Pro-one rack external IO diagram

Planned external inputs and outputs. Top ones are normal analog IO, bottom ones are midi and special keyboard and mod/pitch wheel IO.

Internal signal path diagram for Pro-one rack

Internal signals from card to card in the planned Pro-one rack. As the sheet indicates, the plan is now to build five cards, with the functionality spread as indicated. External signals and signals going to the front panel are not shown, except where indicated by "via switches".

Pro-one rack front panel sketch

Not much to say - just made an initial sketch of the pro one "rack" front panel. The panel is 5U high and 430mm wide, which leaves room for rack ears to be added later - meaning that the machine can be used both as a desktop version and as a rackmounted one.

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.

Trigger

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

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.

Repeat/Ext

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.

Pro one mix and random board finished

Just finished the second board for the Pro-one project. This one contains the oscillator mixer, a modified noise circuit, a sample and hold part and the output buffer. There are so many ins and outs on the upper part of this card that I couldn't find room for the top right screw hole... Oh well.

I think it will take at least two more boards, perhaps three, to get all the necessary circuitry in place. And that excludes the front panel that will hold the pots and switches, and any output boards.