HFnu was considered as an index of modulation
of parasympathetic branch of ANS as it influences sinoatrial node and LFnu was usually viewed as an index of modulation
of the sympathetic division of the ANS.The representation of LF and HF in normalised units emphasizes the controlled and balanced behaviour of the two branches of the autonomic nervous system.
The carrier frequency, c, is altered by the modulating one, m, while the number of partials in the resulting spectrum is determined by an index of modulation, i.
He used [G.sub.4] (392 Hz) as a carrier and G[[sharp].sub.3] (207.65 Hz) as the modulating frequency, and chose 9 for the index of modulation. Example 13 shows the summation and difference tones resulting from the preceding formula.