The Model 28 Programmable Clock is the second computer-based product in the Plan B line and addresses the need for an accurate tap input-based timing clock for analog synthesizers. Unlike other clocks, the Model 28 has no frequency pot -- it's tempo is determined by replicating an interval set by two successive depressions of the TAP INPUT button, or by syncing to it's PHASE LOCK input to an external clock. The M28 may be accurately programmed during a roll-off or three count in live performance - it is meant to be played and it's operation has been intelligently designed to support that in real time.

Along with the quarter note base duration, the M28 provides separate eighth note tuplet and eighth note triplet outputs and approaches these rhythms in the same manner musicians do -- by multipling the tap tempo, not by dividing. The last tap interval read becomes its quarter note base, with the eighth-notes being base x2 and the triplets base x 3. All outputs are held at 50% duty cycle and tempo-accurate within 6 microseconds, with a playback range of a quarter note = 1 (1 beat per minute) to 2500 (2500 beats per minute).

---

Figure 1 - Model 28 TIming Diagram (click for larger view)

Full Programmability

Along with the three parallel event outputs, the M28 is equipped with a single SERIAL OUTPUT which allows the user to construct polyrythmic combinations of the three note values, determined by a voltage level present at the SELECT input.

The Model 28's musicality is further enhanced by a series of five ARTICULATION SWITCHES which introduce expression by either omitting (a rest) or dynamically accenting each degree of the eighth and triplet events (see figure 3). When an accent is selected, a second pulse is sent to independent ACCENT outputs which can be routed as a filter frequency chiff, or to the Velocity Input of a Model 25 VCA, or to the secondary VC Input of a Doepfer A-132, etc. The Accent Outputs are intentionally soft (gates are rounded at the corners) to avoid any pops caused by abrupt changes in amplitude. Click here for a diagram on the usage of these accent outputs.

Understanding that music is sometimes about sound and at other times about the sound of silence, all output events on the Model 28 can be stopped and restarted either manually (RUN/HALT BUTTON) or externally via independent START and STOP inputs. The M28 performs these operations in a make-before-break fashion, completing the full duration of the last gate even if a stop command was received during it's cycle.

Summary of Operation

The user interface of the Model 28 Programmable Tap Clock (PTC) is broken into three regions:

INPUT CONTROLS. The top section addresses playback variables, the primary controls being the manual Tap Input button and Phase Lock input. A READ LED has been provided for visual indication of when a timing sample is being recorded. It lights as a record cycle begins (first depression of Tap In or first phase-lock clock) and remains lit until it receives another Tap or phase-lock clock, indicating it's now replicating that tempo.

The PTC needs only two taps to begin counting. If this is received from an external clock via the Phase-Lock Input, the M28 will respond in real time to accelerandos and retards generated by that signal. Once this signal is removed the M28 will output the last frequency received. An INPUT SELECT switch enables either manual (Tap Input) or external phase correlation input.

Counting may be temporarily halted by either manually hitting the START/STOP pushbutton or by sending a trigger to it's STOP INPUT. To resume counting, an operator may either make another manual depression of the Start/Stop pusbutton or send a trigger to it's START INPUT (click here to how to use a linear controller (such as a keyboard or ribbon controller) to start and stop the Model 28 externally).

PARALLEL OUTPUTS. The center section of the Model 28 contains the parallel clock outputs, the associated accent outputs and expression controls (Articulation Switches). This is where the individual quarter-note, eighth-note and triplets outputs are located. A set of three LEDs indicate the activity of each of the three events, including rests.

SERIAL OUTPUT. The bottom section of the M28 produces the Serial clock output functions, and this is where the fun really begins! This section behaves like a multiply x N generator, with the SELECT INPUT determining which of the three possible note durations will be selected - it generates a user-programmed stream of complex polyrithmic patterns, updated each quarter note, which are determined by the voltage level present at the SELECT INPUT as follows:

---

Figure 2 - Model 28 Slect Input Operation (click for larger view)

SELECT INPUT LEVEL

SERIAL OUTPUT STATE

---

0 to 2.5 volts

Quarter Notes

2.5 to 5 volts

Eight Notes

5 volts and above

Eight Note Triplets

The Select Input defaults to ground, so if no voltage is present the Serial Out returns a quarter-note value. To preserve musicality and assure that cadence changes remain in meter to the base tempo, Select Input voltage sampling occurs only on the downbeat of each quarter note.

A secondary ACCENT OUT on the bottom section returns the same dynamic accents dialed into the Articulation Switches, but decoded so they match the output pattern present at the Serial port. Simply put, if the Serial Out is returning triplets, the accent information coded into the three triplet Articulation Switches will be sent to the Accent Output.. If you switch to eight notes, the Accent out will follows that change by switching to the information coded into the eight=note tuplet switches

---

Audio Examples:

1) The following soundclips (series 1), the basic 'Peter patch' , were done in real time using a single M28, three Model 10's and three Model 13's and shows the multi-rythmical capabilities of the Programmable Tap Clock,.

1A) Highlighted here specifically is the Serial Output - which in this example is being 'selected' by pink noise. There are also some manual amplitude changes happening via the eight and triplet Articulation Switches. click here.

1B) Another variation of the patch above, slower pace and straight out of the Mars movement from Holst's Planets Suite click

1C) A little more involved - two M28's in real time. click

1D) Yet another variation, this one highlighting 1/8 note triplets. click

1E) An example of the Serial Accent output. Same patch, but with accents enabled at the second degree of the duplet and first degree of the triplet. In the first part of this sample, the Accent is further exaggerated as it's also used to pan to the left channel. You can clearly hear the accents decoded with the output rythym currently selected in th Serial out. click


2) Now for something different. In a replication of the basic principle behind the Blacet Binary Zone, the following is an example of gating the output levels of the Model 28 to produce variable control voltages. In this patch (which, outside of the Deopfer A-156 Quantizer was realized completely within the Plan B domain), three Model 15 VCOs which are tuned to octaves and run through a single Model 12 Allpass Filter, are frequency controlled by the outputs of the Model 28, each gated through VCAs. Some of these VCAs are level controlled by keyboards, some by the Stepped output of the Model 24 Heisenberg, which is also controlling the center frequency of the Model 12. The Model 27 Delay has been thrown in at the end for a bit of color. click

Because eahc of the three outputs of the M28 are summed together to create a single CV, the voltage changes

---

Specifications:

ABSOLUTE MINMUMS

---

MIN tempo (mm)

Qtr = 1 beat per minute

MIN Freq (Qtr Notes)

.016 Hz

MIN Freq (Eight Notes)

.033 Hz

MIN Freq (Eight Triplet)

.048 Hz

---

---

Figure 2 - Output patterns -- triplet used for this example
(click diagram for larger view)

---

Model 28 Demo Video by James Cigler: