====== 预滤波 ======

===== - Pre-emphasis And De-emphasis =====
  y(n) = x(n)-a*x(n-1)
  y(n) = x(n) + a*x(n-1)
  a = 0.9~1.0，(speex为:0.98)

===== - 直流陷波 =====
  y(n) = x(n) - 2*x(n-1) + x(n-2) + 1.964 * y(n-1) - 0.964 * y(n-2)
  
H(z) = G*(1 - 2 * z<sup>-1</sup> + z<sup>-2</sup>) / (1 - 1.964*z<sup>-1</sup> + 0.964*z<sup>-2</sup>)

G = 0.9(Fs<12k) / 0.982(fs<24k) / 0.992(fs>24k)