#!/usr/bin/python2.5
from pylab import *
from numpy import *

def pdm(x):
    y = zeros_like(x)
    qe = zeros_like(x)
 
    qe[-1] = 0 # inital [[:en:quantization error]] is 0
    for n in range(x.size):
        y[n] = 2*( x[n] > qe[n-1] ) - 1
        qe[n] = y[n] - x[n] + qe[n-1]
 
    return y, qe

def squarify(x, y):
    halfway = ( x[1:]+x[:-1] )/2
    xo = hstack(( x[0], repeat(halfway, 2), x[-1] ))
    yo = repeat(y, 2)
    return xo, yo
 
t = arange(100.0)
x = sin(2*pi/100 * t)
y, qe = pdm(x)

to, yo = squarify(t, y)
plot( label="PDM sine wave", *squarify(t,y) )
plot( t, x, label="PCM sine wave", linewidth=5 )
 
axis((-5, 105, -1.25, 1.5))
legend(loc='upper right')
 
xticks(fontsize=16)
yticks(fontsize=16)
xlabel("Discrete time (n)", fontsize=18)
ylabel("Signal amplitude",fontsize=18)

savefig("pdm.svg", transparent=True)