The ``leaning sawtooth'' waveforms observed by Helmholtz for steady state bowed strings can be obtained by periodically ``plucking'' the string in only one direction along the string . In principle, a traveling impulsive excitation is introduced into the string in the right-going direction for a ``down bow'' and in the left-going direction for an ``up bow.'' This simplified bowing simulation works best for smooth bowing styles in which the notes have slow attacks. More varied types of attack can be achieved using the more physically accurate McIntyre-Woodhouse theory [307,431].
Commuting the string and resonator means that the string is now plucked by a periodically repeated resonator impulse response. A nice simplified vibrato implementation is available by varying the impulse-response retriggering period, i.e., the vibrato is implemented in the excitation oscillator and not in the delay loop. The string loop delay need not be modulated at all. While this departs from being a physical model, the vibrato quality is satisfying and qualitatively similar to that obtained by a rigorous physical model. Figure 9.55 illustrates the overall block diagram of the simplified bowed string and its commuted and response-excited versions.
In current technology, it is reasonable to store one recording of the resonator impulse response in digital memory as one of many possible string excitation tables. The excitation can contribute to many aspects of the tone to be synthesized, such as whether it is a violin or a cello, the force of the bow, and where the bow is playing on the string. Also, graphical equalization and other time-invariant filtering can be provided in the form of alternate excitation-table choices.
During the synthesis of a single bowed-string tone, the excitation signal is played into the string quasi-periodically. Since the excitation signal is typically longer than one period of the tone, it is necessary to either (1) interrupt the excitation playback to replay it from the beginning, or (2) start a new playback which overlaps with the playback in progress. Variant (2) requires a separate incrementing pointer and addition for each instance of the excitation playback; thus it is more expensive, but it is preferred from a quality standpoint. This same issue also arises in the Chant synthesis technique for the singing voice . In the Chant technique, a sum of three or more enveloped sinusoids (called FOFs) is periodically played out to synthesize a sung vowel tone. In the unlikely event that the excitation table is less than one period long, it is of course extended by zeros, as is done in the VOSIM voice synthesis technique  which can be considered a simplified forerunner of Chant.
Of course, ordinary wavetable synthesis [303,199,327] or any other type of synthesis can also be used as an excitation signal in which case the string loop behaves as a pitch-synchronous comb filter following the wavetable oscillator. Interesting effects can be obtained by slightly detuning the wavetable oscillator and delay loop; tuning the wavetable oscillator to a harmonic of the delay loop can also produce an ethereal effect.
More General Quasi-Periodic Synthesis
The Tonehole as a Two-Port Loaded Junction