Patent Publication Number: US-6218971-B1

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

Description:
BACKGROUND OF THE INVENTION 
     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. 
     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. 
     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. 
     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. 
     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and increasing commercial acceptance of compact disks and other digital audio signal sources, analog mixers are being superseded by digital mixers.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. 
     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. 
     SUMMARY OF THE INVENTION 
     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. 
     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. 
     Still another object of the invention is to make the cascade connection of two mixer units operable on one unit only. 
     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. 
     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&amp; 
     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. 
     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. 
     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. 
     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. 
     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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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; 
     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; 
     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; 
     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 
     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. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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  11   a  and a second digital mixer unit  11   b . The two mixer units  11   a  and  11   b  are 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. 
     Each of the two mixer units  11   a  and  11   b  comprises sixteen-channel analog signal inputs  13   a  or  13   b , two digital signal inputs  14   a   1  and  14   a   2 , or  14   b   1  and  14   b   2 , sixteen analog-to-digital converters (ADCs)  15   a  or  15   b , two digital signal input circuits  16   a   1  and  16   a   2 , or  16   b   1  and  16   b   2 , a digital signal processor (DSP) or digital mixer  17   a  or  17   b , eight digital-to-analog converters (DACs)  18   a  or  18   b , two digital signal output circuits  19   a   1  and  19   a   2 , or  19   b   1  and  19   b   2 , analog signal outputs  20   a  or  20   b , two digital signal outputs  21   a   1  and  21   a   2 , or  21   b   1  and  21   b   2 , a control section  22   a  or  22   b , an operating section  23   a  or  23   b  , a display section  24   a  or  24   b , and a MIDI control signal input/output terminal  25   a  or  25   b.    
     The sixteen-channel analog signal inputs  13   a  or  13   b  of each mixer unit  11   a  or  11   b , to which there may be supplied analog outputs from individual microphones, not shown, are all connected to the DSP  17   a  or  17   b  via the respective ADCs  15   a  or  15   b . The two digital signal inputs  14   a   1  and  14   a   2 , or  14   b   1  and  14   b   2 , of each mixer unit are also connected to the DSP  17   a  or  17   b  via the respective input circuits  16   a   1  and  16   a   2 , or  16   b   1  and  16   b   2 . Each DSP  17   a  or  17   b  has eight outputs connected respectively to the analog signal outputs  20   a  or  20   b  via the DACS  18   a  or  18   b . Each DSP  17   a  or  17   b  has additional outputs connected respectively to the digital signal outputs  21   a   1  and  21   a   2 , or  21   b   1  and  21   b   2 , via the digital signal output circuits  19   a   1  or  19   a   2 , or  19   b   1  and  19   b   2 . Out of the eight analog signal outputs  20   a  or  20   b  of each mixer unit  11   a  or  11   b , two are “stereo” signal outputs, other four are “group” signal outputs, and the remaining two are “effect” signal outputs. 
     The control section  22   a  or  22   b  of each mixer unit  11   a  or  11   b  is connected to all of the DSP  17   a  or  17   b , the operating section  23   a  or  23   b , the display section  24   a  or  24   b , and the MIDI input/output terminal  25   a  or  25   b . It is among the functions of the control section  22   a  or  22   b  to control the associated DSP  17   a  or  17   b  as instructed from the operating section  23   a  or  23   b , to control the associated display section  24   a  or  24   b  in relation to operations taking place elsewhere in the system, and to control signal transmission and reception between the two mixer units  11   a  and  11   b.    
     The MIDI input/output terminals  25   a  and  25   b  of both mixer units  11   a  and  11   b  are interconnected by a MIDI interface cable  12 . Data transfer in packet form is therefore possible between these input/output terminals  25   a  and  25   b  as control input/output means. 
     FIGS. 2 and 3 are explanatory of how the input audio signals travel through the first mixer unit  11   a  and the second mixer unit  11   b , respectively. When these mixer units are used each by itself, the sixteen-channel analog audio signals received at the inputs  13   a  or  13   b  will be digitized by the respective ADCs  15   a  or  15   b . The digital audio signals will then be mixed at the DSP  17   a  or  17   b . Then, after being reconverted into analog signals by the DACs  18   a  or  18   b , the mixed signals will be produced from the outputs  20   a  or  20   b . In this case, as each mixer unit is assumed to be used individually, the “stereo” signals L and R, “group” signals G 1 -G 4 , and “effect” signals E 1  and E 2  will all emerge from the outputs  20   a  or  20   b.    
