Device for Producing Artifical Reverberation

The present invention relates to a device for producing artificial reverberation by means of a one-dimensioned waveguide. Such a device is known from Austrian Pat. Nos. 279,203 and 292,332 (U.S. Pat. Nos. 3,566,310 and 3,697,059).

Devices for the production of artifical reverberation resolve the unreverberated signal dispersively in a random manner. Such devices should, therefore, have a dense, irregular pole/zero distribution in the frequency spectrum to prevent flutter echo and tone shading. Preferably the frequency response of the reverberation time should decrease with increasing frequency.

In addition to the known devices for producing artificial reverberation, such as the reverberant chamber, the plate reverberator and the torsional vibration waveguide, it has become possible in recent times to construct electronic reverberation-producing devices using digital techniques. The advantages of digital devices for the production of artificial reverberation over the conventional electromechanical reverberators are their independence of environmental factors, the programmability of all reverberation parameters, their mechanical sturdiness, and the possibility of integrating them into the digital equipment of a sound studio.

For digital production of artificial reverberation, the use of rapid computers or digital signal processors is known, which simulate the transmission behavior of a room in real time. Such a signal processor consists of an analog/digital converter, which samples the unreverberated input signal and converts it into pulse code modulation, a high speed computer specifically designed for the reverberation which processes the coded sampled data, and a digital/analog conveter which re-converts the computer results into a continuous signal. Until now, reverberation equipment has become known in which high-quality resonators are simulated digitally. A loop composed of a digital delay arrangement, a digital damping element, and an adder is used as a resonator. The feedback is effected by multiplication of the feedback signal by a damping factor, which for reasons of stability must be less than 1. The pulse response is a periodical, exponentially decaying pulse sequence, and the frequency response meets the requirements of a periodical comb filter whose natural resonances occur as integral multiples. This periodicity is the main disadvantage of the resonators, and with long loop delays produces a flutter echo and, with short loop delays, a strong tone color. According to the state of the art, there are the following possibilities for weakening these two undesirable effects:

(1) Tapping the loop delay arrangement at several points, multiplying the signals occurring at the taps by different coefficients, and combining them additively as one output signal.

An equivalent circuit is a series-connected comb filter with irregular frequency response, which diminishes the tone color or timbre of the sound.

(2) Using several parallel- and/or series-connected, different resonators, the individual tone colors mask each other to a certain extent.

(3) Supplementing the arrangement described under point 2 by a feedback loop. This results in a reverberation system with multiple feedback and irregular natural resonance distribution. However, such a circuit structure can be realized only using analog filter techniques. In digital filter techniques, a disturbance referred to as a limit cycle occurs. Limit cycles are numerical instabilities which are caused by a buildup of the rounding error, which is inevitable in a multiplication. They are noticed as soft whistling sounds, which do not disappear even after the signal which triggered them has decayed.

Another disadvantage of resonator reverberation is that the reverberation time frequency response is flat and cannot be influenced. Digital resonators, in fact, have the property that all their natural resonances decay equally fast.

It is an object of the invention to provide an electronic device for producing artificial reverberation which, using digital circuit arrangements, does not have the disadvantages of the known devices.

More specifically, it is an object of the invention to provide a dimensional waveguide, such as a waveguide realized with digital technology circuits, which contains at least two line sections of different length, each line section consisting of two parallel digital delay lines, one for each direction of propagation, and that further at least one three-gate adapter of the digital wave filter type is arranged between two line sections lying side by side, each of two gates of the three-gate adapter being connected with one of the two mutually facing ends of these two line sections, and the dependent gate of at least one three-gate adapter being terminated with a frequency-dependent absorber realized by using digital techniques.

Accordingly, the device of the invention for producing artificial reverberation comprises a digital waveguide which consists of many loss-free sections of different length with equal propagation constants, and over whose total length several frequency-dependent absorbers are distributed. To produce reverberation, the one-dimensional waveguide is energized with the signal to be reverberated, which propagates thereon at constant speed. If it arrives at a junction of a three-gate adapter between two line sections, the signal in the three-gate adapter is not only attenuated but also divided into a passing and a reflected component. This is repeated until only reflected components remain. The absorbers which terminate the three-gate adapters are loss resistances inserted in the one-dimensional waveguide and are bridged by capacitances or inductances for controlling their frequency response. Thus these absorbers determine the reverberation time frequency response of the device.

The advantages of the invention may be summarized as follows:

(a) Due to the mutual coupling of the line sections corresponding to the resonators of the conventional digital reverberators, integral frequency multiples of the natural resonances do not occur.

