The fifth and final part of our series on basic MIDI applications looks at MIDI-controlled audio processors and MIDI data processors. We conclude by connecting the components together into a working system.

SIGNAL PROCESSING DEVICES have become as much a part of modern electronic music as synthesisers and drum machines. Whether the devices in question are audio processors or MIDI data processors, their creative possibilities are being recognised by musicians everywhere.

On the audio side of things, digital reverbs and delays are used extensively to add natural character to electronically generated sounds. MIDI processors, on the other hand, are used for performance effects (like turning pitch-bend information into note data) as well as simplifying and organising complex MIDI systems. In either case, these processors, like the instruments with which they are used, respond to and sometimes generate MIDI messages and can be readily incorporated into a MIDI system.

Before giving specific explanations of these messages, however, it's important to understand the difference between MIDI-controlled audio processors and MIDI event processors. MIDI-controlled audio processors like the Yamaha SPX90, Lexicon PCM70 and Alesis Midiverb II, affect an audio signal fed through the device. They do respond to some MIDI control messages, however. On the other hand, MIDI processors like the Yamaha MEP4, Akai ME100 and Roland MPU-103 manipulate MIDI signals. In fact, they don't even have audio inputs.

Some of the manipulations which MIDI processors perform, however, can produce effects that sound like those of audio signal processors. MIDI delay effects, for example - the intentional ones, that is - can be set up to sound like a audio digital delay, but what actually happens is that the MIDI processor sends out additional note messages to a synth at the chosen delay time. Consequently, the "effected" signal suffers no degradation whatsoever but the delayed messages do use up some of the instrument's available voices.

Other MIDI effects exist, but for now, let's move onto the specific MIDI messages which MIDI-controlled audio processors respond to and generate.

Audio Effects

THE MOST COMMON use of MIDI in audio effects devices is remote patch selection. It's safe to say any signal processor with a MIDI In port will be able to respond to Program Change messages. Most devices only work in Mode 3 (Omni Off), so you'll need to set a specific reception channel. Once you've done that, you can call up different preset effects settings from a master keyboard, sequencer or other controller in the same way that you call up different patches on expander modules.

One problem that you may run into, however, is patch numbering. For example, if the synth acting as a master controller has a string sound in patch 33 and the connected signal processor has a gated reverb effect in the same location, when you select the string patch on the synth the generated Program Change message will call up the gated reverb program on the effects unit - novel, if not particularly useful.

You could rearrange the locations of the patches in the synth or the effects device so that they line up as you'd like them to, but the process can be a real pain. You may also run into situations where you want several different synth patches to be processed by the same effect setting. Another possible solution involves a feature available on some effects devices (and some synths) called "patch mapping". Patch mapping refers to the ability to call up a patch other than the one corresponding to the Program Change number received at the device's MIDI In port. In other words, if a digital reverb with patch mapping receives Program Change number 23, it may call up patch 57 or whatever patch is assigned to number 23 on its Program Change "map" (as in Figure 1).

A few sophisticated master controllers also have a form of patch mapping in which selecting a preset actually sends out a different Program Change message (some send several Program Changes at once). For example, selecting patch 44 causes Program Change number 75 to be sent from the controller's MIDI Out port. Things can get a bit involved if both the controller and the effects device have patch mapping capabilities (selecting patch 21 sends Program Change number 49 which calls up patch 53...), but mapping can be very useful in sophisticated MIDI systems.

In addition to Program Change messages, some effects devices - like Korg's DRV2000 - can change patches with MIDI note messages. In other words, hitting middle C (MIDI note number 60) on the controller can cause the DRV2000 to change to program number 60. If you're in a live performance situation, though, and forget to turn down the volume, hitting what appears to be occasional random notes to change effects patches may not be the best idea.

But you can avoid this problem by using a sequencer. In fact, dedicating one track of a sequencer to sending Program Change messages (or Note On messages) to your effects devices is an excellent way to automate part of your MIDI system. The process involves selecting a MIDI channel to work on (or channels, if you're using multiple units and no patch mapping capabilities are available), setting the controller and effects to that channel, picking a track on the sequencer, and then recording all the Program Changes on that track.