     The DSP  17   a  or  17   b  of each mixer unit  11   a  or  11   b  is shown equivalently to comprise input circuits  30   a  or  30   b  for processing the digitized audio signals, digital data buses  32   a  or  32   b , and level adjusters  31   a  or  31   b . Typically comprising gain controls, three-band equalizers, panpots, and channel faders, the input circuits  30   a  and  30   b  puts out the processed digital audio signals on the buses  32   a  or  32   b . These buses function as mixers, combining the outputs from all the input circuits  30   a  or  30   b . The buses  32   a  and  32   b  are comprised of two “stereo” signals buses, four “group” signals buses, and two “effect” signal buses. The signals L, R, G 1 -G 4  and E 1 -E 2  on the busses  32   a  or  32   b  are individually adjusted by the level adjusters  31   a  or  31   b  and subsequently reconverted into analog signals by the DACs  18   a  or  18   b.    
     Cascaded as in FIG. 1, the two mixer units  11   a  and  11   b  are 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  19   a   1  and  19   a   2 , or  19   b   1  and  19   b   2 , and two digital input a circuits  16   a   1  and  16   a   2 , or  16   b   1  and  16   b   2 . These output circuits are multiplexers, and the input circuits are demulplexers, as set forth in more detail hereafter. 
     Thus, in the first mixer unit  11   a  of FIG. 2, the first digital output circuit  19   a   1  has inputs connected to two “group” signal buses for combining the first and second “group” signals G 1  and G 2  for multiplex transmission from the first digital output  21   a   1 . The second digital output circuit  19   a   2  has inputs connected to two other “group” signal buses for combining the third and fourth “group” signals G 3  and G 4  for multiplex transmission from the second digital output  21   a   2 . The two digital outputs  21   a   1  and  21   a   2  are connected to the digital inputs  14   b   1  and  14   b   2 , FIG. 3, of the second mixer unit  11   b  by way of cables or other transmission paths  26  and  27 , respectively. 
     In the second mixer unit  11   b  of FIG. 3, on the other hand, the first digital output circuit  19   b   1  has 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  21   b   1 . The second digital output circuit  19   b   2  has inputs connected to the two “effect” signal buses for combining the “effect” signals E 1  and E 2  for multiplex transmission from the second digital output  21   b . The two digital outputs  21   b   1  and  21   b   2  are connected to the digital inputs  14   a   1  and  14   a   2 , FIG. 2, of the first mixer unit  11   b  by way of cables or other transmission paths  28  and  29 , respectively. 
     Inputting the multiplex “stereo” signal LR from first digital output circuit  19   b   1  of the second mixer unit  11   b , the first digital input circuit  16   a   1  of the first mixer unit  11   a  separates 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  11   a  on two of the buses  32   a  carrying such signal. Also, inputting the multiplex “effect” signal E 1 E 2  from the second mixer  11   b , the second digital input circuit  16   a   2  of the first mixer unit  11   a  separates the input signal into the two original “effect” signals E 1  and E 2 . These signals will then be combined with the like signals E 1  and E 2  of the first mixer unit  11   a  on two others of the buses  32   a  carrying such signals. 
     Consequently, as indicated in FIG. 2, the first mixer unit  11   a  will produce from four of its analog outputs  20   a  the “stereo” signals L and R and “effect” signals E 1  and E 2  which have been recreated from both the sixteen-channel inputs of the first mixer unit  11   a  and the sixteen-channel inputs of the second mixer unit  11   b.    
     On the other hand, inputting the multiplex “group” signal G 1 G 2  from the first digital output circuit  19   a   1  of the first mixer unit  11   a , the first digital input circuit  16   b   1  of the second mixer unit  11   b  separates the input signal into the two original “group” signals G 1   1  and G 2 . These signals will then be combined with the like signals G 1  and G 2  of the second mixer unit  11   b  on two of the buses  32   b  carrying such signals. Also, inputting the other multiplex group signal G 3 G 4  from the second digital output circuit  19   a   2  of the first mixer unit  11   a , the second digital input circuit  16   b   2  of the second mixer unit  11   b  separates the input signal into the two original “group” signals G 3  and G 4 . These signals will then be combined with the like signals G 3  and G 4  of the second mixer unit  11   b  on two others of the buses  32   a  carrying such signals. 
     Thus, as indicated in FIG. 3, the second mixer unit  11   b  will produce from four of its analog outputs  20   b  the “group” signals G 1 -G 4  which have been recreated from both the sixteen-channel inputs of the first mixer unit  11   a  and the sixteen-channel inputs of the second mixer unit  11   b.    
     It is understood that the two cascaded mixer units  11   a  and  11   b  are controlled for synchronous production of outputs. The “stereo” signals L and R and “effect” signals E 1  and E 2  put out by the first mixer unit  11   a  and the “group signals G 1 -G 4  put out by the second mixer unit  11   b  are in synchronism with one another. 
