Multichannel digital mixer designed for cascade connection, and a cascade connection of two such mixers

[0002] This invention relates generally to mixers, more particularly to a multichannel digital mixer suitable for handling audio signals, and still more particularly to such a mixer designed explicitly for cascade connection with another mixer of identical make, beside being capable of use as an independent unit. The invention also pertains to a system of two such multichannel digital mixers in cascade connection.

[0003] Sixteen-input mixers are in widespread use for mixing audio signals from as many individual microphones. Audio engineers are, however, not always satisfied with sixteen channels but sometimes desire more channels. Conventionally, for fulfillment of this desire, it has been practiced to connect two sixteen-input mixer units of identical make in cascade mode by means of cables in cases where a more-than-sixteen-input mixer is not available. The cascaded mixer system provides a total of thirty-two inputs.

[0004] The cascading of two analog mixer units is easy if each one is fabricated with that mode of use in mind, complete with a set of cascading inputs in addition to the regular signal inputs. One mixer unit has its cascading inputs left unused but has its combined signal outputs cabled to the cascading inputs of the other mixer unit.

[0005] The audio outputs from microphones or the like are directed into the respective input circuits of the two mixer units thereby to be variously conventionally processed and routed to provide, for instance, left and right “stereo” signals, four-channel “group” signals for monitoring, and two-channel “effect” signals for echo and other acoustic effects. The output signals from the first mixer unit are directed into the cascading inputs of the second unit thereby to be combined with like signals. The combined signals are produced from the outputs of the second mixer unit.

[0006] Recently, with the advent and increasing commercial acceptance of compact disks and other digital audio signal sources, analog mixers are being superseded by digital mixers. Being functionally equivalent to analog mixers, digital mixers also lend themselves to cascade connection, provided, however, that each unit is furnished with digital output circuits and digital input circuits for cascading.

[0007] An objection to the prior art digital mixer units designed for cascade connection is that the provision of many such digital output circuits and input circuits have rendered each unit very costly. The mixer system constituted of two such prior art digital mixer units in cascade connection is itself objectionable, too, because of the necessity for operating the control boards of both units.

[0008] The present invention aims at the provision of a digital mixer unit explicitly designed for use either singly or in cascade connection with another unit of like construction.

[0009] Another object of the invention is to attain the first recited object by making the construction of each mixer unit, as well as interconnections between two such units, as simple as feasible without impairment of their intended functions either as independent mixers or as a cascade mixer system.

[0010] Still another object of the invention is to make the cascade connection of two mixer units operable on one unit only.

[0011] Briefly summarized in one aspect thereof, the present invention provides a multichannel digital mixer unit for use either singly or in cascade connection with another mixer unit of identical make. The mixer unit comprises: (a) a plurality of analog inputs for inputting as many analog signals to be mixed; (b) at least one digital input for inputting a digital output signal from a second mixer unit of identical make if such a unit is connected in cascade with the instant unit; (c) a plurality of analog-to-digital converters connected one to each analog input for digitizing the input analog signals; (d) a digital signal processor connected to the digital input and the analog-to-digital converters for producing a plurality of digital output signals by mixing the digital input signal, if any, from the second mixer unit and the outputs from the analog-to-digital converters; (e) a plurality of digital-to-analog converters connected to the digital signal processor for converting the digital output signals therefrom into analog signals; (f) a plurality of analog outputs connected one to each digital-to-analog converter for putting out the analog output signals therefrom; (g) at least one digital output connected to the digital signal processor for putting out at least one of the digital output signals therefrom for application to the digital input of the second mixer unit if such a unit is cascaded with the instant unit; (h) operating means for manually inputting instructions indicative of instructions to be performed by the digital signal processor on the signals input thereto; (i) control means connected between the operating means and the digital signal processor for causing the latter to process the input signals according to the instructions from the operating means; and (j) control input/output means for connecting the control means to like control means of the second mixer unit if such a unit is cascaded with the instant unit, in order to permit control of both mixer units by either mixer unit.

[0012] Another aspect of the invention concerns a digital cascade mixer system comprising two digital mixer units, each constructed as in the foregoing, in cascade connection with each other. The two mixer units are cascaded by connecting the digital output or outputs of a first unit to the digital input or inputs of a second unit, the digital output or outputs of the second unit to the digital input or inputs of the first unit, and by interconnecting the control input/output means of both unit&

[0013] In the preferred embodiment to be disclosed subsequently, two sixteen-channel mixer units, each constructed as in the summary above, are cascaded to provide a thirty-two-channel mixer system for processing as many analog audio outputs from individual microphones. Only four selected outputs (e.g. “group” signals) from the digital signal processor of one mixer unit are directed to the digital inputs of the second unit, therein to be mixed with like signals. Another four selected outputs (e.g. two “stereo” signals and two “effect” signals) from the digital signal processor of the second unit are directed to the digital inputs of the first unit, also therein to be mixed with like signals.