(b) Despite the close coupling of the resonators, limit cycles are not possible.

(c) Through the absorbers it is possible to influence the frequency responses of the reverberation time.

Three-gate adapters are known from German DE-OS No. 20 27 303, see FIGS. 18 and 20. But that publication does not show a device for producing artificial reverberation.

As another possibility, the device for producing artificial reverberation may further contain at least one two-gate adapter of the digital wave filter type, arranged between two line sections lying side by side, each of the two gates of the two-gate adapter being connected with one of the two mutually facing ends of these two line sections. In this manner, too, reflection at one point can be achieved. Further, line sections of different wave resistance can be realized.

Again it should be mentioned that two-gate adapters are known from DE-OS No. 20 27 303, see FIG. 15 (corresponds to U.S. Pat. No. 3,976,099, FIG. 30). A preferred form of the device according to the invention is characterized in that the two ends of the waveguide are connected together, possibly with insertion of a two-gate adapter or a three-gate adapter. By providing a "circular" design of the device for producing artificial reverberation, a still much better diffusivity of the reverberation can be achieved, because not only are the two directions of propagation coupled to each other via the three-gate adapters and/or the two-gate adapters, but also each direction of propagation in itself is coupled in feedback. The input of a non-"circular" form of the device of the invention is such that the device is provided with an input terminal by means of an adder unit, the output of this adder unit being connected with the input for one direction of propagation at one end of the waveguide and the output for the other direction of propagation at the same end of the waveguide with a second input of the adder unit.

The output of this form of the invention is such that, at the other end of the waveguide the output is provided with an output terminal which is connected with the input at the same end of the waveguide either with insertion of an absorber or directly.

The input of a "circular" form of the device of the invention is such that between two line sections lying side by side a first and a second adder unit is provided, one for each direction of propagation, where for each direction of propagation the output of the delay line of one line section is connected to a first input of the adder unit and the output of this adder unit is connected to the input of the delay line of the other line section, and an input signal can be supplied to a second input of both adder units.

The output of this "circular" form is designed according to the invention in such a way that, seen in one direction of propagation, the output of a delay line of a line section is connected to a first input of a third adder unit and viewed in the other direction of propagation the output of a delay line of a line section is connected to a second input of the third adder unit, the output signal being available at an output of this third adder unit.

In a preferred form of the "circular" device according to the invention, the two input signals for the two inputs of the third adder unit are tapped from the delay lines in such a way that the time delay between the first adder unit and the third adder unit seen in the respective direction of propagation is equal to the delay time between the second adder unit and the third adder unit seen in the opposite direction of propagation. This symmetrical design has the advantage that the unreverberated signal and its echoes occurring periodically with the delay of the transit time in the device, can be compensated. To this end, the reverberated signals tapped from the third adder unit are supplied to a differential amplifier, so that after the difference formation essentially only diffuse signal components are present.

The frequency-dependent absorber may correspond to a one-gate circuit composed of a parallel connection of the digital type, of an ohmic resistance and a capacitance. The frequency-dependent absorber may alternately correspond to a one-gate circuit which is formed by a series connection of the digital type, of an ohmic resistance and a capacitance or of an ohmic resistance and an inductance. A digital one-gate circuit which obtains the parallel connection of an ohmic resistance and an inductance may consist, for example, of a chain composed of an adder, a delay section, a multiplier, and a second adder, the output of the multiplier being connected to the second input of the first adder and the output of the delay section being connected via a sign inverter with the second input of the second adder.

Another variant for simulating the parallel connection of an ohmic resistance and a capacitance has the same chain connection as described above, but it differs in that a sign inverter is inserted between the first adder and the delay section.

The series connection of an ohmic resistance and a capacitance can be obtained by a digital one-gate circuit consisting of a chain composed of an adder, a multiplier, a second adder, a sign inverter and a delay section, the output of the one-gate circuit being connected via a sign inverter to the second input of the first adder, and the input of the one-gate circuit being connected via a sign inverter to the second input of the second adder.

A further possibility of the digital simulation of the series connection of an ohmic resistance and an inductance by means of a digital one-gate circuit differs from the above described chain connection in that the output of the second adder is connected directly to the input of the delay section.