     Let us imagine that the two mixer units  11   a  and  11   b  were to be manipulated independently. Then the final level adjustment of the “stereo” signals L and R and “effect” signals E 1  and E 2  would have to be done by the level adjusters  31   a  of the first mixer unit  11   a , and that of the “group” signals G 1 -G 4  by the level adjusters  31   b  of the second mixer unit  11   b . 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  22   a  and  22   b  of both mixer units so that the final level adjustment of the outputs from the second mixer unit  11   b , too, can be done on the first mixer unit  11   a.    
     It is toward that end that the control sections  22   a  and  22   b  of both mixer units are interconnected by the cable  12  meeting the MIDI interface requirements. The level adjusters  31   b  of the second mixer unit  11   b  are therefore operable from the first mixer unit  11   a  via the control sections  22   a  and  22   b  of both mixer units. More will be said presently on this subject. 
     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  22   a  or  22   b  of each mixer unit  11   a  or  11   b  controls the DSP  17   a  or  17   b , the display section  24   a  or  24   b , and the intercommunication of the two mixer units via the MIDI interfacing, all in response to instructions from the operating section  23   a  or  23   b . The DSP  17   a  or  17   b  responds to command programs from the control section  22   a  or  22   b  by processing the incoming digital audio signals as schematically illustrated in FIGS. 2 and 3. 
     The operating section  23   a  or  23   b  of each mixer unit  11   a  or  11   b  comprises manual control means  41   a  or  41   b  for inputting instructions on the equalizers, faders, muting circuits, pans, “solo” switches, etc., and an input microcomputer  42   a  or  42   b . The manual control means  41   a  or  41   b  when 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  11   a  or  11   b . Receiving these coded signals, the input microcomputer  42   a  or  42   b  delivers corresponding commands to the control section  22   a  or  22   b.    
     The display section  24   a  or  24   b  of each mixer unit  11   a  or  11   b  may 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  23   a  or  23   b . The level indicators indicate the digital audio signal levels as such information is supplied from the DSP  17   a  or  17   b.    
     As has been stated, the two mixer units  11   a  and  11   b  may be used either independently or in cascade connection. In order to make such selective use possible, the control sections  22   a  and  22   b  and input microcomputers  42   a  and  42   b  of both mixer units  11   a  and  11   b  are 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  23   a  or  23   b . The mixer units  11   a  and  11   b  operate 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. 
     The digital mixer system  10  can be constructed to permit the following six different kinds of information transfer when operating in the cascade mode: 
     1. Mixing information transfer for the first mixer unit  11   a , over the path comprising the operating section  23   a , control section  22   a , and DSP  17   a  of the first mixer unit  11   a.    
     2. Display information transfer over the path comprising the operating section  23   a , control section  22   a , and display section  24   a  of the first mixer unit  11   a.    
     3. Information transfer for discarding unnecessary information, over the path comprising the operating section  23   a  and control section  22   a  of the first mixer unit  11   a.    
     4. Mixing information transfer for the second mixer unit  11   b , over the path comprising the operating section  23   a  and control section  22   a  of the first mixer unit  11   a , the cable  12 , and the control section  22   b  and DSP  17   b  of the second mixer unit  11   b.    
     5. Display information transfer for indicating the conditions of the second mixer unit  11   b  on the display section  24   a  of the first mixer unit  11   a , over the path comprising the control section  22   b  of the second mixer unit  11   b , the cable  12 , and the control section  22   a  and display section  24   a  of the first mixer unit  11   a.    
     6. Information transfer for controlling the DSP  17   a  of the first mixer unit  11   a  by instructions from the second mixer unit  11   b , over the path comprising the control section  22   b  of the second mixer unit  11   b , the cable  12 , and the control section  22   a  and DSP  17   a  of the first mixer unit  11   a.    
     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: 
     1. The first and fourth kinds of information transfer: 
     Adjustment of the output levels of the “group” signals G 1 -G 4  of the second mixer unit  11   b  from the operating section  23   a  of the first mixer unit  11   a.    
     2. The first and fourth kinds of information transfer: 
     Audibly checking any desired channels of signals of the first mixer unit  11   a  by operating the “solo” switches of the first mixer unit, or any desired channels of signals of the second mixer unit  11   b  by operating the “solo” switches of that unit. Manipulation of any particular solo switch on each mixer unit causes the control section  22   a  or  22   b  to mute all but the desired channel. 
     3. The fourth and sixth kinds of information transfer: 
     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  22   a  of the first mixer unit  11   a  is to control the DSP  17   a  under command from the unshown selector switch of the second mixer unit  11   b . The fourth kind of information transfer will also be used in this case as the second mixer unit  11   b  will have to be notified of the operations taking place in the first mixer unit  11   a.    
