quantization errors along a frequency swap

#-*- coding: utf8 -*-
from __future__ import division
import numpy as n, pylab as p

nbits=4
fa=8000 # Hz
nperiodos=1000.

passoFreq=20 # Hz

freqInit=20
freqFin=20000


ngrados=2**nbits
rd=[]
for freq in xrange(freqInit,freqFin,passoFreq):
    afreq=2*n.pi*freq
    sinal=n.sin(n.linspace(0,afreq*nperiodos,passoFreq*2*n.pi,endpoint=False))
    sinal_=((sinal+1)/2)*ngrados
    sinal__=n.array(sinal_,dtype=n.int)
    sinal_quantizado=sinal__/ngrados
    erro_quad=(sinal-sinal_quantizado)**2
    relacao_db=10*n.log10(sum(erro_quad)/sum(sinal**2))
    print relacao_db
    rd.append(relacao_db)

rmedia=n.mean(rd)
rdesvio=n.std(rd)
print rmedia, rdesvio