Mercury-4 can play up to 16 simultaneous polyphonic voices. The Assign Mode section determines the total number of available voices and how these voices are played.
Detune- Sets the amount of pitch spread of voices when using Unison 1 or Unison 2 mode. This can be used for mild chorusing effects, or vintage rave "hoover" sounds.
Assign Mode- These determine voice allocation.
IMPORTANT: Unlike just about any other poly synth we can think of, the Jupiter-4 arpeggio section does not have an on/off switch. This means Jupiter-4/Mercury-4 is always in either one of the live play modes described below, OR it's in one of the arpeggio modes. In other words, the various up, down, etc. arpeggio should be thought of as additional voice modes, just like unison, poly, etc. If you've never had your hands on an original Jupiter-4, only one of the various arpeggiator and Assign Mode buttons can be on at any time - pressing one pops the others off like an old Zenith television. Hooray for the 70s!
Unison 1- One note plays at a time (i.e. "mono mode") with the number of stacked simultaneous voices determined by the Number Of Voices setting. Use the Detune knob to "un-tune" the voices for fat sounds.
Unison 2- If single notes are played, this behaves the same as Unison 1, but if more than one note is played simultaneously, voices are distributed dependent upon the number of notes currently played and the Num Of Voices setting. This is sometimes referred to as "unison share" mode.
For example, If Num Of Voices was set to 4, and a single note is played, all four available voices will sound for that note. If two notes are played, two voices will sound for each note. If four notes are played, one voice will sound for each note.
The Detune knob applies here as well - it affects voices regardless of the number of notes currently played under each key - this can lead to really out-of-tune chords, so go easy on this guy, especially if the Drift knob is turned up.
Check out the main chord figure in the vintage Human League tune "Seconds" for a fabulous demonstration of Jupiter-4 Unison 2 mode.
Mono- One note can be played a time with one voice sounding. Though the knob position won't change, the Number Of Voices is always 1 when Mono is selected. Mono mode uses last-note priority - this means the most recently played note will always sound.
Poly 1- This is the standard polyphonic mode. Unlike most polyphonic synths, if the number of currently held notes exceeds the number of available voices (i.e. the Num Of Voices setting), notes are not "stolen." (It will steal notes with longer release times that are still ringing.) Please have a look at the (Lack Of) Voice Stealing and The Jupiter-4 section below for more information.
The original Jupiter-4 had only four-voices, so if four notes were held down, no new notes would be heard until one of the original four keys was released, but you're unlikely to run into this if Mercury-4's Num Of Voices control is set above 8 (unless you have a freakishly high number of fingers).
Poly 2- Poly 2's allocation is optimized for use with Mercury-4's portamento (glide) function - it does a good job of "knowing" which notes to glide from and to when holding notes while playing melodies (regardless of whether notes are played legato or separately).
Num Of Voices- Sets and displays the maximum number of notes that can simultaneously play, from 2 to 16.
Chord- Allows chords to be memorized and played with single keys. To memorize a chord, click the Chord button. The LED flashes showing that it's in learn mode. To learn a chord, play up to 16 notes (this is limited by the current Number Of Voices setting); when the last note is released, the LED stops flashing and the chord is memorized. To disable Chord mode, click the button again.
Note that Chord mode isn't just for "chords" per se; it can be used for octaves, fifths, or any desired combination of notes.
Hold- Notes play continuously. VCF and VCA envelopes operate as normal, with the caveat that their release phases are ignored. Hold can also be used in conjunction with the Arpeggio section, ideal for "standing-there-looking-fabulous-while-the-synthesizer-plays" New Wave 80s moments.
Portamento On/Off and Speed knob- Also known as glide, Portamento causes notes to slide smoothly from one pitch to the next. Toggling the switch enables Portamento, and the Speed knob slows glide speed as it's increased. Portamento works in all voice modes; if you'd like to use it polyphonically, we recommend using Poly 2 mode.
(Lack Of) Voice Stealing and The Jupiter-4
This is important, hence all the bigness and italics. When assigning keyboard notes to voices, most analog poly synths makes use of "voice stealing" in order to optimally utilize a limited number of voices (i.e. how many notes can sound at once).
For example, a Prophet-5 synthesizer can simultaneously play a maximum of five notes. If five notes are held and a sixth note is played, the earliest note played gets shut off, and the most recent played note "steals" that voice in order to sound. This makes the instrument feel more responsive, and often gives the audio illusion of an instrument that can play more simultaneously playable notes than the actual number of onboard voices (five, in the case of the Prophet-5).
However, because the Jupiter-4 is a very early poly synth with limited computing power, it doesn't have any kind of voice stealing. Being a four-voice synth, if four notes are held and a fifth note is played, the fifth note will not sound until one of the previous four notes is released - hence, no voice stealing.
When playing a real Jupiter-4, one tends to adjust their playing technique to account for the lack of voice stealing. In the interest of authenticity, Mercury-4 retains this "no voice stealing" behavior. But unlike the original Jupiter-4, Mercury-4 quadruples the maximum number of simultaneously sounding voices - up to 16 notes (depending on the current Num Of Voices setting), so the lack of voice stealing is far less noticeable than with the original instrument. That said, if you want the true Jupiter-4 experience, set the Num Of Voices control to 4 and dig into the limited-fingers world of early 80s Simple Minds/Human League/Yaz(oo).