Patent GB-1288959-A (GB)
IPC: G10H1/043
Filed: 1969-10-30
Abstract: 1288959 Phase modulation FAMILLER and BOKEY 30 Oct 1969 [30 Oct 1968] 53223/69 Heading H3R [Also in Division G5] Electrical input signals are modulated by apparatus comprising a plurality of parallel filters of different characteristics wherein signals representative of said input signals are applied to the filters with different phases and the frequency response of at least one of the filters is varied. In a circuit to produce a tremulant effect on sound from a musical instrument, Fig. 3, the electrical signal 30 representative of the sound is fed to the primary winding of a transformer 32, the secondary winding of which has an earthed centre tap 34. The signal from one end of the secondary winding is fed to a low pass filter consisting of a capacitor 38 and a photosensitive resistor 36 and the signal from the other end, which is 180 degrees out-of-phase, is passed to a high pass filter comprising capacitor 42 and photosensitive resistor 44. After passing through the filters the signals are combined by resistors 40, 46 to give an output. If the resistors 36, 44 are of high value, the output will be in phase with the signal applied to the high pass filter. Conversely if the resistors are of low value, the output will be in phase with the signal applied to the low pass filter. Therefore as the light source 48 is driven by signal generator 50 to vary the resistance of the resistors 36, 44 the phase of the output changes, producing a tremulant effect. If the crossover point (56) of the high- and low-pass filters is at the 50% level (Fig. 4, not shown) the output amplitude will remain constant. If however the crossover point is not at the 50% level the amplitude of the output will also be modulated with the varying light source 48. In order to produce a sharper roll-off characteristic which will produce a more marked tremulant effect, several basic circuits may be connected in series (Fig. 6, not shown). The output from the combining resistors (80, 86) passes through a capacitor to a transistor (90) forming a buffer amplifier stage which drives the primary winding of the transformer (74) in the next stage. In a complete system, Fig. 9, the output signal from an instrument is fed to a transistor 252 acting as a pre-amplifier. From the emitter thereof the signal is split into three parallel paths. The first path leads to a high pass filter fo-med from capacitors 254, and resistors 256. From this filter the signal passes through four identical series-connected tremulant circuits 262-268. The tremulant circuits each consist of a transistor 272 from the collector of which a signal is fed through a second transistor to a high pass filter and from the emitter of which a 180-degree out-of-phase signal is fed to a low pass filter. The rest of the circuit is as previously described. From the fourth tremulant circuit the signal passes through a resistor 302 and capacitor 304 to a two transistor output stage. The second path passes to a low pass filter formed from resistors 258 and capacitors 260. The signal then passes through three identical series - connected tremulant circuits 320-324. These circuits are similar to those in the first path except the signals are fed from the collector and emitter of the transistor 328 to the high- and low-pass filters respectively through capacitors 330, 336 instead of transistors. The output from the third tremulant circuit 324 passes through a resistor 356 and capacitor 304 to the output stage. The third path leads to a circuit producing spring type reverberation of the modulated signal, the output of the circuit is fed via a resistor 394 and capacitor 304 to the output stage. A generator 50 for producing a varying light source 48 (Fig. 7. not shown) uses a unijunction transistor (126) to generate a series of spike pulses which trigger a bi-stable multivibrator formed from two transistors (134, 136). The output square wave thereof is integrated by a network formed from capacitors (156, 158) and a photosensitive resistor (160). The triangular waveform so produced is fed to transistors (148, 150) which drive a lamp (152) giving a sine wave variation in illumination due to the integrating effect of the lamp filament. The integration of the multivibrator output is controlled by the resistor (160) which is illuminated by a lamp (172) driven by transistors (162, 166, 170) whose bias is set by the voltage input to the unijunction transistor (126). This input voltage controls the frequency of the tremulant signal and has limits defined by potentiometers (112, 114). In an otherwise similar circuit (Fig. 8, not shown) Zener diodes are used to control the limit of the input voltage to the unijunction transistor (188) and a switch (230) is provided to selectively control the time constant of the integrator network. Reference has been directed by the Comptroller to Specification 1,088,131.