[0014] The mixing of thirty-two input audio signals is possible in the above described manner even though the two cascaded mixer units are each greatly simplified in construction compared to the noted prior art mixers designed for cascading.

[0015] For even simpler connection of the two mixer units according to the invention, it is recommended that the desired digital audio signals be transferred between the two mixer units by multiplex transmission. Each mixer unit incorporates two digital output circuits in the preferred embodiment, each for multiplexing two outgoing digital audio signals, and two digital input circuits for demultiplexing the two incoming digital audio signals into four. Only half as many audio signal paths are then required between the two mixer units as when they are sent separately.

[0016] The present invention also proposes the interconnection of the control sections of both mixer units, preferably by means meeting the standard MIDI interface criteria. The cascade mixer system will then become operable on one mixer unit by establishing master-slave relationship between the two units.

[0017] The above and other objects, features and advantages of this invention and the manner of achieving them will become more apparent, and the invention itself will best be understood, from a study of the following description and attached claims, with reference had to the accompanying drawings showing the preferred embodiment of the invention.

[0018] FIG. 1 is a block diagram of two sixteen-channel digital mixer units, each constructed according to the present invention, cascaded into a unitary thirty-two-channel mixer system also in accordance with the invention;

[0019] FIG. 2 is a more detailed schematic electrical diagram showing in particular those parts of the first mixer unit of the FIG. 1 mixer system which are related to the audio signals being processed therein;

[0020] FIG. 3 is a diagram similar to FIG. 2 but showing in particular those parts of the second mixer unit of the FIG. 1 mixer system which are related to the audio signals being processed therein;

[0021] FIG. 4 is a block diagram showing those parts of the FIG. 1 mixer system which are related to the signals for controlling the operations of both mixer units; and

[0022] FIG. 5 is a flow chart explanatory of how master-slave relationship is established between the two units of the FIG. 1 mixer system for manual control of both units from one unit.

[0023] The present invention is believed to be best embodied in the digital mixer system diagramed in FIG. 1. Generally designated 10, the representative mixer system is essentially a tandem connection of a first digital mixer unit 11aand a second digital mixer unit 11b. The two mixer units 11aand 11bare of identical make, each constructed in accordance with the invention, and may be put to use either singly or, as pictured here, in cascade connection with each other to make up a streamlined mixer system.

[0024] Each of the two mixer units 11aand 11bcomprises sixteen-channel analog signal inputs 13aor 13b, two digital signal inputs 14a₁and 14a₂, or 14b₁and 14b₂, sixteen analog-to-digital converters (ADCs) 15aor 15b, two digital signal input circuits 16a₁and 16a₂, or 16b₁and 16b₂, a digital signal processor (DSP) or digital mixer 17aor 17b, eight digital-to-analog converters (DACs) 18aor 18b, two digital signal output circuits 19a₁and 19a₂, or 19b₁and 19b₂, analog signal outputs 20aor 20b, two digital signal outputs 21a₁and 21a₂, or 21b₁and 21b₂, a control section 22aor 22b, an operating section 23aor 23b, a display section 24aor 24b, and a MIDI control signal input/output terminal 25aor 25b.

[0025] The sixteen-channel analog signal inputs 13aor 13bof each mixer unit 11aor 11b, to which there may be supplied analog outputs from individual microphones, not shown, are all connected to the DSP 17aor 17bvia the respective ADCs 15aor 15b. The two digital signal inputs 14a₁and 14a₂, or 14b₁and 14b₂, of each mixer unit are also connected to the DSP 17aor 17bvia the respective input circuits 16a₁and 16a₂, or 16b₁and 16b₂. Each DSP 17aor 17bhas eight outputs connected respectively to the analog signal outputs 20aor 20bvia the DACS 18aor 18b. Each DSP 17aor 17bhas additional outputs connected respectively to the digital signal outputs 21a₁and 21a₂, or 21b₁and 21b₂, via the digital signal output circuits 19a₁or 19a₂, or 19b₁and 19b₂. Out of the eight analog signal outputs 20aor 20bof each mixer unit 11aor 11b, two are “stereo” signal outputs, other four are “group” signal outputs, and the remaining two are “effect” signal outputs.

