Mono Mode (Part 6)
Getting the Most from Mono Mode
You may have your heart set on a specific MIDI guitar controller, but do you know which kind of synth voice unit to combine it with? Rick Davies offers some interactive advice.
In past issues we've looked at how various instruments behave when receiving MIDI data in Mode 4. This month we'll do a bit of the same, but with a specific breed of controller in mind: the guitar.
MIDI AND GUITARS have yet to sort out all their differences, but there are many MIDI guitar controllers available now, and for guitarists daring enough to dive into such systems, there is more to consider than just the type of controller; the synthesiser at the other end of the MIDI cable has to get along with your guitar controller in ways which may not be obvious at first.
Any MIDI controller can generate note events, and any MIDI-equipped synth will interpret these notes with little problem (usually). But to make the synth respond to the nuances of a musician's guitar-playing, other aspects of MIDI data come into play besides the Note On/Off messages which get the sounds happening.
Before continuing, I'd better stress that this article does not address the tracking delay problems for which so many guitar controllers are notorious. What is of importance is how well-suited some synths are to guitar controllers, and what to listen for when you're out looking either for a new system, or for some additional synth voices.
Some synth manufacturers have started implementing features which make their machines suitable for guitar controllers, generally making expressive playing more effective. Other synths may not be ideally set up for guitar controllers, but with a bit of consideration, and understanding MIDI Mode 4 (or Mono Mode, as it is often called), satisfying results can be obtained.
There are, of course, guitar synths which feature built-in voices like the Roland GR700 and, most recent, the Stepp DG1. Yet even with these systems, it may be of interest to add other sound sources via MIDI.
It's helpful to understand that until recently, most synthesisers have been designed with keyboard controllers, built-in or external, in mind. Guitars are able to play many notes at different positions on the fretboard, unlike keyboards which have only one key per note, so there are many times when guitarists will play the same note on two strings at the same time. If only one of these strings is muted, only one of the identical notes stops, not both.
One would hope, then, that a synthesiser reacts in a similar manner while playing a guitar controller. This may happen, but there is a catch: the synth must either be a bit clever, or you must tell it which string is being played or muted with each note message sent out over MIDI. We'll consider these two possibilities one at a time.
As mentioned earlier, some manufacturers did not have guitar controllers in mind when they designed the MIDI implementation of their synths, so it is often assumed that a sustaining note must be turned off before it can be re-triggered. That is, you have to stop playing a note before you can play it again. On a guitar, however, that is only the case if you are playing the same note on the same string. Some keyboard synthesisers can sound a note twice if it is played from the keyboard and also received over MIDI at the same time. But if two identical Note On events are received one after the other, as may happen when using a guitar controller, then some synths may play the note twice, while others may play the note only once.
To make matters even stranger, if either of the identical notes is muted, some synths will respond by switching off both notes. But if a synth has its wits about it, it keeps track of incoming notes, and switches them off one by one as Note Off messages are received. The Oberheim Matrix 6/6R is one such synth.
There's good news and bad news, though. The good news is that some synths like this can play several identical notes even while in MIDI Mode 1 (Omni On/Poly), which is convenient because as soon as you start working in Mode 3 or Mode 4, you have to start watching your MIDI channels, and this may be more work than many first-time MIDI users will want to involve themselves with. The bad news is that when a synth has received several identical Note On messages, the ensuing Note Off messages are typically applied to sustaining notes in the order in which they were triggered. Thus, even if the last note played is the first to be muted on the guitar, the first note played may be the one turned off on the synth. If the dynamics of identical notes are similar, this effect may not be perceived, but if one string is played harder than the other, the dynamics of the synth's voices will not follow your playing accurately.
There is a way around all of this, of course. As mentioned earlier, the misunderstandings between guitar controllers and synthesisers can be sorted out if, along with the note number and velocity, each Note On/Off message also specifies the string played. Fortunately, many MIDI guitar controllers can assign each string to an individual MIDI channel (1-16), and more synths than ever are now implementing MIDI Mode 4 (Omni Off/Mono).
