Simple hybrid systems for accurate synthesis and analysis of harmonic spectra

This paper discusses a simple hybrid approach to the problems of synthesizing and analyzing harmonic spectra. Most of the attention is given to the synthesis problem but a very brief account is given of magnitude and phase analysis of harmonic spectra. The hybrid approach is shown to possess several important advantages over pure analog or pure digital techniques. The digital portion of the system computes exact values for samples of N harmonics of a fundamental frequency f0, and produces pulses whose duty cycles are exactly proportional to the sampled values. The sampling pulses are multiplexed and gate an analog switch whose output is an amplitude and pulse width modulated signal representing a weighted sum of harmonics. To eliminate aliasing of frequencies above Nfo, a low pass filter is used. The arithmetic is based on a simple harmonic sampling algorithm, and some variations are briefly discussed which can be used for phase shifting and generating subharmonics of the existing spectra. A detailed block diagram for a 16 harmonic synthesizer is given whose only active components are fourteen standard TTL integrated circuits and some op amps (for the low pass analog filter). Finally, we discuss very briefly a possible application of the system to speech synthesis.