Using MIDI Guitars
Except for brief tests in music stores, my own experience is exclusively with the Casio MIDI Guitar system, so there are a few Casio-specific hints here. However, the principles apply to all systems which use a hex pickup to sense the vibrations of guitar strings.
Guitar Setup: At the risk of stating the obvious, the first step in configuring a MIDI guitar system is to make sure the guitar is set up to your satisfaction as a guitar. It doesn't make any sense to start adjusting hex pickup height and sensitivity until the truss rod, string height above fretboard, and intonation adjustments have been made, since any change in these conditions will require new hex pickup settings.
There's nothing wrong with low action, but fret buzz can thoroughly confuse the pitch detector, so make sure it's not too low.
I've read a lot of different opinions about optimum string gauges for MIDI guitar. Here's mine: Use what's consistent with your musical objectives. Heavier gauges theoretically can improve the signal presented to the pitch detector, and are less likely to be stretched sharp by overly enthusiastic fretting (one of my own bad habits). Even so, Casio MIDI guitars were shipped with .009-.042 strings, and they work fine for me; I've used gauges up to .011-.050, and they work fine, too.
MIDI Configuration: Once the guitar is set up to your satisfaction, configure the MIDI guitar and the receiving synthesizer(s) with matching MIDI Mode and bend range settings. (On the synthesizer, bend range may have to be set for each MIDI channel, and even within each patch. It may not be something you can do once, then forget.)
The default MIDI Mode setting for MIDI guitars is the Poly mode. Most synthesizers are also set to Poly by default, with "Omni On." This means the unit will receive MIDI information on all channels, cutting down on phone calls from disgruntled new users. The default settings mean your MIDI guitar will usually make a synthesizer talk as soon as the two units are connected. This is fine if all you want to do is play the same sound from all six strings, all the time-- and if having the synthesizer track string bending isn't a high priority. (Poly mode tracks bending of only one string at a time.)
Mono mode, in most MIDI guitars, is actually Multiple Mono mode, with each string triggering a different MIDI channel. This allows bending all strings at once (at least in theory); it also allows each string to trigger a different sound. If your synthesizer supports this mode, this is probably the best way to go. The guitar transmits on six adjacent MIDI channels. The settings on the synthesizer end are the same as if it were being controlled by a sequencer. But do make sure the synthesizer is expecting the same six channels the guitar is using. Also, make sure that it's set to "Omni Off," if that option is available.
MIDI Adjustments: Once all these factors have been addressed, adjust the height of the hex pickup. I've found that a good starting point is as close as you can get it to the strings, without the strings actually hitting it. Casio recommends a distance of one millimeter; I personally consider that a maximum.
Now, you can tackle hex pickup sensitivity. On Casio MIDI guitars, this can be adjusted for each string, using six little trim pots located under a rubber cover on the back of the guitar. These are very tender, so be gentle! The Casio PG-380 allows use of the built-in LED display as a sensitivity meter. This is handy for getting in the ball park, but you'll have to make final adjustments by ear, through trial and error, and it will take time-- likely several weeks. I've found that a synthesizer sound which varies dramatically with velocity changes is very useful for making these adjustments. The Steel Drum sound in the Casio VZ synthesizers is my favorite for this purpose.
I find the pitch tracking pretty bad when I play chords in "normal" configuration-- that is, with pitch bend enabled. I keep my guitars set for "chromatic" response almost all the time. On the rare occasions when I double a guitar lead line with, say, a fretless bass-type sound, bend tracking is quite good. I've read that a lot of Roland users have the same approach-- "Disable pitch bend unless absolutely necessary!"
Tracking down the sources of bad notes triggered by a MIDI guitar can be difficult, because there are so many possible causes. I had truly astonishing percentages of really bad sounds emanating from my speakers when I was first getting started. After long and dedicated exploration, I had to conclude that most of them were my own doing. In a way, it was a bitter pill to swallow--I hadn't realized my technique was that bad! In another way, though, it was a good thing, because it meant the solution was (literally) in my hands.
Hardware Problems: Defective equipment is possible, though, and needs to be ruled out. This may be difficult if the problem is in the digital circuitry, but Occam's Razor is your best tool, and sometimes the problems are easily fixed. An intermittent break in the MIDI cable, a bad contact in a Casio guitar's MIDI out port, a broken solder joint where the jack attaches to the PC board inside the guitar-- these are all problems I've personally encountered.
