The LFO (low-frequency oscillator ) generates sub-audio range signals intended for modulation purposes.
Sync- When the Sync button is enabled, LFO speed will lock to host tempo when using Mercury-4 within DAW software, or to the current tempo in the top menu bar when using the standalone version.
Mod Wheel- This is a really handy, but potentially confusing feature, so don’t skim this section. If the button is in the Off position, the LFO is always “on.” Its effects will be immediately audible by moving the LFO sliders in the VCO, VCF, and VCA sections. Clicking to the On position enables control of LFO depth with an external keyboard controller mod wheel. If the mod wheel is all the way down, LFO depth is zero. This makes setting up a mod wheel to add vibrato or wah effects really easy. If it seems like the LFO isn't working, check if the Mod Wheel switch is engaged.
Waveform- Selects the repeating pattern of the LFO. Available waveforms are sine, square, ramp, and inverted ramp. One time I skated an inverted ramp, and it was super gnarly.
Rate- The Rate slider sets the speed of the LFO, from 0.25 to 830 Hz (with Sync switch off) or from 8 beats up to 1/64th note triplets (Sync switch on). We've replicated the insanely fast LFO speed of the original Jupiter-4 (well into the audio range) enabling all manner of clangorous, raunchy sounds to be created. The LED beside the Rate slider flashes at the current modulation rate.
Delay- Moving this slider up gradually delays the onset of LFO depth. The delay time can be set from 0 to 5 seconds. Because Mercury-4 includes separate LFO’s for each polyphonic voice, onset delay is independent for each note, which is a nice effect when playing melodies or arpeggiated note passages.
When the Mod Wheel switch is in the on position, the Delay slider grays out and is disabled (because delaying the onset of LFO mod when turning up the mod wheel would be super confusing).
JP-4, The "Modifier" Section, and the Confusing "LFO Bend" Controls
This section isn't essential to using Mercury-4, but if you're familiar with the original instrument, it'll clarify some of the changes we made to the front panel. Either way, it's interesting if you're into synthesizer history. When we say "JP-4" we're always referring to the original Roland Jupiter-4, btw.
The original Jupiter-4 was one of the first instruments produced with digital patch storage. It had extremely limited computing power and digital memory (a whopping 1K - half of the average Atari game cartridge of the era), and the general concepts of how a poly synth would handle patch storage and recall weren't firmly established. Notably, the Jupiter-4 had no "edit" mode. This is such a ubiquitous and obvious feature in modern synths that most people probably don't even know what an "edit" mode is, but to illustrate, here's how patch storage and recall works in the original Jupiter-4:
To create and edit sounds, the Manual button must be pressed - this means that the sound conforms to the current front panel settings. To save a sound, the user presses the Write button, followed by one of the eight Compu-Memory user preset buttons. Pressing any of the user or factory preset buttons mechanically deactivates the Manual button, and all controls in the Programmable section go dead (i.e., almost all of the synthesis parameters). You read that right... the front panel goes DEAD. Wanna tweak the filter cutoff of a saved patch and re-save it? Tough luck. The only way to accomplish this is to carefully reverse-engineer the control positions by switching back and forth between Manual mode and the saved patch, and then re-save the patch. Super-duper not fun, and it should be clear why we decided to skip this part of vintage Jupiter-4 (non) functionality.
In just about any other modern synth, moving a control causes its value to instantly jump to the current control position (or increase/decrease if it uses endless rotary encoder-type pots). For all intents and purposes, modern synths are always in edit mode, which is why "edit mode" is rarely acknowledged in the first place. It should go without saying that this is how Mercury-4 works - we didn't think anybody would miss the original instrument's paint-in-the-butt, no-edit-mode implementation.
Modifier vs. Programmable Parameters
With this "parameter lockdown" in mind, the designers of the Jupiter-4 wanted to give users the ability to manipulate modulation destination, amount, and speed for factory and user patches. This explains the impressive flexible implementation of the joystick for controlling pitch bend, filter cutoff, VCA amplitude, and oscillator, filter, and amp LFO modulation - these left-hand control settings are always live and not stored with patches on the original JP-4 (but they do get stored with Mercury-4 patches).
With this in mind, the Jupiter-4's top panel controls are divided into two sections: Modifier controls at the left of the panel (i.e. always-live controls not stored with patch memory), and Programmable controls at the right of the panel (i.e. controls stored with patch memory that go dead when a preset is selected). Like the left-hand mod controls described above, the idea behind the Modifier section controls was that these would still be live and usable for stored patches. These consist of the Trigger section (still present in Mercury-4, see the Trigger section for more info), and the Delay/Bend section, which has been (mostly) removed from Mercury-4.
LFO Bend Controls
Here's where it gets interesting. The Modifier Delay/Bend section of the JP-4 included a Delay Time slider which simply delays LFO mod onset; we relocated this to Mercury-4's LFO section. It also had two LFO Bend controls - a center-detented slider with 0 in the middle, along with a two-position Norm/Wide switch. These essentially acted as additional LFO speed controls that could add or subtract from a preset's saved LFO rate value - in this way, you weren't stuck with the stored LFO rate. If you wanted the stored LFO rate, the LFO Bend slider could be clicked into its center-detented 0 position, or it could be sped up or slowed down by moving the slider. Combining this control with the left-hand control settings allowed full control of LFO speed and depth, even for saved "dead panel" sounds.
(As an aside, the Jupiter-4 didn't have a mod wheel, so the "Bend" part of the Delay/Bend section was meant to imply that the bender joystick controlled the overall amount of LFO mod, not that anything was actually being "bent." Confusing, right? Don't feel bad, I've owned my JP-4 for roughly 15 years and only properly figured all this out when we had to design and document Mercury-4.)
How Does This Affect Me? I Just Want To Use Mecury-4!
Glad you (didn't) ask. Since every single parameter of Mercury-4 is programmable and saves with stored patches, all this clunky and confusing Modifier vs. Programmable parameter business goes riiiiight out the window. The extra LFO "Bend" adder/subtractor slider and range switch are redundant, so we eliminated them. With that said, they expand JP-4's already-huge LFO speed range even further, so we extended the range of Mercury-4's LFO section Rate slider to reflect this. In other words, the minimum speed of Mercury-4's LFO section Rate slider is equivalent to JP-4's LFO Rate slider set to its slowest setting and the LFO Bend slider at its lowest setting, and Mercury-4's fastest LFO Rate setting is equivalent to JP-4's LFO rate slider set to its fastest setting and the LFO Bend slider at its fastest setting. And since the LFO Rate slider has such a large range, we kept the Norm/Wide switch to make it easier to dial in.