| 摘要 |
There is disclosed a circuit for controlling the expression of an electronically controlled keyboard instrument and is an improvement on the circuit disclosed in application Ser. No. 680,996. The circuit works on the principle that by switching a solenoid on and off at a rapid rate and then varying the time on versus the time off, the energy supplied to the solenoid varies and therefore the striking force of the piano is changed. In accordance with the present invention, precise control over the width of the pulses is achieved by first setting a set voltage level and then adding thereto increments of set voltage according to a binary weighting. These voltages are then added and compared with a triangular voltage in a comparator. Both the up ramp portion and the down ramp portion of the triangular waveform are utilized and compared against the sum voltages. The pulse width of the comparator output is thus a function of the intersection of the ramp voltage, both up and down ramps, with the sum voltage. By varying the sum voltage in digital increments the width of the pulses is varied and these pulses which are supplied to the solenoid thus have the power or energy of the solenoid varied to vary the expression effects on the playback of the piano, for example. |