The actual recording procedure is the same as for recording notes except that instead of playing keys on the keyboard, you hit patch select buttons at the appropriate times. If you've got extremely good timing or if you only need to send a few Program Changes you can probably record in real time, but otherwise you may want to use step-time recording for accuracy. One other thing to keep in mind is that some devices do not instantly respond to Program Change messages (particularly if they receive an audio input signal right after they receive the Program Change), so you may have to enter these messages a bit earlier in the sequence than you'd otherwise expect.

"Dedicating one track of a sequencer to Program Change messages is an excellent way to automate part of your MIDI system."

Useful as it is to perform remote patch changes over MIDI, it seems that you should be able to do even more with an industry standard interface. Happily, a number of manufacturers feel the same way and have developed more advanced MIDI implementations. One of these involves a different use for note messages.

On pitch-shifting devices equipped with a MIDI input - like the Roland VP70 and Korg DVP1 - you can use note messages to adjust the amount of pitch shift made to the incoming audio signal. For example, on both the VP70 and the DVP1, if you play a major chord on the MIDI controller, the monophonic input signal will be harmonised to a major chord. But if you suddenly switch to a minor 7th chord, the harmonies being created with the input signal will instantly change to minor 7th as well. This greatly increases the effects you can create with these devices. Prior to this, changing the pitch shifting amount required manually adjusting the intervals.

Another type of real-time MIDI control for signal processors can be found on the Lexicon PCM70. They call it Dynamic MIDI - other manufacturers have their own names for similar features - but the idea is to use MIDI controller messages or MIDI note messages to adjust individual parameters within a given preset effect as you play. For example, just as the pitch-bend and mod wheels are used to make real-time "adjustments" to the current patch on a synth, they can be used to alter delay time, reverb decay and so on. Again, like the MIDI control available for pitch shifters, this adds a whole new level of possibilities and expression to effects devices, because they can be "played" like a musical instrument.

Though all MIDI-controlled audio effects respond to MIDI messages, not too many actually generate them. Some devices send Program Change commands when you select a program from the front panel and some will even send a different numbered command upon receiving one over MIDI (another form of patch mapping); but that's about the extent of their capabilities. The one exception to the rule is that a number of effects boxes are starting to include internal memory dumps using System Exclusive messages to generic librarian programs and sequencers which record SysEx data.

MIDI Processors

UNLIKE AUDIO PROCESSORS, most MIDI processors generate more types of data than they respond to. By their very nature they receive MIDI messages and then alter and/or re-route this data before they output it. Depending on the sophistication of the processor, these "alterations" can be as simple as changing the channel the information is sent on or as complex as deriving a completely different set of messages from the original input.

Describing the specific capabilities of all the MIDI processors would be long-winded and boring, so we'll discuss some general principles and describe some of the differences between MIDI "utility boxes" and creative MIDI effects - the two basic types of MIDI processors.

MIDI utility boxes are devices used to organise the flow of MIDI signals and overcome problems in the routing of these signals to various components of the MIDI system. Thru boxes, MIDI switchers and basic MIDI mergers all fit into this category. (If you've been following this series, you're aware that a MIDI switcher has been a prominent part of our typical MIDI studio over the last four articles.)

Though they're often seen as accessories, these utility boxes can play an extremely important role in MIDI systems because they make certain MIDI connections - and therefore, applications - possible. For example, if you want to record a sequence into a sequencer that's synced to a drum machine, you need a MIDI merger to combine the System Real Time clock messages from the drum machine with the Channel Voice note messages from the controller. Similarly, if you're working with a number of different synths that don't have MIDI Thru ports, you need a MIDI Thru box or switcher to be able to hook them all together to create big MIDI layers.

Another benefit of switchers is that they allow you to reconfigure your MIDI system at the touch of a button - without having to resort to unplugging and reconnecting MIDI cables. And let's face it, the more time you waste messing with a tangle of cables, the less time you have to make music.

A number of commercially available switchers and mergers have more sophisticated MIDI processing functions.

"Though they're often seen as accessories, some MIDI processors can serve an extremely important role in MIDI systems."

JL Cooper's MSB products (MIDI Switch Boxes) and 360 Systems' MIDIMerge+ have selective MIDI filtering features (like those found on some sequencers) if you need to lighten the data stream or filter out unwanted information. If you want to prevent say, memory-hungry aftertouch messages from reaching your sequencer, you simply engage the aftertouch filter and it will remove these messages from the data stream. Additional features found on other devices include transposition, which is self-explanatory, and channel-shifting or rechannelising, which simply changes the channel on which the input MIDI messages are sent.