A different MIDI cable is a quick thing to try, and you need a backup anyway.
I had to resolder all the input and output connections on an (ab)used MG-510 I bought in a pawnshop. Fortunately, the clues were obvious: The synthesizer would suddenly jump to program #1 (indicating a momentary loss of power), the guitar output signal would cut out briefly, and/or I'd get synthesizer notes stuck on (indicating that the MIDI output was disconnected briefly). Fortunately, the MG-510's port panel and circuit board are easily removed, and the connections were easily resoldered.
I received a note from another MIDI guitarist who had the same problem with a PG-380. This was a more complex repair because the PG-380 has two stereo audio output ports, compared to a single mono jack in the MG-510. Nonetheless, he was able to make the repair himself.
The quick test here is to wiggle the MIDI connector, audio plug and power connector while sounding one or more notes through the external synth; if bad things happen, and you're certain the cable is OK, a resoldering inside may well be in order.
Recognizing Operator Error: The most difficult part will be analyzing your own playing to see if it's responsible for unwanted sounds-- and, if so, how. Making written notes of what usually sounds good and what never does can be helpful. Recording yourself can help even more. A four-track is ideal for this purpose, because you can listen to the guitar, the internal synth if you have one, and the external synth, together or in isolation. But any stereo recorder (cassette deck, stereo VCR, even a jambox with external inputs) will let you compare guitar sounds with synthesizer sounds.
Expect this process to take time-- weeks, perhaps. Play in all the different ways you can imagine-- single notes and chords, strumming and plucking (two-hand tapping too, if that's part of your bag of tricks), hard and soft. Sooner or later, patterns will begin to appear ("I always get that sour note at this point in this song!") Then you can make some real progress.
Identifying Problems: The nature of the unwanted notes can give you clues:
Harmonics, in my experience, are often generated by the right hand. A rough edge on a flatpick or fingernail can be a culprit; so can picking location.
A big problem for me was notes a semitone sharp while playing chords. I almost always traced this to the side of a fretting finger touching an adjacent string.
An imperfectly barred chord can produce unpredictable results. (In my own case, a majority of wrong notes come from my bad left-hand technique.)
Notes that jump around wildly before settling down are freqently cured by a sensitivity adjustment.
Occasionally, when playing a chord, I get a note in the octave below the sound I'm playing, but not exactly an octave down. I have absolutely no idea why-- it seems to violate both common sense and the laws of physics. Fortunately, this is very rare. It usually happens on an upstrum (which is when most of my right-hand bad notes occur).
In the next section, I describe some of the playing techniques I use to try to avoid these problems.
Why Can't I Play This Thing?: Nobody plays MIDI guitar as a first instrument. We all started off playing a regular guitar, whether acoustic or electric. By the time we're ready for MIDI capability, we know how to play guitar. We not only know it intellectually, but we've practiced enough that most of the mechanics of playing the instrument are accomplished at a subconscious or unconscious level. And any poor technical habits we may have developed also operate in this non-conscious area-- habits which may not sound bad at all on the guitar, but which thoroughly confuse the MIDI guitar system's pitch detection circuit. For this reason, I strongly suspect it would be much less frustrating for a guitarist to tackle an instrument with a completely different playing interface-- trombone, say, or accordion-- than MIDI guitar, which requires us to relearn what our bodies already "know" about dealing with strings and frets.
How Bad Things Happen To Good Players: Let me give a couple of examples: I've already mentioned my problem with the side of a fretting finger touching an adjacent string. When I do this, the sound of that string as it comes from the guitar is muffled, and quiet enough that it's really not even heard. The pitch detector, though, often "hears" the note a semitone sharp, so it passes the word on to the tone generator. Depending on the dynamic configuration of the particular patch, this inadvertent note that's likely not noticeable from the guitar can be as loud as any other note coming from the tone generator.
Another left-hand problem area for me is triggering notes unintentionally with my left hand, when I lift fingers from strings. Again, it doesn't sound too bad coming from the guitar, but can produce pretty awful noises from the tone box.
Both of these problem areas are most noticeable when using sounds with on/off, organ-type amplitude envelopes, and can be much less obvious with highly dynamic sounds, such as steel drum, vibraphone, trumpet, etc. tend to be. Since my main approach to MIDI guitar primarily involves pad sounds with organ-type envelopes, obviously I'm keenly aware of these particular technical shortcomings on my part! The best solution for me is to mute with my right hand while changing chords, and it helps enormously.