The MIDI Page: Channel Voice Messages

by Todd Albertson

This month we get to look at the most useful and powerful types of MIDI commands. It's been a long haul, but if you have managed to stayed with me this far, your reward is in receiving powerful knowledge today.

CHANNEL VOICE MESSAGES:

As the name implies, these messages are understood and acted upon only by the instruments assigned to the channel being sent. Like a television filtering out unwanted stations, MIDI instruments ignore extraneous signals on other channels. There is an exception to this, and we have already discussed it: OMNI mode. We will assume here that OMNI is OFF, and that all instruments are responding to their assigned channels.

NOTE OFF and NOTE ON commands are simple enough. Since computers have no way of knowing how long to sound a given note, you must tell them; thus we have a message for turning on a sound, and another for turning it off. These messages include the NOTE NUMBER to affect. For instance, Middle C is note number 60, C below Middle C is note number 48, C above Middle C is note number 72. As you can see, adding or subtracting 12 from any note number will affect the note exactly one octave from your original position. Note numbers range from 1 to 127. This is an ENORMOUS range! Many instruments currently will not even respond to note numbers throughout the entire range available, but this is changing. "Velocity" information is also transmitted in NOTE ON and NOTE OFF events. Velocity means: "The force with which the key was struck." That is how it is possible to make a note sound different when played with different forces by the keyboardist. The computer is able to read and interpret his playing through the use of this velocity information. Like note numbers, velocities can range 1 127. Understanding NOTE ON and NOTE OFF com-mands is an absolute MUST for sequencer editing, since most sequencers present such information in numeric form rather than in music notation (The software "Finale" is the notable exception). Sequencers often have the ability to move these events on the "track" to line them up perfectly with the beats of the tempo. Listening to old music tracks from the 70's and before can really throw you if you've grown accustomed to listening to music in the 80's, because that is just about the time sequence editing began to be widely accepted. When musicians talk about a feel that is "in the pocket", or "in the groove" they usually refer to getting the timing "just right". The funny thing is that nowadays, with this incredible technology at our fingertips, most of us tend to use it VERY selectively. "Quantization" (perfectly lining up the notes to the tempo) is really nice for the drums, bass, and other rhythm instruments, but melodies lose something intangible when quantized. There is a strong move these days among musicians to insure quality in their work by quantizing only certain lines in any given piece. All this is done by manipulating NOTE ON and NOTE OFF events.

PROGRAM CHANGE events are what we use to access patches stored within the memory banks of a given instrument. By sending a Program Change, one can cause an instrument to instantaneously change its sound to another previously stored sound within the instrument. It's exactly the same as though you were standing right over the instrument and pushing the patch selector buttons.

AFTERTOUCH is yet another attempt to personalize and de mechanize the sound of modern music. "Aftertouch" refers to the ability of an instrument to alter its sound (usually with an LFO) according to pressure exerted by the keyboardist AFTER he has already pressed the note. In other words, let's say I wanted a flute vibrato to begin about .5 seconds after a certain long sustained note begins. This is precisely the way a real flautist would handle the same note. Try listening carefully to solo instruments, and you will see that vibrato comes and goes in most long notes. Since I wish the vibrato to begin late, I press the key, listen as the note sounds, and then add pressure to the key. As I do so, the vibrato is introduced into the sound in direct proportion to the amount of pressure I exert. This is VERY handy for getting a note right on the first try, rather than going back and sequence editing the track to add things like this later. There are two kinds of Aftertouch: CHANNEL and POLYPHONIC. Channel Aftertouch means that all the notes on that channel are affected by the pressure introduced to any note after playing it. Polyphonic Aftertouch means each individual note can be played with a different pressure for a different amount of vibrato (or whatever you have assigned to Aftertouch). You need not worry when buying an instrument if it doesn't support Poly Aftertouch. Most don't. In fact, I have yet to meet the musician that can competently handle this feature (myself included). It's just a lot to do. On top of that, many sequencers have difficulty handling that much data, especially on an already "busy" track. You tend to fill up memory very fast using this feature, and editing can be an absolute nightmare. If you are not sure you positively need to use this feature, I would just turn it off. I don't recommend this for beginners!

PITCH BEND events are just the information passed down the MIDI line to control "bending" notes, the way a guitarist would bend certain notes or chords. Usually these pitch bend numbers are generated by a wheel located on the left side of the keyboard. The trick when using a pitch bend wheel is to remember to "unbend" your keyboard after using it. In other words, if you are recording with a sequencer and you bend your sounds, you must try to release the wheel before stopping the recording. If you forget to do this, every time you recreate your performance, the keyboard will bend at the appropriate spot, your recording will end, and there you will be with a keyboard that plays in the wrong key! You can always edit the track of course, but it's easier and faster to remember to release the wheel before stopping the recording. Having said that, let me add that on many of the tracks I've recorded, I have had to bend a note up or down on the last note of the piece. Since I can't really release the wheel before stopping the sequencer in these situations, I usually edit such tracks to send "PB=0" at the beginning of the song. That way the pitch bend circuitry is reset each time the track starts -- Just a tip.

Sheese! Looking back over this article, I realize how much information there is this time. Rather than dump more into your laps, I'll wait until next month to cover CONTROL CHANGES. This is appropriate since the functions of the various Control Changes are so varied anyway. Thanks for reading.

Add comment

Comments

Subscribe:
  • No comments found