Patent GB-1458966-A (GB)
Title: Waveform processing
IPC: G10L19/00
Filed: 1972-12-22
Abstract: 1458966 Waveform synthesizers ELECTRONIC MUSIC STUDIOS (LONDON) Ltd 19 Dec 1973 [22 Dec 1972] 59540/72 Heading H4R A synthesizer includes a plurality of digital waveform-producing means each operable to produce for a given frequency band a set of digital signals defining successive points of a waveform at a specific frequency, a store containing digital signals representative of successive, desired amplitude or frequency values of one of the waveforms, and an interpolator for causing the corresponding parameter of the waveform to change in a plurality of increments from the current to the stored value. The digital signals are combined with control of amplitude, phase, waveshape and/or frequency to produce a composite waveform. The synthesizer may be used to transpose music or reconstitute speech or music analysed by a channel vocoder in which the spectral energy in predetermined frequency bands is analysed and subsequently synthesized as above in terms of digitally generated discrete frequencies. In a vocoder (Fig. 1) the input waveform is analysed once every 1400 Ás in 64 frequency channels 6, converted to digital form 7 and routed via 12-bit general purpose computer 2 to store 4. A special purpose computer 1 receives the stored data and interpolates the information to produce data at 46,000 points/sec. by, e.g. inverse fourier transformation. The computer 1 may receive spoken inventory enquiries over a telephone line, perform any computations necessary and compose a synthesized reply. The data in store 4 includes digitally coded words defining both the frequency band and the "address" of the corresponding filter 6 in the analyser since in certain applications the frequency bands may be adjustable through the audio range. The synthesizer 1 is initially programmed from store 4 with data concerning amplitude, frequency and waveform characteristics and subsequently receives from computer 2 for each waveform generator a series of words defining the required amplitude of the associated waveform at a particular time, the words in the series being interleaved. The synthesizer itself (Fig. 2) includes a bank of 64 oscillators 8 generating sawtooth waveforms in digital fashion and at unit amplitude. A programmed input f 2  changes at 10 the waveform (e.g. to sine-wave) and modulator 11 changes the amplitude according to the output of a programmed f 3  function generator 12. The thus generated waveforms are combined 9 and fed to a D-A converter and exponential modulator 13 to produce an analogue output in real time, the overall level being controlled at 15. The oscillators 8 are frequency controlled by programmed f 1  function generators 20 which receive instructions from present value stores 19 and linear interpolators 18. Inputs 16, 17 respectively denote the new frequency required and the ("slew") time required to make the change, the interpolators 18 creating further frequency values at 46,000 points/sec. between present and new values. A similar arrangement 21-24 controls amplitude variation. The interpolator is more fully described in Specification 1,449,812. The above elements may be timeshared by all the oscillators, and may be arranged in three groups with separate waveform and amplitude control to produce three different "voices". The oscillator frequencies are accurate to 0À25 Hz over the audio range by using 16-bit frequency-determining words defining 64,000 possible frequencies. It is stated that only 120 frequencies are necessary for diatonic music bit 512 (i.e. 50 steps per octave) are preferable for broader applications and may denote any suitable ones of the 64,000 frequencies available. In this way only 9 bits require transmission. The generation of the oscillator frequencies is performed by sixty four 12-bit storage locations (25, Fig. 3, not shown) which hold the momentary values of the sawtooth waveforms. Each store is addressed in turn over a bus (26) and a value M is read in equal to the sum of the last calculated value N and three times the instantaneous frequency value F of the oscillator, F normally being constant. Each store resets to zero at maximum count thereby generating a sawtooth waveform whose gradient depends on the current value in store 19. The Specification also outlines an interface (Figs. 4-9, not shown) and table of instructions controlling the routing of signals between the computer 2 and synthesizer 1. The computer operates in a closed loop until instructed by the interface to transfer a new value from its AC register to the synthesizer. Circuit panels using standard circuit blocks are shown for various stores, buffers, drivers, master clock and timing circuits, function generators and the D-A converter and exponential modulator of the synthesizer. The latter (Figs. 26, 27, not shown) comprise a pair of D-A converters (A1, A2) differentially combined and switched (A3), when the combined output is steady, to an unfiltered output (UFOP) and through filters (A5-A9) and differential amplifier (A10) to a filtered output (FOP). The exponential modulation is effected by a luminescent diode (42) acting on a resistor (40) in the filter output path and a further resistor (41) in the feedback path of amplifier (All) receiving a control signal which is the exponential of a signal fed to a long-tail pair (43) from analogue versions of the computer AC outputs and signals controlling the "voice" to be reproduced. A further output (A10) controls the overall level of the output.