[0026] The control section 22aor 22bof each mixer unit 11aor 11bis connected to all of the DSP 17aor 17b, the operating section 23aor 23b, the display section 24aor 24b, and the MIDI input/output terminal 25aor 25b. It is among the functions of the control section 22aor 22bto control the associated DSP 17aor 17bas instructed from the operating section 23aor 23b, to control the associated display section 24aor 24bin relation to operations taking place elsewhere in the system, and to control signal transmission and reception between the two mixer units 11aand 11b.

[0027] The MIDI input/output terminals 25aand 25bof both mixer units 11aand 11bare interconnected by a MIDI interface cable 12. Data transfer in packet form is therefore possible between these input/output terminals 25aand 25bas control input/output means.

[0028] FIGS. 2 and 3 are explanatory of how the input audio signals travel through the first mixer unit 11aand the second mixer unit 11b, respectively. When these mixer units are used each by itself, the sixteen-channel analog audio signals received at the inputs 13aor 13bwill be digitized by the respective ADCs 15aor 15b. The digital audio signals will then be mixed at the DSP 17aor 17b. Then, after being reconverted into analog signals by the DACs 18aor 18b, the mixed signals will be produced from the outputs 20aor 20b. In this case, as each mixer unit is assumed to be used individually, the “stereo” signals L and R, “group” signals G₁-G₄, and “effect” signals E₁and E₂will all emerge from the outputs 20aor 20b.

[0029] The DSP 17aor 17bof each mixer unit 11aor 11bis shown equivalently to comprise input circuits 30aor 30bfor processing the digitized audio signals, digital data buses 32aor 32b, and level adjusters 31aor 31b. Typically comprising gain controls, three-band equalizers, panpots, and channel faders, the input circuits 30aand 30bputs out the processed digital audio signals on the buses 32aor 32b. These buses function as mixers, combining the outputs from all the input circuits 30aor 30b. The buses 32aand 32bare comprised of two “stereo” signals buses, four “group” signals buses, and two “effect” signal buses. The signals L, R, G₁-G₄and E₁-E₂on the busses 32aor 32bare individually adjusted by the level adjusters 31aor 31band subsequently reconverted into analog signals by the DACs 18aor 18b.

[0030] Cascaded as in FIG. 1, the two mixer units 11aand 11bare intended to transfer the digital audio signals therebetween. Toward this end, as indicated in FIGS. 2 and 3, each mixer unit comprises two digital output circuits 19a₁and 19a₂, or 19b₁and 19b₂, and two digital input a circuits 16a₁and 16a₂, or 16b₁and 16b₂. These output circuits are multiplexers, and the input circuits are demulplexers, as set forth in more detail hereafter.

[0031] Thus, in the first mixer unit 11aof FIG. 2, the first digital output circuit 19a₁has inputs connected to two “group” signal buses for combining the first and second “group” signals G₁and G₂for multiplex transmission from the first digital output 21a₁. The second digital output circuit 19a₂has inputs connected to two other “group” signal buses for combining the third and fourth “group” signals G₃and G₄for multiplex transmission from the second digital output 21a₂. The two digital outputs 21a₁and 21a₂are connected to the digital inputs 14b₁and 14b₂, FIG. 3, of the second mixer unit 11bby way of cables or other transmission paths 26 and 27, respectively.

[0032] In the second mixer unit 11bof FIG. 3, on the other hand, the first digital output circuit 19b₁has inputs connected to the two “stereo” signal buses for combining the first and second “stereo” signals L and R for multiplex transmission from the first digital output 21b₁. The second digital output circuit 19b₂has inputs connected to the two “effect” signal buses for combining the “effect” signals E₁and E₂for multiplex transmission from the second digital output 21b. The two digital outputs 21b₁and 21b₂are connected to the digital inputs 14a₁and 14a₂, FIG. 2, of the first mixer unit 11bby way of cables or other transmission paths 28 and 29, respectively.

[0033] Inputting the multiplex “stereo” signal LR from first digital output circuit 19b₁of the second mixer unit 11b, the first digital input circuit 16_a1of the first mixer unit 11aseparates the input signal into the two original “stereo” signals L and R. These signals will then be combined with the like signals L and R of the first mixer unit 11aon two of the buses 32acarrying such signal. Also, inputting the multiplex “effect” signal E₁E₂from the second mixer 11b, the second digital input circuit 16a₂of the first mixer unit 11aseparates the input signal into the two original “effect” signals E₁and E₂. These signals will then be combined with the like signals E₁and E₂of the first mixer unit 11aon two others of the buses 32acarrying such signals.