In Mode 4, the receiving synth assigns one voice to one MIDI channel, and thus acts as several monophonic synths. This is similar to the way a guitar behaves; each string can be considered a monophonic voice. When the guitar controller transmits note messages on an individual channel for each string, and the receiving synth is in Mode 4, there is no confusion about which note to turn off.
A synth in Mode 4 can still get confused if two identical notes are received on the same channel, though. So make sure that your guitar controller transmits on separate channels, because if it doesn't, one of the synth's voices is going to go crazy trying to play all the polyphonic information coming in.
Note also that there are some interesting (non-MIDI spec, perhaps?) implementations of Mode 4 going around. For example, a Prophet 2000 in Mode 4 can handle polyphonic information received on one channel by assigning it to any available voices, so even if the guitar controller transmits on separate channels, each string is treated as a polyphonic source. You never know; it could come in useful sometime.
Typically, guitar controllers and synths also differ in the ways they can deal with several channels of MIDI data, specifically the channel numbers themselves. Currently, most synths assign their internal voices to consecutive MIDI channels when in Mode 4. For example, a six-voice synth with its basic channel set to 3 would transmit and receive over channels 3-8. Many guitar controllers, on the other hand, can assign any channel to each string. This can be useful for driving several synths over MIDI, or if the receiving synth has more than six voices. For example, since the Prophet 2000's 16 samples are assigned to MIDI channels 1-16 in Mode 4, a guitar controller could play any six of these samples simply by assigning each string to the appropriate channel.
Mode 4 also comes in handy when dealing with pitch-bend messages. Pitch wheels, joysticks, and the like are examples of monophonic performance controls on synths which affect all voices simultaneously. The equivalent on a guitar is the whammy bar. Some MIDI guitar controllers, such as the Ibanez MC1, feature assignable whammy bar-like controllers which don't actually change the string tension, but can be programmed to create the desired effect over MIDI. To control a synth's pitch, these bars should be assigned to the pitch-bend controller over MIDI. When the receiving synth is in Mode 1, this does the job just fine. But put the synth into Mode 4, and the MIDI channel assignment of the pitch-bend controller becomes crucial, as does the synth's Mode 4 implementation.
For a start, many synths in Mode 4 respond to pitch-bend messages on individual MIDI channels by pitch-bending individual voices. Such synths are well suited to guitar control because they can respond to individual string bending. For example, if the guitar controller and synth have matching MIDI channels, then bending a string assigned to channel 4 will cause the synth voice assigned to that channel to shift in pitch. The same would apply to the remaining five strings and synth voices.
So if pitch-bend messages on the six channels affect each synch voice individually, how can the whammy bar bend all six voices at once?
Answer 1: by transmitting pitch-bend messages on six channels. (Slow, sloppy, and too obvious.) Answer 2: by transmitting a pitch-bend message on the channel just below the synth's basic MIDI channel. (Fast, simple, and perhaps unexpected.) That is, if the synth receives on channels 3-8, then a pitch-bend message on channel 2 will affect all six voices simultaneously.
Pitch-bend messages received on channels 3-8 still affect each voice individually. Only a handful of newer synths, such as the Prophet VS, use this "Basic Channel minus 1" method in Mode 4, but word has it that other instruments will adopt it in future.
To summarise, Mode 4 plays a major part in getting the most expression out of any synth being driven by a MIDI guitar controller. It's worth checking out synths and controllers for some of the features mentioned in this article, so listen for notes being cut off, and demand individual string-bending capability as well if possible.
Mode 4 may not be essential for everything you intend to do with your synth, but it can make all the difference in the world when combined with a versatile MIDI controller. If you're just beginning to look into guitar synths, take the time to check out a synth's MIDI implementation before taking the plunge, regardless of the guitar controller you choose.
Feature by Rick Davies
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