Инициализация:
   X = 1;
   R = exp(j*2*pi/Fs*Fc)
    Рекурсия:
   X = X * R

Application Notes: The contents of the structures NSP?ToneState
and the particular equations used to calculate the tone are
implementation-dependent. The tone is calculated using a structure that
implements the following second-order transfer function:

X(z)  = z^–1/ (1 - az^–1 + z^–2),  a = 2cos(2pi × rfreq)

This system has two complex conjugate poles on the unit circle. The angle
of the poles is determined by rfreq. There are several possible equations
to implement this system. The particular equation used is
implementation-dependent because the relative speed and harmonic
distortion depends on the particular processor.