function ejemplotonos(N,P);
if(nargin==1)
    P=1;
end
M=1024;
if(N>M)
    M=N;
end
x=zeros(N,P);
for i=1:P,
    x(:,i)=cos(0.35*pi*[0:N-1]'+2*pi*(rand(1,1)-0.5))+cos(0.4*pi*[0:N-1]'+2*pi*(rand(1,1)-0.5))+randn(N,1);
end
S=abs(fft(x,M)).^2/N;
SM=mean(S');
subplot(211);
plot(linspace(0,0.5,512),10*log10(S(1:512,:)));
ylabel('Densidad Spectral Potencia (dB)')
xlabel('frecuencia');
grid on;
subplot(212)
plot(linspace(0,0.5,512),10*log10(SM(1:512)));
grid on;
ylabel('Densidad Spectral Potencia (dB)')
xlabel('frecuencia');