[0034] Consequently, as indicated in FIG. 2, the first mixer unit 11awill produce from four of its analog outputs 20athe “stereo” signals L and R and “effect” signals E₁and E₂which have been recreated from both the sixteen-channel inputs of the first mixer unit 11aand the sixteen-channel inputs of the second mixer unit 11b.

[0035] On the other hand, inputting the multiplex “group” signal G₁G₂from the first digital output circuit 19a₁of the first mixer unit 11a, the first digital input circuit 16b₁of the second mixer unit 11bseparates the input signal into the two original “group” signals G1₁and G₂. These signals will then be combined with the like signals G₁and G₂of the second mixer unit 11bon two of the buses 32bcarrying such signals. Also, inputting the other multiplex group signal G₃G₄from the second digital output circuit 19a₂of the first mixer unit 11a, the second digital input circuit 16b₂of the second mixer unit 11bseparates the input signal into the two original “group” signals G₃and G₄. These signals will then be combined with the like signals G₃and G₄of the second mixer unit 11bon two others of the buses 32acarrying such signals.

[0036] Thus, as indicated in FIG. 3, the second mixer unit 11bwill produce from four of its analog outputs 20bthe “group” signals G₁-G₄which have been recreated from both the sixteen-channel inputs of the first mixer unit 11aand the sixteen-channel inputs of the second mixer unit 11b.

[0037] It is understood that the two cascaded mixer units 11aand 11bare controlled for synchronous production of outputs. The “stereo” signals L and R and “effect” signals E₁and E₂put out by the first mixer unit 11aand the “group signals G₁-G₄put out by the second mixer unit 11bare in synchronism with one another.

[0038] Let us imagine that the two mixer units 11aand 11bwere to be manipulated independently. Then the final level adjustment of the “stereo” signals L and R and “effect” signals E₁and E₂would have to be done by the level adjusters 31aof the first mixer unit 11a, and that of the “group” signals G₁-G₄by the level adjusters 31bof the second mixer unit 11b. The mixing engineer would have to reach for both mixer units for such level adjustment. The present invention overcomes this inconvenience by designing the control sections 22aand 22bof both mixer units so that the final level adjustment of the outputs from the second mixer unit 11b, too, can be done on the first mixer unit 11a.

[0039] It is toward that end that the control sections 22aand 22bof both mixer units are interconnected by the cable 12 meeting the MIDI interface requirements. The level adjusters 31bof the second mixer unit 11bare therefore operable from the first mixer unit 11avia the control sections 22aand 22bof both mixer units. More will be said presently on this subject.

[0040] Reference may be had to FIG. 4 for a consideration of how the cascaded mixer system of FIG. 1 is controlled. Constituted of a microcomputer or central processor unit, the control section 22aor 22bof each mixer unit 11aor 11bcontrols the DSP 17aor 17b, the display section 24aor 24b, and the intercommunication of the two mixer units via the MIDI interfacing, all in response to instructions from the operating section 23aor 23b. The DSP 17aor 17bresponds to command programs from the control section 22aor 22bby processing the incoming digital audio signals as schematically illustrated in FIGS. 2 and 3.

[0041] The operating section 23aor 23bof each mixer unit 11aor 11bcomprises manual control means 41aor 41bfor inputting instructions on the equalizers, faders, muting circuits, pans, “solo” switches, etc., and an input microcomputer 42aor 42b. The manual control means 41aor 41bwhen manipulated generate coded electric signals indicative of the desired operations to be performed on the various channels of digital audio signals being input to the mixer unit 11aor 11b. Receiving these coded signals, the input microcomputer 42aor 42bdelivers corresponding commands to the control section 22aor 22b.

[0042] The display section 24aor 24bof each mixer unit 11aor 11bmay comprise a liquid-crystal character display and a set of visual level indicators typically in the form of light-emitting diodes. The character display may exhibit, for example, the various working conditions of the system and the instructions being input from the operating section 23aor 23b. The level indicators indicate the digital audio signal levels as such information is supplied from the DSP 17aor 17b.