Further details of the invention will be explained below with reference to the drawing, in which;

FIG. 1 is a diagram of a line section of a digitally realized one-dimensional waveguide;

FIG. 2 illustrates a discontinuity between two waveguides;

FIG. 3a is a diagram of a serial three-gate adapter;

FIG. 3b is a diagram of a shunt or parallel three-gate adapter;

FIG. 4a and 4b are diagrams of an absorber with parallel-connected reactance and ohmic resistance respectively;

FIG. 4c and 4d are diagrams of an absorber with series-connected reactance and ohmic resistance respectively;

FIG. 5 is a diagram a line consisting of two sections of different wave resistance;

FIG. 6 is a block diagram of the first embodiment of the invention;

FIG. 7a is a diagram of the digital circuit of the recording point in the first embodiment;

FIG. 7b shows the digital circuit of the reverberation pickup in the first embodiment;

FIG. 7c shows a variant of the first embodiment with symmetrical reverberation recording and pickup;

FIG. 8 is a block diagram of a second embodiment of the invention;

FIG. 9a is a diagram of the digital circuit of the recording point in the second embodiment; and

FIG. 9b is a diagram of the digital circuit of the reverberation pickup in the second embodiment.

Referring to the drawings, FIG. 1 shows the propagation of signals on a one-dimensional waveguide and a digital circuit thereof according to the invention. As a waveguide 1 transmits the signals in both directions, its digital circuit consists of two parallel delay arrangements, the delay sections 2 (labelled T) transmitting the signal propagating from left to right while the delay sections 3 transmit the signal propagating from right to left. The signals entering the line sections in the forward direction are marked a1 and a2, the signals leaving the line section (reflected signals) are marked b1 and b2.

The discontinuity shown in FIG. 2 shows the division of the signal a1 into a signal component b1 reflected at the discontinuity, and a signal component b2 passing through it. Such a division of the signal occurs both at the waveguide taps or three-gates due to an inevitable mismatch and at the junctions between waveguides of unequal wave resistance.

FIG. 3a shows a serially tapped line section 11, 12 (the ideal transformer coil 13 is not necessary in principle and has been inserted only for the sake of illustration). The digital realization implementation of a serial tap is termed a serial three-gate adapter and consists of the adders 14, 15, 16 and 17, the sign inverter 18, and the multiplier 19. The wave resistance ratio (or characteristic impedance ratio) between the tapped and tapping lines is determined by the multiplier coefficient ##EQU1## where Z.sub.0 is the characteristic impedance of the tapped line, and Z.sub.1 the characteristic impedance of the tapping line at 10.

FIG. 3b shows the parallel tapped line section 20, 21.

The digital implementation of a parallel line linkage is termed a parallel three-gate adapter and consists of the adders 22, 23, 24, 25 and 26 the sign inverters 27, 28, 29, 30, and the multiplier 31. The multiplier coefficient is given by the ##EQU2## value (G.sub.0 and G.sub.1 are the conductivities of the tapped and tapping lines corresponding to the wave resistances or characteristic impedances Z.sub.0 abd Z.sub.1). Naturally the three-gate adapters can be inserted also between line sections of different wave resistance. But such an arrangement offers no particular advantages and has an adverse effect on the computing time and the S/N (signal to noise) ratio, as in this case the three-gate adapter requires an additional multiplier.

FIG. 4a shows an absorber with rising absorption frequency response, which consists of the parallel connection of an inductance 33 and an ohmic resistance 32 and whose digital circuit is composed of the adders 34, 35, the sign inverter 36, the delay section 37, and the multiplier 38.

FIG. 4b shows a absorber with falling absorption frequency response, which consists of the parallel connection of a capacitance 39 and of an ohmic resistance 40, and whose digital circuit differs from the above described circuit only by an additional sign inverter 42. The circuit of FIG. 4b otherwise includes adders 40, 41, sign inverter 43, delay section 44 and multiplier 45.

FIG. 4c shows an absorber with rising absorption frequency response, composed of an ohmic resistance 46 and a capacitance 47 connected in series therewith.

FIG. 4d shows an absorber with falling absorption frequency response composed of a resistance 55 and an inductance 56 connected in series therewith.

In FIG. 4c, the digital circuit comprises adders 48, 49, sign inverters 50, 51, 52, time delay 53 and multiplier 54. The digital circuit of FIG. 4d comprises adders 57, 58, sign inverters 59, 60, time delay 61 and multiplier 62.

