The core of the single sample feedback network consists of two oscillators that cross-modulate each other. At each new sample the phase of the two oscillators is computed by adding the actual phase of each oscillator to the one sample old output of the two oscillators, as follows:
𝑥(𝑡) = 𝑐𝑜𝑠(𝑤𝑡 + 𝐴𝑥(𝑡 − 1) + 𝐵𝑦(𝑡 − 1))
𝑦(𝑡) = 𝑐𝑜𝑠(𝑤𝑡 + 𝐶𝑥(𝑡 − 1) + 𝐷𝑦(𝑡 − 1))
In frequency modulation synthesis A, B, C and D are usually called mod- ulation indexes and their value determines the amount of modulation that is applied to the signal. As described in the previous section, these indexes are dependent on the analysis performed by the first network. In particular, they are in an inverse relation with each other: modulation indexes increase when the output is soft, and decrease when this is louder or more noisy.
Other major components of the network are an adaptive comb filter, a zero crossing frequency estimator and a square wave generator. All these elements are organised in a feedback loop whose recirculating signals are reorganised through a 8x4 feedback matrix. This matrix is responsible for combining different sources and rescaling the amount of signal that is fed back to the process in the successive iteration. A simplified representation of the network is in Figure 2.5.