Akai MPX-820
A rack-mountable 8-input stereo mixer that stores every control position - faders, pans, sends, even EQ - as digital snapshots recalled by MIDI program change.
Overview
The MPX-820 arrived as what Akai's own advertising in Mix (June 1986, p.82) called a fully programmable 8-channel mixer, and the claim had teeth. It was the first mixer to fold channel and master levels, pans, send and receive levels, and even EQ into internal registers addressable by standard MIDI commands. Earlier MIDI mixers handled channel on/offs and effects routing. Others used proprietary codes loosely based on MIDI to move individual levels. The MPX-820 stored the whole control surface - up to 99 snapshots at a time - and let you recall them from a numeric keypad, footswitches, a tape sync track, or a MIDI program change.
It fits a standard 19-inch rack at 7U, roughly 12¼ inches high. The layout is conventional apart from the numeric keypad and two-digit LED display in the upper right corner. The eight channel inputs on the rear panel offer both low-impedance balanced and high-impedance unbalanced connectors, with per-channel pad switches. Each channel carries a pre-fader monitor send, a post-fader effects send, a panpot, and three EQ controls labelled High, Mid and Low. It also has direct input and output jacks and a front-panel trimpot, alongside green (-10) and red (+10) peak lights. Channel levels ride on 100mm slide faders calibrated 0 to 10, with extra markings between 7 and 8 that suggest where Akai thought the optimum operating range sat. The master section has a fader per channel, left and right effects-receive controls, two aux input knobs each with its own pan, two vertical 12-segment LED bargraphs marked -20 to +8, an uncalibrated fade-time knob, and a headphone jack with level control. Rear-panel jacks cover main stereo outputs, mono effects send, stereo effects receive, monitor send, aux inputs with pad switches (all unbalanced), footswitch jacks for incrementing programs up and down, tape sync in and out, and MIDI In, Out and Thru.
Programming is simple on purpose. Kill the memory-protect switch, hit the manual button (or press zero twice - register 00 is the manual register), set the controls where you want them, then press A followed by the two digits of the target register. Every control on the board except the input trims and headphone level gets recorded. To recall, punch the register number, use the program-increment functions, or send a MIDI program change. Modifying a stored program only changes the settings of controls you actually moved. Akai pushed the snapshot idea further than most by allowing crossfades between snapshots, which makes dynamic mixing of all console settings possible. The snapshot and crossfade data live inside the console rather than the sequencer, so the MIDI cable only carries program change instructions and rarely gets overloaded.
The completeness of that data storage is what set the MPX-820 apart, and Paul D. Lehrman's review in Recording Engineer framed it that way: the unit was the first mixer to make channel and master levels, pans, send and receive levels, and EQ accessible by standard MIDI commands. Two software houses, Opcode Systems and Hybrid Arts, announced editors that would let those settings be modified on an external computer and stored to disk, then reloaded into the mixer's internal memory. Opcode's ran on the Apple Macintosh, Hybrid Arts' on the Atari ST series. Programs were stored in banks of 99, and selecting one brought up a simulation of the mixer's control surface on screen. A mixing-automation table in the same publication listed the Opcode Akai MPX820 editor alongside products from Digital Music Services, Sellmark, Soundcraft and Steinberg.
The EQ numbers are where the manual and the bench disagreed. The High control is rated +10/-10 at its extreme positions, and the manual quotes a 10kHz corner frequency, but measurement put the real corner at 8kHz. The Low control is +18/-16dB with a 50Hz corner at the extreme settings. Both controls are so broad at full tilt that each has a +5dB effect at 1kHz. The treble's influence reaches down to 150Hz and the bass's up to 2kHz. The Mid knob centres at 1.4kHz (the manual says 1.5kHz, close enough), +12/-15dB, with ±5dB points at 400Hz and 4kHz. That much equalization across that much of the audio spectrum can play havoc with levels. The built-in indicators only monitor the post-EQ signal, so pulling the midrange all the way down makes it easy to overload the input preamp without ever lighting the +10 lamp. Worse for synthesists, none of the EQ controls or panpots have centre detents. The circuit is relatively forgiving of small changes near centre, but nulling the EQ out of the path entirely becomes an exercise in frustration. That is an important deficiency in a mixer pitched at electronic musicians who often prefer to work with no EQ at all and make tonal changes on the synths.
