4ms Ping
A quad pingable LFO that turns a tap of your finger into four channels of modulation synced to your patch.
Overview
The 4ms Ping - formally the Quad Pingable LFO, or QPLFO - is a four-channel tap-tempo and clock-syncable LFO for Eurorack. You set the modulation speed by tapping a button or feeding it a clock, and four independent LFOs stay locked to that pulse. Each channel runs its own speed and its own waveshape, but they all obey the same ping. According to the module's own documentation, the QPLFO was designed to be useful both in small portable systems that need maximum functionality in minimal space and in large systems that need maximum modulation sources. That dual purpose explains a lot about how it's built.
The frequency of each LFO is set by the time between pulses - the "ping." Tap the tempo button and the internal clock takes over. Feed a signal into the Ping jack and the module switches to external clock automatically. The manual notes that as soon as an external signal is recognized on the Ping jack, the internal tap-tempo clock is turned off, and the Ping button displays the incoming signal instead. Tap a new tempo to re-enable the internal clock. You never have to flip a switch or dig into a menu to change clock source.
The speed range is wide. The slowest period is 71 minutes - about 0.0002 Hz - and the fastest is roughly 500 Hz, with marginal tracking up to 1 kHz. A single channel can crawl through a modulation cycle that lasts over an hour, or run fast enough to push into audio-rate territory.
Each of the four channels is fully independent, with its own tap tempo button, Ping jack, On/mute button, Skew knob and CV jack, Reset jack, and output jack. The tap tempo button is free-running - the LFO runs indefinitely when driven by the internal clock - but external clocking is auto-stopping, meaning the LFO stops when the external clock stops. With an external clock, the modulation freezes when your sequencer stops. With tap tempo, it keeps running whether you want it to or not.
The Skew control is where the module gets interesting. A single knob and CV jack reshape the output continuously from a 10ms trigger pulse at 0% skew, through a near-exponential "pluck," into a linear ramp-down, then a triangle at 50%, and finally a ramp-up at 100%. According to the manual, the Skew knob provides offset to the incoming CV signal, so any range of CV is acceptable. The full range of the Skew jack is 10V, and the module won't switch to the trigger or exponential curves unless Skew was near zero before the start of the curve. That keeps the shape from jumping unexpectedly mid-cycle. One LFO can serve as a clock source, an envelope-ish pluck generator, a triangle modulator, or a ramp.
The Reset jack restarts the LFO without changing the tempo, triggered on a rising edge. The Reset Lock Point is relative to the ping in terms of length of time, not proportion. If you give the QPLFO a reset pulse 500ms after the ping, the LFO will always start 500ms after the ping, even if you slow the ping time down. Speed the clock up past that 500ms window and the reset time becomes meaningless, defaulting to 0ms, in phase with the ping. The manual notes this is a quick way to clear the Reset Lock Point: speed up your clock, then slow it back down.
Outputs are jumper-selectable between unipolar (0V to +10V) and bipolar (-5V to +5V), with one jumper controlling all four channels. Factory default is unipolar. The LED on each channel flashes and dims to the signal on the output jack, and the tap tempo button flashes to the internal tap-tempo clock or to incoming pulses on the Ping jack.
The Ping and Reset inputs use comparators, so any waveshape can be used for triggering - audio waves, complex LFOs, even the QPLFO itself. A trim-pot on the PCB adjusts the comparator threshold value. At fully counterclockwise the threshold is 5.0V, at center it's 2.5V, and at fully clockwise it's 0V and the unit will not respond to Ping or Reset. Factory default is about 2V. The manual notes the QPLFO can be interfaced with LZX-level compatible equipment by setting this trimpot to about 0.8V.
A handful of advanced features are hidden behind jumpers and button combinations. "Fire-on-unmute" is a per-channel jumper that, when installed, causes the On button to fire a quick pulse into the Reset jack when switching the channel on. It's useful for one-shot behavior or manual re-syncing. Factory default is no jumpers installed. One-shot mode lets the Reset jack trigger a single envelope when the external ping clock is stopped. With tap tempo, holding the Tap button for two seconds stops the internal clock, and the manual recommends a "Tap-Tap-Hold" sequence to avoid double-triggering. Floating Reset mode is entered by holding a gate high on the Reset jack - or simply plugging a bare cable into it - which tells the QPLFO to stop re-syncing the waveshape on each ping. The LFO still follows the ping's timing, but it drifts freely rather than snapping back into phase. The manual treats this as a subtle but important feature, useful when you want the modulation to wander rather than lock.
A set of headers on the back lets the QPLFO connect to other modules by ribbon cable. Outputs can connect to the 4ms VCA Matrix, the 4ms SISM, or the Toppobrillo Mixiplexer, and the normalization of the ping jacks can connect to the 4ms Quad Clock Distributor. The manual also documents a self-contained quadrature LFO patch that uses the QPLFO alone. Set the threshold trimpot to 5V with unipolar outputs, then patch channel 1's output to channel 2's Ping input, channel 2 to channel 3, and channel 3 to channel 4. Tap a tempo on channel 1, and the four outputs run 90 degrees out of phase with each other.
The mute buttons are latching, the tap tempo is immediate, and the Skew knob gives you continuous control over shape without menu diving. The manual suggests a patch of four VCOs through a VCA Matrix, with the QPLFO driving the FM inputs, which the documentation says produces an endless variety of complexly changing sounds. The four channels can cross-trigger, reset each other, and drift in and out of phase.
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