Patent GB-1308070-A (GB)
Title: Oscillator keyboard with roll and double strike control
IPC: H03M11/02
Filed: 1970-12-15
Abstract: 1308070 Keyboard SPERRY RAND CORP 15 Dec 1970 [15 Dec 1969] 59551/70 Heading G4H An electronic keyboard comprises a number of keys, a number of switch circuits actuated by respective keys and connected in parallel to a constant current source, an encoding circuit for generating coded outputs in response to actuation of the switch circuits, a circuit for sensing when one of the switch circuits is actuated and generating a gating signal for gating the coded signals to an output. The keyboard may include a further sensing circuit for generating an output when two of the switch circuits are actuated and thereby inhibiting the production of the gating signal. The system is arranged so that if two keys are actuated within 12 m. sees. the gating signal is inhibited and an alarm is sounded. Each key 3 is connected to an inductance core 5 having ferrite and brass portions so that actuation of the key varies the inductance in a tank circuit and causes a respective transistor Q1 to switch. The outputs from the key circuits 21 are selectively connected to a number of NOR gates 37 which produce on lines KB1-5, a coded representation of the actuated key. In order to allow the outputs of the NOR gates to stabilize a delayed gating pulse KB6 is generated to gate the output to, e.g., a computer. The pulse KB6 is produced by a circuit which includes a Schmitt trigger 39 which is fired at a voltage somewhat less than that required to trigger the NOR gates. When a key is depressed the voltage at the collector of the corresponding key switch transistor drops to fire the trigger and generate pulse KB6 which is passed via NOT circuit 47 and delay circuit 49 to the output gating arrangement (not shown).  The transistors of each key circuit are connected in parallel to a constant current source 25 so that when two keys are actuated simultaneously the current divides between the respective transistors whose collector voltages drop to a value less than if only one key is actuated. A further Schmitt trigger 55 monitors the common collector voltage at point 57 and is designed to fire at a voltage between those which signify that one and two keys have been actuated and before the encoding NOR gates 37. If two keys are actuated at the same instant or within 12 m.secs. the circuit operates as follows. When the first KB6 signal is generated the output of NOT 47 is differentiated by capacitor 66 and triggers multivibrator 69 which produces a logical 1 output for 12 m.secs which is supplied to NAND 65. When the second key is actuated trigger 55 fires and its output drops to logical 0. This output is supplied via inverter 61 to NAND 65 which then sets flip-flop 71 and activates a buzzer. The output of inverter 61 is inverted at 63 and drives the input of NOT 47 to logical 0 independently of the state of trigger 39. The output of NOT 47 is inverted by NOT 51 and a second KB6 pulse is not produced. When two keys are rolled, i.e. their actuations overlap but are spaced by more than 12 m.secs. the output of multivibrator 69 drops to logical zero before the actuation of the second key so that the buzzer is not actuated. When the second key is depressed trigger 55 is activated and the output of inverter 61 rises to logical 1. This signal is inverted at 63 and drives the input of NOT 47 to logical 0. When the first depressed key is released the voltage at point 32 holds trigger 39 off (due to the still actuated second key) but the voltage at 57 rises above the threshold of trigger 55 which is turned off. The output of inverter 61 drops to logical 0 and the output of NOT 63 rises to logical 1. The output of NOT is logical 1 (due to trigger 39) so that NOT 49 produces a logical 0 output which via delay 49 and NOT 51 becomes the KB6 gating pulse for the second key.