MONO, POLY and OMNI ON/OFF
Welcome back, my friends. Are you ready to begin? Heh, heh... sort of sounds like the dentist, doesn't it? Hopefully, this column is a LITTLE easier to take than that! I dunno, though. Some of the calls and letters I'm getting sound like maybe I'm giving some of you headaches... Oh, by the way: I finally got my R-8 (drum box) expander card from Roland -- heh.
This month we dig deeply into the complexities of what first appears patently simple - MONO, POLY, OMNI ON/OFF interactions. As you may recall, OMNI mode is used to cause a receiving instrument to respond to ALL channel messages. Consider how important this can be: IF you make an error, and turn on OMNI when you are sending multiple channels down your line, your poor instrument will try to play all the parts at once. This is rather like trying to listen to all bands of radio frequencies at once. It only works if there is only one station transmitting! If, on the other hand, OMNI is accidentally turned off when you were depending on it, you might get no sound at all, depending on which channel you were receiving, and which was being sent. The basic principle once again is, if OMNI is on, the receiving instrument will respond to ALL channels. If OMNI is off, the receiver will respond only to its assigned channel.
What about the transmitter? What does OMNI do for the transmitting instrument? The answer is "nothing" most of the time. The last part is the clincher. You see OMNI mode is easy to understand when you consider it only as it applies to polyphonic instruments in polyphonic mode. POLY and MONO modes can be downright confusing at times though.
Let's first define what we are NOT talking about. We are not talking about "multi-timbral" abilities. Recall that a multi-timbral instrument has the ability to play not only many notes, but many voices on separate channels. For instance, you may assign the new Roland U-220 to deliver piano when messages arrive on channel #1, but strings when messages arrive on channel #2. As with all truly multi-timbral instruments, the U-220 can handle both at the same time. Since purely MONO instruments are so rare nowadays, some manufacturers have started to refer to multitimbral capability as POLY mode. This doesn't help anyone though. I think Ensoniq has the answer when they call this "multi" mode. POLY and MONO modes are clearly defined by the MIDI creators to mean polyphonic and monophonic.
Fancy words like "monophonic" deserve to be explained. No, it's not the same thing as "monaural," which was what we listened to before we bought "stereos." "Monophonic" simply means that no matter how many notes you play, only ONE will sound at a time. "Polyphonic" means, the ability to sound more than one note at a time. Again, "multitimbral" is something entirely different.
Believe it or not, just ten years ago, MONO ONLY instruments were still being made and sold successfully! These days, MONO mode is just one facet of an instrument's abilities. This takes us back to our original question concerning the behavior of OMNI mode in the transmitting keyboard. When we set our instrument to OMNI ON / MONO, it will transmit as we would expect, sending our single note down the assigned transmission channel. IF however, we set our instrument to OMNI OFF / MONO, we could be in for a shock. Some transmitters will separate our chords into single notes sending each note in the chord to the next numbered MIDI channel! Surprise! That's what the creators of MIDI intended. Now for another surprise: No one I know of has ever successfully implemented this! What's going on here?!?!
The MIDI standard says keyboard behavior when transmitting can be summed up as follows:
OMNI ON / POLY: All playing is sent to one channel.
OMNI OFF / POLY: All playing is sent to one channel.
OMNI ON / MONO: Each single note sent to one channel.
OMNI OFF / MONO: Chords are "broken" into single notes and sent to a separate MIDI channel each. The notes are assigned one after the other to the next ascending MIDI channel number. Pretty wild, huh?
Keyboard behavior when receiving can be summed up as follows:
OMNI ON / POLY: All channels received and played in POLY.
OMNI OFF / POLY: Normal operation. Receives on one channel only, and plays polyphonically (chords).
OMNI ON / MONO: All channels received, played one note at a time. (Imagine ALL channels trying to control one note!)
OMNI OFF / MONO: The assigned channel is received and sounded one note at a time.
It's VERY confusing at first to try to sort out what benefit could be derived from the implementation of the MIDI standard as it has been defined. I have YET to see chords broken into single notes and assigned to a separate channel each. It seems to be one of those weird anachronisms that no one attempts to implement, but it keeps showing up in owner's manuals and charts with confusing explanations.
Why in the world would the MIDI gods seek to make us so miserable with such a weird brew as this? The answer lies once again, in history. In the early days, when the MIDI standard was being conceived, MONO ONLY instruments were still common. Someone thought, "Let's set it up so a guy could have seven different MONO ONLY instruments in his chain, and then play chords!" Everyone thought this was a WONDERFUL idea, so it was added. No one realized that as the standard was released, MONO ONLY instruments were already joining the ranks of other notables such as brontosauruses, pterodactyls, and mammoths. So we have a standard that is just not useful in some ways.
Here's what most manufacturers seem to be doing now: Full implementation of the first three in each group, and implementation of the OMNI OFF / MONO mode to sound single notes at the LOCAL keyboard but send all notes to whatever channel is assigned for transmission. Receiver behavior likewise, simply responds to one channel with only one voice. This is certainly much more intuitive! MIDI is sometimes a prisoner of its primitive past, and in this case, most manufacturers just choose to abandon it for a more reasonable approach.
Having thoroughly destroyed any previous understanding you might have had about these matters, I'll try to find something a little less painful for next month. Perhaps that lower right molar...