If you've been paying attention you'll have noticed the discussion slowly drifting towards more creative MIDI data processing. The reason is that certain features are useful as utilities and creative tools. For example, devices like the Voyce LX4 and LX9 offer a form of MIDI mapping, which is similar to patch mapping in that it "maps" or reassigns certain incoming MIDI messages before they're sent out again. With these units, however, you can assign different sections, or zones, of your master controller to send on different MIDI channels. This is achieved by having certain ranges of MIDI note messages sent from the controller redirected. In other words, even though the controller may only be able to transmit on one MIDI channel, with a MIDI processor of this type you could have the notes from the bottom two octaves sent on one channel, the next octave on another and so on. In this case, a utilitarian function is being applied to a creative end.

MIDI mapping allows for even more dramatic processing of MIDI data, though, an example of this principle is The Mapper from an American firm, Axxess Unlimited. This allows you to take any type of incoming MIDI message and convert it to any other type of MIDI message. So, for example, if you want to make a sustain pedal transmit a short melody, you could convert switch controller number 64 (that is, stepping on the sustain pedal) into a string of Note On messages. Like I said, creative.

Finally, MIDI Delay features represent another form of creative MIDI processing. In addition to basic delay times, some devices allow you to control the number of repeats, their velocity level and the MIDI channel on which they're sent. By adjusting this last parameter, you could have the "echoes" occur on different instruments with completely different timbres. Neat.

The System

NOW THAT WE'VE examined the basic components of a typical MIDI system individually, it's time to consider their combined capabilities. (To refresh your memory, Figure 2 shows a diagram of one possible setup.)

As mentioned in MIDI Basics part two, the real beauty of a MIDI system is that it allows various components of your choice to work together as a music-making system. (Half the fun of working with a MIDI system is planning the next few additions to it...). No matter how many or how few pieces of MIDI gear you have, if you use their MIDI capabilities to the fullest, the potential of the system will definitely exceed the sum of its parts.

If you compose and/or arrange your own music, and you're primarily interested in recording, a MIDI studio can prove a tremendously exciting tool. For this application, the sequencer would form the heart of the system and with it you could record all the musical parts, as well as synchronise the drum machine and automate the effects processors. You could even record drum patterns into the sequencer and have the drum machine act as another sound module, giving the sequencer complete control and increasing the memory available for drum patterns. (If you don't have a drum machine but you do have a sampler and some good drum sounds, this method will work as well.) But if you prefer to control the tempo from the drum machine, you can, you'll simply need to add a MIDI merger to the system. The master controller would serve as an input device in this example, generating and sending the various MIDI messages to the sequencer.

If, on the other hand, your primary interest is live performance, the heart of your system will be the master controller. You can certainly use a sequencer and drum machine, they'll probably be used as playback devices in this context. In other words, the sequencing and programming would be done ahead of time, and you'd hit the Start button and play along with the pre-programmed data. Expander modules, MIDI-controlled audio processors and MIDI effects processors, however, would play an extremely important role, because in combination with the master controller, they will be your instrument. Depending on the sophistication of the MIDI implementation of your controller - or the MIDI features added courtesy of a MIDI processor - this combination of MIDI-equipped boxes could become an extremely expressive one too. Hit one patch select button and an entire new group of patches and corresponding effects with different real-time control capabilities would instantly be called up. Impressive indeed.

This brings us to alternative MIDI controllers. To make life simple nearly all of the examples in this series have referred to keyboard controllers. But if you're a guitarist, wind player or drummer, you obviously want to know if they apply to controllers designed for players of your respective instruments. The answer is yes. Though different types of MIDI controllers have specific features unique to them, and though they may use different techniques to produce certain MIDI messages, MIDI data is MIDI data, no matter where it comes from. Thus, an expander module doesn't really care if the aftertouch data it receives comes from a MIDI guitarist pushing down an expression pedal, a wind synthesist blowing harder into a controller or a keyboard player pushing down harder on the keyboard. All it does is respond to those messages.

So where does this all lead us (and leave us)? Over the course of this series I've discussed what MIDI is, how it works and, through examples, how you might take advantage of it to help you more efficiently create your own music. Making sophisticated music has never been this easy or this much fun and MIDI can take the credit (or blame) for this amazing revolution.

What a great time to be a musician.