%
%?Example?of?sound?aliasing
%
clear?all;

fc?=?1.8e3;???????????????%?tone?freq
Fs?=?1e3*8;???????????????%?sample?freq
Ts?=?1/Fs;????????????????%?sample?time
timelen?=?3;??????????????%?sample?file?length

n?=?0:ceil（timelen/Ts）-1;
x?=?0.9*cos（2*pi*fc*n*Ts）;

disp（[‘tone?freq?=?‘num2str（fc）‘?sampling?frequency?=?‘num2str（Fs）]）;
wavplay（xFs）;

%
%?downsampling
%
d2=3/4;
disp（‘press?any?key?to?continue‘）;
pause
disp（[‘tone?freq?=?‘num2str（fc）‘?sampling?frequency?=?‘num2str（Fs*d2）]）;
x_d2?=?0.9*cos（2*pi*fc*n*Ts/d2）;
wavplay（x_d2（1:floor（length（x_d2）*d2））Fs*d2）;

d3=1/3;
disp（‘press?any?key?to?continue‘）;
pause
disp（[‘tone?freq?=?‘num2str（fc）‘?sampling?frequency?=?‘num2str（Fs*d3）]）;
x_d3?=?0.9*cos（2*pi*fc*n*Ts/d3）;
wavplay（x_d3（1:floor（length（x_d3）*d3））Fs*d3）;

%
%?write?wav?files
%
%wavwrite（xFs16‘output01.wav‘）;
%wavwrite（x_d2（1:floor（length（x_d2）*d2））Fs*d216‘output02.wav‘）;
%wavwrite（x_d3（1:floor（length（x_d3）*d3））Fs*d316‘output03.wav‘）;

%
%?show?plot
%
Xw?=?fft（xlength（x））;
L?=?0:length（Xw）-1;
figure（1）;
subplot（211）;
plot（Fs*L/length（Xw）/1e3?20*log10（abs（Xw）））;?grid;
xlabel（‘Frequency?KHz‘）;?ylabel（‘Gain?dB‘）;
title（[‘Tone?Freq?=?‘num2str（fc）‘?Sampling?Freq?=?‘num2str（Fs）]）;
subplot（212）;
stem（1:30x（1:30））;
xlabel（‘n‘）;?ylabel（‘Magnitude‘）;

Xw_d2?=?fft（x_d2length（x_d2））;
L_d2?=?0:length（Xw_d2）-1;
figure（2）;
subplot（211）;
plot（（Fs*d2）*L_d2/length（Xw_d2）/1e3?20*log10（abs（Xw_d2）））;?grid;
xlabel（‘Frequency?KHz‘）;?ylabel（‘Gain?dB‘）;
title（[‘Tone?Freq?=?‘num2str（fc）‘?Sampling?Freq?=?‘num2str（Fs*d2）]）;
subplot（212）;
stem（1:30x_d2（1:30））;
xlabel（‘n‘）;?ylabel（‘Magnitude‘）;

Xw_d3?=?fft（x_d3length（x_d3））;
L_d3?=?0:length（Xw_d3）-1;
figure（3）;
subplot（211）;
plot（（Fs*d3）*L_d3/length（Xw_d3）/1e3?20*log10（abs（Xw_d3）））;?grid;
xlabel（‘Frequency?KHz‘）;?ylabel（‘Gain?dB‘）;
title（[‘Tone?Freq?=?‘num2str（fc）‘?Sampling?Freq?=?‘num2str（Fs*d3）]）;
subplot（212）;
stem（1:30x_d3（1:30））;
xlabel（‘n‘）;?ylabel（‘Magnitude‘）;