DCOAt the beginning of the signal chain is the DCO. There are two identical DCOs, so we'll just look at one I've dubbed "OSCA".
At the bottom left is the sync pulse generator. When a pulse or square wave is fed into the "OSCA_SYNC" net a short pulse is output at pin 4 on U20B. The width of this pulse is determined by the RC network formed by R29 and C9. Together they cause a small delay between pins 5 and 6 changing logical state. While these inputs are unequal the output will be high.
This sync pulse enables transistor Q2 to sink the charge stored in C10/2 thereby "resetting" it to VSS. The capacitor is then able to charge back up via the exponential current source. This cycle results in a sawtooth wave that is present on pin 5 of U18B.
U18B serves as a buffer for the sawtooth, but can also be switched to generate a squarewave. Nets OSCA_WF_SW and OSCA can be connected or disconnected via multiplexer U12. If connected the opamp is configured as a noninverting amplifier outputting the sawtooth. If unconnected the opamp is configured as a comparator, comparing the pulse width CV, OSCA_PWM_CV, on pin 6 and the sawtooth on pin 5. This results in a PWM squarewave as pictured below.
Next in the chain is the mixer. The mixer is made of two simple VCAs, one for each oscillator. Each VCA works off of a single linear CV. The mixers serves only to change the relative volume of the oscillators, not to apply the volume envelope.
After the mixer is the filter. It's closely related to the OSCar filter that is, in turn, based off of the Wasp filter. It should be noted that the inverting stage present in the OSCar has been removed from the Bass Station. This stage allowed the filter to be switched into a high-pass mode. Without it, the Bass Station is stuck in lowpass mode.
|Bass Station VCF|
I find it interesting that the resonance is under CV control, but cannot be modulated. It does respond to MIDI though.
Last in the chain is the main VCA. It's another simple, one-OTA VCA. Unlike the mixer VCAs it includes an exponential current source and responds to modulation from one of the envelopes. It also has a trimmer to adjust the offset voltage.
The final thing of note is an unlabeled header. It breaks out most of the internal CV and audio outputs. Almost all of them tap directly after a buffer and, with the addition of a 1K resistor, are safe to connect to other devices.
Unfortunately none of them are really suited to take a signal as input. Nearly every signal would have to be disconnected from its destination and fed through a summing-amp instead. With this summing-amp an external CV signal could then be mixed in with the existing CV.
Here is the majority of the analog section of the Bass Station. I've omitted the dull CV multiplexer and buffers.
|Bass Station Schematic|