[0043] As has been stated, the two mixer units 11aand 11bmay be used either independently or in cascade connection. In order to make such selective use possible, the control sections 22aand 22band input microcomputers 42aand 42bof both mixer units 11aand 11bare so constructed are understood to be selectively conditioned by the operator for either independent mode or cascade mode. Either mode is selectable by actuation of a mode select switch, not shown, of each operating section 23aor 23b. The mixer units 11aand 11boperate individually as sixteen-channel mixers when the independent mode is chosen, and conjointly as a streamlined thirty-two-channel mixer when the cascade mode is chosen.

[0044] The digital mixer system 10 can be constructed to permit the following six different kinds of information transfer when operating in the cascade mode:

[0045] 1. Mixing information transfer for the first mixer unit 11a, over the path comprising the operating section 23a, control section 22a, and DSP 17aof the first mixer unit 11a.

[0046] 2. Display information transfer over the path comprising the operating section 23a, control section 22a, and display section 24aof the first mixer unit 11a.

[0047] 3. Information transfer for discarding unnecessary information, over the path comprising the operating section 23aand control section 22aof the first mixer unit 11a.

[0048] 4. Mixing information transfer for the second mixer unit 11b, over the path comprising the operating section 23aand control section 22aof the first mixer unit 11a, the cable 12, and the control section 22band DSP 17bof the second mixer unit 11b.

[0049] 5. Display information transfer for indicating the conditions of the second mixer unit 11bon the display section 24aof the first mixer unit 11a, over the path comprising the control section 22bof the second mixer unit 11b, the cable 12, and the control section 22aand display section 24aof the first mixer unit 11a.

[0050] 6. Information transfer for controlling the DSP 17aof the first mixer unit 11aby instructions from the second mixer unit 11b, over the path comprising the control section 22bof the second mixer unit 11b, the cable 12, and the control section 22aand DSP 17aof the first mixer unit 11a.

[0051] The foregoing six kinds of information transfer, with the associated transfer paths, will be employed, either singly or in combination, as the cascade mixer system 10 is put to use in various ways. The following are some examples:

[0052] 1. The first and fourth kinds of information transfer:

[0053] Adjustment of the output levels of the “group” signals G₁-G₄of the second mixer unit 11bfrom the operating section 23aof the first mixer unit 11a.

[0054] 2. The first and fourth kinds of information transfer:

[0055] Audibly checking any desired channels of signals of the first mixer unit 11aby operating the “solo” switches of the first mixer unit, or any desired channels of signals of the second mixer unit 11bby operating the “solo” switches of that unit. Manipulation of any particular solo switch on each mixer unit causes the control section 22aor 22bto mute all but the desired channel.

[0056] 3. The fourth and sixth kinds of information transfer:

[0057] It is recommended from the standpoints of cost reduction and less space requirement of each unit that operating means for some optional mixer function or functions (e.g. auxiliary equalization) be provided not for each channel but in common for all the channels and selectively connected to each channel by a selector switch, not shown. The sixth kind of information transfer is used for this purpose in the case where the control section 22aof the first mixer unit 11ais to control the DSP 17aunder command from the unshown selector switch of the second mixer unit 11b. The fourth kind of information transfer will also be used in this case as the second mixer unit 11bwill have to be notified of the operations taking place in the first mixer unit 11a.

[0058] 4. The fifth kind of information transfer:

[0059] The exhibition, on the display section 24aof the first mixer unit 11a, of the signal levels of the “group” buses of the second mixer unit 11b.

[0060] For adjustment of the output levels of the “group” signals G₁-G₄of the second mixer unit 11bfrom the operating section 23aof the first mixer unit 11a, listed first above, the mixer system 10 will operate as flowcharted in FIG. 5 according to the program introduced into the control sections 22aand 22bof both mixer units.

[0061] After interconnecting the two mixer units 11aand 11bas shown in FIG. 1, the unshown mode select switch on the operating section 23bmay be operated to select the cascade mode. Then those level adjusters 31aof the first mixer unit 11awhich are connected to the group buses G₁-G₄thereof may be operated on the operating section 23a.

[0062] Now will start at S₁the subroutine of FIG. 5. Next comes the node S₁which asks whether the cascade mode has been chosen or not. The answer “no” to this question will result in operation of both mixer units in independent mode. If the answer is “yes,” on the other hand, then it is dictated by the block S₃that the first mixer unit 11aoperate as master and send its self-identification signal to the second mixer unit 11b. Receiving this signal at the block S₄, the second mixer unit 11bconditions itself for operation as slave at the next block S₆and further sends its self-identification signal back to the first mixer unit 11a, together with a query as to whether the identity of the second mixer unit has been ascertained by the first mixer unit. The first mixer unit 11areplies to the second mixer unit 11bthat it has duly received the self-identification signal of the second mixer unit and identified it, at the block S₆. The cascade connection of the two mixer units 11aand 11bhave now been completed, making them ready for operation as master and slave, respectively.

