
L = length(c);
k=1100;
plot(1:k, c(1:k), "color","red", k+(1:900), c(k+(1:900)), "color","green",...
        "linewidth",2, (k+900+1):xmax, c((k+900+1):xmax), "color","red")
xlim([0 xmax]); ylim([0 2*max(a1+b1)])
text(200, 1.6, 'X_N[n] * h[n]') % can't use "interpreter","none" here

 text(1263, -.6, "X[n] * h[n]", "fontsize",16)
%text(1274, -.6, "X[n] ∗ h[n]", "interpreter","none", "fontsize",16) % requires PERL post-processor

% After a call to annotation(), the cursor coordinates change to the units used below.
annotation("line", [.367 .367], [.113 .022])
annotation("line", [.664 .664], [.113 .022])