The frequency-dependent absorption is due to the fact that, for example when the dissipative resistance is bridged or shunted by an inductance, the low-frequency signal components flow over the inductance more or less unhindered, whereas higher-frequency components are forced into the dissipative resistance and are partially absorbed there. The opposite reverberation time frequency response is obtained by replacing the inductance by a capacitance. It would be possible to add two further absorber types with a serial or parallel resonant circuit as reactance, but they are of no practical significance because of their unnatural absorption frequency response. The basic theory on absorber circuits is found in the "Journal of the Franklin Institute", vol. 300, No. 1, July 1975, p. 41-58.

FIG. 5 illustrates the digital implementation of a discontinuity which is formed at the junction between line section 63, with wave resistance Z.sub.0, and line section 64, with wave resistance Z.sub.1. The circuit, referred to as two-gate adapter, consists of the adder circuits 65, 66 and 67, sign inverter 68, and multiplier 69. The multiplier serves to multiply the signals entering the point of discontinuity by the reflection factor .alpha.=(Z.sub.p -Z.sub.1)/(Z.sub.0 +Z.sub.1).

FIG. 6 shows an embodiment which consists of a reflectingly terminated line fed from a current source, composed of the sections 70, 71, 72, 73, 74, 75 and 76 of different wave resistance and different length. In one section, 71, 72, an absorber consisting of the parallel connection of a resistance 77 and inductance 78 is serially inserted, while another section 74, 75 is connected in parallel with the series connection of a resistance 79 and capacitance 80.

The digital recording circuit of the first embodiment shown in FIG. 7a consists only of an adder 81, which forms the sum of the reflected signal S.sub.R and the recorded signal S.sub.T and whose output is connected to the input of the one-dimensional waveguide.

FIG. 7b shows the digital termination circuit and the reverberation pickup of the first embodiment, the reverberated signal S.sub.H being picked up at the output of the waveguide and returned into the line via the connection 82. It is also possible to return the signal S.sub.H via an absorber 83 as noted above.

FIG. 7c shows a variant of the first embodiment where the signal is recorded and picked up at both ends of the waveguide. The actual output signal S.sub.H is obtained by the formation of the difference of the two output signals in adder 84. The advantages of the symmetrical arrangement are discussed in reference to the next embodiment.

FIG. 8 shows the second embodiment of the invention, which consists of a "circular" one-dimensional waveguide composed of several line sections 85 to 96 of different lengths and different wave resistances. In section 86, 87, an absorber consisting of an inductance 98 and resistance 97 is inserted serially, while in section 89, 90 an absorber consisting of a resistance 99 and capacitance 100 is inserted serially, and at section 93, 94 an absorber composed of a resistance 102 and capacitance 101 is connected.

The unreverberated signal S.sub.T is applied at a point of the ring line 103 and propagates in both directions. At the discontinuity junctions it is partially reflected and partially absorbed in the dissipative resistances shunted by inductances and capacitances. If, for example, a pulse is impressed on the ring line, it is split into two pulses traveling through the ring line in opposite directions. The two pulses simultaneously reach the taps 104 and 105, arranged symmetrical to the feed point, and from there they are supplied to a subtractor 106, which eliminates them due to the fact that they are subtracted from each other. The reflected or reverberated components of the pulse, however, are not eliminated by the subtractor 106, as the discontinuity points ae arranged non-symmetrically to the feed point 103. After several revolutions the two oppositely running pulses are completely frayed out at the discontinuity points into reflected components, so that they supply the reverberated signal S.sub.H. It is easy to see that with so few discontinuity points as are shown in the embodiment of FIG. 8 one cannot produce a very diffused reverberation. For good reverberant quality the ring line must be composed of at least fifty line sections of different wave resistance and statistically distributed length. Also a larger number of uniformly distributed absorbers is necessary to prevent the formation of isolated points on the line in which the energy is not greatly absorbed.

FIG. 9a shows the digital circuit of the recording point for the second embodiment. It consists of the adder circuits 110 and 111, through which the unreverberated signal S.sub.T is fed symmetrically into the "circular" waveguide 112, 112.

FIG. 9b shows the digital circuit of the reverberation pickup for the second embodiment. It consists of an adder circuit 116 which is connected to two pickup points 113, 114 symmetrical which are to the recording point, of opposite direction of signal propagation, and which compensates the unreverberated signal and the echoes thereof, with the possibility that the number of symmetrically arranged pickup points may be more than two.

To summarize, it may be stated that due to the digital value of a one-dimensional waveguide with discontinuity points and absorbers it is now possible to reverberate a signal diffusely and with programmable reverberation time frequency response, without the disadvantage of a reduction of the S/N ratio by limit cycles.