A few words about Serge sequencers. Note that Serge/Sound Transform does not make a traditional keyboard like you see on most other synthesizers, such as Moog, Roland, etc. You can get a traditional vintage control voltage keyboard (Roland, Moog, Polyfusion, etc.) and interface it to the Serge, since the Serge adheres to the 1 volt/octave standard. Or you can generate a control voltage from your MIDI keyboard and an interface.
Sound Transform follows the Buchla model of offering a touch sensitive keyboard/sequencer unit, as well as more traditional simple sequencers. Originally, they offered a sequencer whose stage was selected by push buttons. This is what's called a sequencer-programmer: you could use it in an automatic mode as a traditional sequencer, or use it manually to select/program voltages you set. So you can mimic a traditional keyboard if you want by tuning each stage to various notes. Or discard the traditional keyboard concept and simply use it to fine-tune different parameters in your patch for later recall.
The TKB is the hub of the sequencers - each stage has a corresponding output that goes high when it's selected. This complements the other sequencers, which have an input that, when high, causes that stage to be selected. So the TKB can be used to control another sequencer, or whole sets of other sequencers, regardless of the number of stages they might have.
Sound Transform offers a set of Serge sequencing programmers, from four to eight stages, plus the Sequencer with eight stages, and the TKB with sixteen stages. Confused? What they call their regular Sequencer is not a sequencing programmer in that each stage is selectable by a button but not by a stage select input. The sequencing programmers have stage select inputs. And the TKB has stage select outputs.
The sequencers are a little 'bare' in that none of them have an internal clock. You must use another module as a clock source, and typical sources are the DSG, DTG, and the Smooth section of the SSG, patched to oscillate. But they do have cute features such as step trigger inputs for RESET, UP/DOWN, and HOLD.
Most of the Serge sequencers are optionally available with an internal connection to a quantizer. Why would you want that? The quantizer forces the range of the stage pots into stepwise note intervals. So as you turn the pot, it isn't a continuous rise or fall, but steps up and down a scale. This makes tuning much easier and more reliable. The variable outputs are still available on quantized sequencers also.
Touch Keyboard Sequencer (TKB)
This little miracle is a 16 stage sequencer with 4 layers, five simultaneous outputs, sixteen capacitive touch-sensitive keys. It is a sequencer and performance keyboard rolled into one. A four-stage vertical sequencer steps through each of the four layers, allowing sequences up to 64 steps in length. The Reset input brings the sequence back to stage 1. A switch and trigger input allow the sequence to go forward or reverse, a hold input stops the sequence. A random select input randomly selects from the sixteen stages. Sequences can range from 2 to 16 steps, or multiples of 4 through 64 (8,12,16,20..) steps. The capacitive keys have a pressure output that produce a voltage proportional to the key area your finger covers. A separate KV output provides an equal-tempered voltage (16 equal steps) which is independent of the stage setting pots. A KEYBOARD switch lets you select each 4-layer stage with the keys or disconnect the key selection of the stages.
Pro: Extremely versatile. You can explore this sequencer for years and not exhaust what it's got. Can be clocked at audio rates. Easily chained to other (Serge) sequencers. The Pressure output is very nice, especially in conjunction with attenuators to make the pressure touch effects more subtle.
Con: The vertical clock doesn't reset when the Reset input goes high. If it did, it would allow more sequencing patterns. Also when 'KEYBOARD' switch is turned off, touching keys will still affect the starting stage in a running sequence, but only when you've patched one of the stage trigger outputs to the reset jack(!). Also, it needs to be externally clocked (like all the Serge sequencers I know of) so you need to use another module or external source to do this.
Wizardry: An advantage to external clocking is, you can experiment with clocking with waveforms that are not perfect pulses. The voltage range of each stage is around zero to 3.5v. When the pots are set to zero, they truly are at zero. The maxima are all different, however. Unlike some sequencers I've played with, there is no interstage interaction - turning a pot on one stage does not affect another stage.
Tweaks: The TKB design now uses bright amber LED's, which apparently are better than red ones in low-light applications. One of the Reset jacks was replaced with a toggle switch that sets forward/reverse sequencing in addition to the forward/reverse jack. The keypad material was also updated so it's more durable.