[0063] The setting of the first mixer unit 11ain master mode at the block S₃, and of the second mixer unit 11bin slave mode at the block S₅, are both not an absolute requirement. Such settings might be made instead after the block S₆.

[0064] The next block S₇calls for buss reallocation. Being the master, the first mixer unit 11amay have the channel numbers one through sixteen of its inputs left unchanged. The channel numbers of the slave unit 11bmust have its channel numbers redesignated from one through sixteen to seventeen through thirty-two.

[0065] Then, at the block S₈, the operator may operate the level adjusters 31aof the master unit 11afrom the operating section 23athereof in order to cause signal transmission to the DSP 17aover the first recited path for adjustment of the “stereo” signals L and R and the “effect” signals E₁and E₂. The “stereo” signals L and R and “effect” signals E₁and E₂will then be put out as adjusted by the operator.

[0066] Although the “group” signals G₁-G₄are being processed in the slave unit 11b, the adjustment of their levels are now being performed on the master unit 11a. The instructions that have been input from the operating section 23aof the master unit 11afor processing the “group” signals are therefore transferred at the block S₉to the slave unit 11bover the fourth recited path above. The DSP 17bof the slave unit 11bresponds at the block S10 to the instructions thus transferred from the master unit 11a, by processing the “group” signals G₁-G₄accordingly, and waits for the next instruction at the block S₁₁.

[0067] The advantages gained by the cascade mixer system 10 may be summarized as follows:

[0068] 1. The two constituent mixer units 11aand 11bof the system can be used either individually, as sixteen-channel mixers, or in combination as a thirty-two-channel mixer.

[0069] 2. The mixer units 11aand 11bdo not have all their eight outputs interconnected; instead, the four “group” signals G₁-G₄of the first unit are send over the paths 26 and 27 to the second unit, and the two “stereo” signals L and R and two “effect” signals E₁and E₂of the second unit are sent over the paths 28 and 29 to the first unit. Consequently, for cascade connection, the first unit 11arequires only two digital input circuits 16a₁and 16a₂and two digital output circuits 19a₁and 19a₂, and the second unit 11bonly two digital input circuits 16b₁and 16b₂and two digital output circuits 19b₁and 19b₂, in addition to the preexisting parts for use as independent mixers. Moreover, one digital input circuit and one digital output circuit have conventionally existed in digital mixers. By utilizing these preexisting circuits for the purposes of the instant invention, only one digital input circuit and one digital output circuit need to be added to each mixer unit for transfer of eight different signals between the two units. Each digital output circuit functions to multiplex two signals, and each digital input circuit to demultiplex the input multiplex signal into the two original signals, in the illustrated embodiment of the invention.

[0070] 3. A master-slave relationship can be established between the two cascaded mixer units 11aand 11b, it being necessary to manipulate only the first mixer unit 11afor operating both units in any desired manner.

[0071] 4. The transfer of control signals between the two mixer units 11aand 11b, needed for controlling the second mixer unit from the first, is inexpensively accomplished by taking advantage of the familiar MIDI interfaces customarily incorporated in mixers.

[0072] Notwithstanding the foregoing detailed disclosure it is not desired that the present invention be limited by the exact showing of the drawings or the description thereof. The following, then, is a brief list of possible modifications or alterations of the illustrated embodiments:

[0073] 1. Control of both mixer units 11aand 11bby the first unit 11ais possible even when the two units are cascaded in other than the illustrated way, for example, when all the digital outputs from the DSP 17bof the second unit 11bare directed into the DSP 17aof the first unit 11a.

[0074] 2. The microcomputer 42ashown included in the operating section 23aor 23bof each mixer unit 11aor 11bin FIG. 4 could be omitted if the microcomputer of the control section 22aor 22bwere equipped to perform its functions.

[0075] 3. The control sections 22aand 22bof both mixer units 11aand 11bcould be interconnected via dedicated signal paths other than MIDI interfacing.

[0076] 4. Each mixer unit could have other than the indicated numbers of input channels and output channels and process the input audio signals in other than the indicated ways.

[0077] All these and other changes of the illustrated embodiment are intended in this disclosure. It is therefore appropriate that the invention be construed broadly and in a manner consistent with the fair meaning or proper scope of the annexed claims.