%		     The University of Queensland
%     School of Information Technology & Electrical Engineering
%	     COMS3100/7100 Introduction to Communications

% Read a section of an audio clip and display three plots illustrating
% the difference between continuous-time/analogue, discrete-time and
% digital signals

% Read in the clip

[x, Fs] = wavread('clean-clip.wav');

% Select a bit that looks interesting

start = 50000;
len = Fs/20;
ct = x(start:(start+len), 1);

% Plot the continuous-time signal

figure(1);
clf;
plot(0:(1/Fs):(len/Fs), ct);
ylim([-0.8, 0.8]);
xlabel('\itt');
ylabel('\itx\rm(\itt\rm)');

% Convert to discrete-trime by sampling (actually, by grossly undersampling,
% but we'll come to that later in the course).

dt = ct(1:100:length(ct));

% Plot it

figure(2);
clf;
stem(dt, 'filled', 'g');
axis tight;
h = gca;
set(h, 'XTick', 1:length(dt), 'XGrid', 'on');
ylim([-0.8, 0.8]);
xlabel('\itn');
ylabel('\itx\rm[\itn\rm]');

% Now quantise the amplitudes to create a `digital' signal (of course, all
% the data is already in stored in the computer and so must be digital
% already, but we exaggerate the quantisation here).  Here, we quantise
% to the nearest multiple of 0.2.

dig = 0.2 * round(dt / 0.2);

% plot it again

figure(3);
clf;
stem(dig, 'filled', 'r');
axis tight;
h = gca;
set(h, 'XTick', 1:length(dt), 'XGrid', 'on', 'YTick', -0.8:0.2:0.8, ...
    'YGrid', 'on');
ylim([-0.8, 0.8]);
xlabel('\itn');
ylabel('\itx\rm[\itn\rm]');