tirsdag 8. mai 2012

Pro-one gate and seq/arp

The Pro-one has a nifty system for controlling the arpeggiator and sequencer. It all runs through the gate circuit, so here are a few details about the gate and trigger circuit, deducted from the schematics (as I have no real Pro-one to compare with).

Sequencer and arpeggiator

First of all, the sequencer/arp speed is controlled by the speed of the LFO, which is not uncommon on old synths. The triangle output of the LFO runs through HALF of switch S125 (Seq) or S127 (Arp) when these are in active modes. The other half of the switches are connected to the microcontroller, to read the sequencer or arpeggiator mode. From the switch, the triangle wave is connected to the Gate output, and thus controls the gate directly. So, in sequencer or arpeggiator mode, a new gate signal is sent for every cycle of the LFO. The gate signal from the LFO is NOT sent to the Gate out jack. I don't yet know the shape of the LFO-gate signal when it arrives at the gate. It starts out as a triangle wave, but runs through a transistor and a capacitor on the way.

The square wave of the LFO is connected to the microcontroller as the "clock". One can assume that this is used to control the arpeggiator up/down motion and sequencer advance. More on the clock a little later.

The trigger signal is normally controlled from the microcontroller, and is probably syncronized to the clock programatically when the synth is in sequencer or arpeggiator mode.

The clock signal may be overriden by both an external gate input and the -ext and Audio Gate inputs. how the sequencer/arpeggiator works in these cases is unknown to me at the moment.

Gate/Clock input, Ext and Audio gate

An external gate signal input is available. It is connected to the clock input of the microcontroller and not directly to the gate. As such, the fact that the gate signal comes from an external source is unknown to the microcontroller, which is the reason I don't know how the arpeggiator and sequencer act in this mode.

As the ext and audio gate signals are connected to the clock in, the same is true for these. However, if no external gate/clock is connected, one must probably turn down the LFO speed to 0, to prevent the LFO from interfering.


The trigger sets off the envelope generators that control the volume and filter frequency (The envelope generators then run through the Attach and Decay-parts of their cycle, and stays at Sustain level untill the gate signal goes away, which triggers the Release-mode). The trigger is controlled directly by the microcontroller in normal mode, but can be overridden by Mode drone or Mode repeat/ext.

Mode drone

The mode drone switch is connected between the microcontroller and the trigger and gate signals (the gate out signal is tapped before the switch and is not affected by mode drone. When the switch is in the drone-position, both trigger and gate are permanently connected to +5v, keeping them "open" at all times, which lets through any sound coming from the mixer.  When the Mode drone switch is in the off position, the gate and trigger signals are unaffected.


Glide is not directly a part of the gate circuit, but it is controlled by the gate. As with gate out, the control signal is tapped before the mode drone switch and thus not affected by mode drone. When the glide switch is in auto-mode, the gate is connected to the glide circuit, when it is in normal mode, the glide circuit is fed a constant signal of +5v through the switch.


The Mode repeat/ext switch is connected between the microcontroller and the trigger/gate. As it is connected BEFORE the gate out, it affects the gate out signal. When in normal mode, both trigger and gate are passed through directly from the microcontroller. When the switch is in repeat mode however, both trigger and gate are connected to the clock in. This means that they are controllable directly by the Audio gate, gate/clock in and LFO! In LFO mode, notes are retriggered on every LFO cycle, regardless of wether or not a key is currently pressed. It also means that the gate and trigger cannot be controlled by the microcontroller when in repeat mode, which in turn means that the keyboard keys only affect the pitch, not the triggering of notes.

Gate out

The gate out jack is tapped before the mode drone switch, but after the repeat/ext switch. It is thus not affected by Mode drone, but will be affected by any external clock input when the repeat switch is in repeat mode. The gate signal from the Seq/Arp switch (LFO triangle wave) cannot reach the gate out directly, so in seq/arp mode the gate out is connected to the microcontroller. I do not know if the microcontroller outputs any gate signal when in seq/arp mode.

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