     4. The fifth kind of information transfer: 
     The exhibition, on the display section  24   a  of the first mixer unit  11   a , of the signal levels of the “group” buses of the second mixer unit  11   b.    
     For adjustment of the output levels of the “group” signals G 1 -G 4  of the second mixer unit  11   b  from the operating section  23   a  of the first mixer unit  11   a , listed first above, the mixer system  10  will operate as flowcharted in FIG. 5 according to the program introduced into the control sections  22   a  and  22   b  of both mixer units. 
     After interconnecting the two mixer units  11   a  and  11   b  as shown in FIG. 1, the unshown mode select switch on the operating section  23   b  may be operated to select the cascade mode. Then those level adjusters  31   a  of the first mixer unit  11   a  which are connected to the group buses G 1 -G 4  thereof may be operated on the operating section  23   a.    
     Now will start at S 1  the subroutine of FIG.  5 . Next comes the node S 1  which 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 S 3  that the first mixer unit  11   a  operate as master and send its self-identification signal to the second mixer unit  11   b . Receiving this signal at the block S 4 , the second mixer unit  11   b  conditions itself for operation as slave at the next block S 6  and further sends its self-identification signal back to the first mixer unit  11   a , 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  11   a  replies to the second mixer unit  11   b  that it has duly received the self-identification signal of the second mixer unit and identified it, at the block S 6 . The cascade connection of the two mixer units  11   a  and  11   b  have now been completed, making them ready for operation as master and slave, respectively. 
     The setting of the first mixer unit  11   a  in master mode at the block S 3 , and of the second mixer unit  11   b  in slave mode at the block S 5 , are both not an absolute requirement. Such settings might be made instead after the block S 6 . 
     The next block S 7  calls for buss reallocation. Being the master, the first mixer unit  11   a  may have the channel numbers one through sixteen of its inputs left unchanged. The channel numbers of the slave unit  11   b  must have its channel numbers redesignated from one through sixteen to seventeen through thirty-two. 
     Then, at the block S 8 , the operator may operate the level adjusters  31   a  of the master unit  11   a  from the operating section  23   a  thereof in order to cause signal transmission to the DSP  17   a  over the first recited path for adjustment of the “stereo” signals L and R and the “effect” signals E 1  and E 2 . The “stereo” signals L and R and “effect” signals E 1  and E 2  will then be put out as adjusted by the operator. 
     Although the “group” signals G 1 -G 4  are being processed in the slave unit  11   b , the adjustment of their levels are now being performed on the master unit  11   a . The instructions that have been input from the operating section  23   a  of the master unit  11   a  for processing the “group” signals are therefore transferred at the block S 9  to the slave unit  11   b  over the fourth recited path above. The DSP  17   b  of the slave unit  11   b  responds at the block S 10  to the instructions thus transferred from the master unit  11   a , by processing the “group” signals G 1 -G 4  accordingly, and waits for the next instruction at the block S 11 . 
     The advantages gained by the cascade mixer system  10  may be summarized as follows: 
     1. The two constituent mixer units  11   a  and  11   b  of the system can be used either individually, as sixteen-channel mixers, or in combination as a thirty-two-channel mixer. 
     2. The mixer units  11   a  and  11   b  do not have all their eight outputs interconnected; instead, the four “group” signals G 1 -G 4  of 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 E 1  and E 2  of the second unit are sent over the paths  28  and  29  to the first unit. Consequently, for cascade connection, the first unit  11   a  requires only two digital input circuits  16   a   1  and  16   a   2  and two digital output circuits  19   a   1  and  19   a   2 , and the second unit  11   b  only two digital input circuits  16   b   1  and  16   b   2  and two digital output circuits  19   b   1  and  19   b   2 , 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. 
     3. A master-slave relationship can be established between the two cascaded mixer units  11   a  and  11   b , it being necessary to manipulate only the first mixer unit  11   a  for operating both units in any desired manner. 
     4. The transfer of control signals between the two mixer units  11   a  and  11   b , 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. 
     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: 
     1. Control of both mixer units  11   a  and  11   b  by the first unit  11   a  is possible even when the two units are cascaded in other than the illustrated way, for example, when all the digital outputs from the DSP  17   b  of the second unit  11   b  are directed into the DSP  17   a  of the first unit  11   a.    
     2. The microcomputer  42   a  shown included in the operating section  23   a  or  23   b  of each mixer unit  11   a  or  11   b  in FIG. 4 could be omitted if the microcomputer of the control section  22   a  or  22   b  were equipped to perform its functions. 
     3. The control sections  22   a  and  22   b  of both mixer units  11   a  and  11   b  could be interconnected via dedicated signal paths other than MIDI interfacing. 
     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. 
     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.