The resonant circuits of the synthesizer can be arranged either in parallel, the outputs of the various circuits being summed to produce the final output, or in series, the output of each resonant circuit being fed into the next. The series synthesizer is a closer approximation to the acoustical behavior of the vocal tract, and incorporates in its design Fant's (1956) observation that if certain assumptions are made about the glottal source spectrum and the formant bandwidths, the relative amplitudes of vowel formants can be predicted from their frequencies. Thus a series synthesizer requires fewer parameters and can be expected to produce more natural vowels. On the other hand, parallel synthesizers are far more flexible, and simplify synthesis strategy for sounds with complex spectra, like voiced fricatives. The relative merits of parallel and series synthesizers are far best summed up by Flanagan (1957). Lawrence's (1953) PAT was the first example of a parallel resonance synthesizer; Fant's (1958) OVE II, the first full-scale experimental series synthesizer. Highly reliable resonance synthesizers of both types are now available. 3

A parametric control scheme should have made synthesis by rule simpler, since the parameters to be specified are precisely the dimensions of speech in terms of which it is convenient to state acoustic rules. Ingemann (1960), in fact, reformulated her rules for use with the Edinburgh series version of PAT (Anthony and Lawrence 1962). But in order to control a resonance synthesizer, some means of changing the parameter values dynamically is required. At first this was accomplished with a function generator: for example, parameter functions for the Edinburgh PAT were represented in conductive ink on parallel tracks of a moving plastic belt (Fourcin 1960). But applying the rules (as distinct from stating them) was if anything more troublesome with a function generator than with the Pattern Playback. Fortunately, digital computers now began to become available for phonetic research. Kelly and Gerstman (1961) demonstrated that the computer not only could apply a set of rules (i.e., calculate the parameter values) quickly and accurately but also could be used to simulate the synthesizer itself. 4  Other investigators showed that if an actual, rather than a simulated synthesizer is used, the computer could also play the role of function generator. 5
__________
3.  Other parallel resonance synthesizers are described in Borst 1956, Holmes et al. 1964, Mattingly 1968b and Glace 1968; other series synthesizers are described in Coker 1965, Tomlinson 1965, Liljencrants 1968, Kacprowski and Mikiel 1968, Kato et al. 1968, Dixon and Maxey 1970, Shoup, pers. comm.

4.  The advantage of a simulation is that it can be completely reliable and accurate, and the design of the synthesizer can be readily modified; the disadvantage is that an extremely powerful computer is required and such computers are too expensive to permit extended real-time operation. Recent simulations of resonance synthesizers (all series) include those described in Flanagan et al. 1962, Rao and Thosar 1967, Rabiner 1968, Saito and Hashimoto 1968.

5.  On-line transmission of stored parameter values can be performed by a laboratory computer at a cost low enough to permit the investigator to experiment at length; it is easy to program other convenient facilities such as routines for editing or displaying the stored parameter values. Schemes of this sort include those of Tomlinson (1965), Denes (1965), Coker and Cummiskey (1965), Scott et al. (1966), Mattingly (1968b). Off-line control schemes, in which the computer produces a record, such as a paper tape, which is then used to control a function generator, are also practical, though less convenient (Holmes et al. 1964; Iles 1969).