Plotting and comparing modified periodograms

Hello guys,

I have plotted the modified periodogram of a signal using periodogram with the Hamming window, and tried to compare it with Hayes' mper() function from Statistical Digital Signal Processing and Modeling, 1996, pp 394. But the result isn't the same.

Could anyone point me where I am wrong?

clear all
clc
close all

ap1 = -2.737;
ap2 = 3.746;
ap3 = -2.629;
ap4 = 0.922;
a = [1, ap1, ap2, ap3, ap4];          
b = 1.0;                   

figure(1)
Pz4=0;
Pz=0;
for j=1:50
S2 = filter(b,a,randn(2048,1));
[Px4,F4] = periodogram(S2,hamming(length(S2)),length(S2),1);
Pz4 = Pz4 + Px4;
Px=mper(S2,2);
Pz = Pz + Px;
end
plot(F4,10*log10(Pz/50));
hold on
plot(F4,10*log10(Pz4/50));


function Px = mper(x, win, n1, n2)

x = x(:);
if nargin == 2
    n1 = 1; n2 = length(x);
end

N = n2 - n1 + 1;

switch win
    case 1
        w = rectwin(N);
    case 2 
        w = hamming(N);
    case 3
        w = blackman(N);
end

xw = x(n1:n2).*w/norm(w);
Px = N*periodogram(xw);
end