The pan controls suffer even more from the missing detents because they are less forgiving near centre. In the review unit the centre position produced output consistently 1dB to 2dB too high on the left, though the fault likely lay in the summing amplifiers rather than the pots. There is no way to balance left and right outputs except by using the panpots themselves. The same null-point problem plagues recall. The display only tells you when you have moved past a stored setting, not when you are on it, and a physical control will not begin to work at all until you move it past the null point. A large intended change can do nothing if you happen to be moving the wrong direction. In a live situation where a level must come down quickly, it first has to go up.
Measured performance, once the EQ is nulled out, is genuinely good. Effective input noise came in at -118dB at the mic inputs and -90dB at the line inputs, unweighted. Frequency response is quite flat from 20Hz with a 2dB rolloff at 20kHz. Crosstalk is better than 70dB, both between left and right outputs and across the inputs. Maximum output level is +20dBm, though some of the gain-structure choices are odd. The effects returns are 20dB less sensitive than the line outputs, so an outboard unit needs to produce a reasonable signal, and the individual channel outputs are heavily attenuated before reaching the main mix bus. It is impossible to drive a single channel hard enough to clip the main output. That is not a bad thing in itself, but it means the usable range of the channel faders is fairly small and sits on the high side.
In use within a MIDI studio, the drawbacks soften. Editing a pre-programmed mix is usually a matter of nudging one or two controls, and at that level the system is reasonably usable. The automation functions, as far as they go, work remarkably well, the sound quality is good, and the low noise figures are particularly impressive. For quick-and-dirty production chores the MPX-820 is fast to set up and very easy to learn - a typical three-minute instrumental with seven synths and a drum machine can sound pretty good with only 15 or 20 moves. The design does require thinking differently, and occasionally a desired move is essentially impossible, so you fake it. The snapshot approach itself drew a dissenting verdict, though. The reviewer found the idea of a mix as a series of snapshots restrictive, detracting from the sense that a mix should be a living, breathing thing. The programmable fade time helps, but assumes every control should move at the same relative rate during a crossfade. The 99-snapshot ceiling per piece of music also limits subtlety, partly because there is not room to store a large number of moves and partly because storing each snapshot pretty much requires stopping the music. The input trims are not automated, so precisely reproducing a mix - especially an old one - can be far more of a chore than it should be, which in some ways defeats the whole purpose of the mixer. The gain structure is restrictive and could benefit from higher levels throughout, and the fade-time control desperately needs calibration. The editing system is clumsy and slow, and the reviewer hoped someone would eventually develop an editor/librarian for the many computers in general use by studios and musicians.
The verdict in Recording Engineer was that the MPX-820 is an important first step toward MIDI-based studio automation, but the snapshot approach is the wrong way to go, especially given that MIDI has hundreds of potential discrete controllers that, used properly, could credibly run a complex mixer in real time. The advantages were never limited to the studio. Live, the mixer can be reconfigured instantaneously from any MIDI keyboard. In a conventional small studio, the tape-sync function supports time-based automation provided an extra tape track is free for the sync tone. It makes the most sense alongside a sequencer in a MIDI-based studio. There, moving or deleting an event is as simple as altering the position of a MIDI program change and has no effect on any other events. Changing a tune's tempo needs no retiming of individual mixer moves, and a sequencer with cut-and-paste or loop functions manipulates the mixer's instructions the same way it handles musical data. The aux inputs are ostensibly for ganging multiple units, and while Akai's U.S. distributor literature states up to eight units can be linked, there seems no reason the number could not be higher - the manual itself mentions no limit.
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