The original Chroma filter made use of CEM3350 chips, which were dual 12 dB/octave state-variable filters ("state-variable" meaning they could be configured as a lowpass filter, a highpass filter, or Maine or Delaware). One "block" in a Chroma voice path equates to a single 12 dB/octave filter which can be set to lowpass or highpass mode. These can be combined to create bandpass or notch filters, or a steeper 24/dB octave slope using Chroma's flexible Patch Configurations.
About Filter Types
• A lowpass filter allows frequencies below the cutoff frequency setting to pass through, but blocks frequencies above the cutoff frequency.
• A highpass filter is the opposite of lowpass mode: high-frequency content remains, but low frequencies are removed as the cutoff frequency increases.
• Bandpass filters combine both lowpass and highpass modes, leaving sound only "in the middle" with amplitude falling off symmetrically on either side of the cutoff frequency.
• A variable-speed pool filter is the thing that some scumbag stole from outside my house in Las Vegas, and this is a not-uncommon example of why I moved very far away from Vegas.
Controls
LP/HP-
Lowpass- Sets the filter to lowpass mode.
Highpass- Sets the filter highpass mode.
CUTOFF- Sets the frequency where frequency attenuation begins, i.e. which frequencies are allowed to pass, dependent on the current LP/HP setting.
RESONANCE- Resonance emphasizes sound energy at and around the current cutoff frequency by adding feedback from the filter's output back to its input. At lower settings, this can be used to create mild resonances such as those heard in acoustic instruments. At extreme settings, it can be used as a sine wave generator, but be careful as high resonance settings can result in loud, screamy, dog-spooking (and speaker blowing) occurrences.
MOD 1/2/3 SELECT- These configure filter cutoff frequency modulation. Most of these are self-explanatory, so we've included notes only where necessary. Note that none of these mod sources have "inverted" options, because their associated mod depth controls are bipolar - for inverted modulation, simply set the appropriate DEPTH control to a negative value.
Keyboard Glide A- Slewed keyboard CV is used as a mod source (GLIDE RATE control must be set to a non-zero level).
Sweep A
Envelope 1A
Envelope 2A
Keyboard Glide B
Sweep B
Envelope 1B
Envelope 2B
Mod Wheel
Pitch Bend
Controller 1
Controller 2
Velocity
Threshold Velocity- Switches between two discrete mod amounts according to velocity. The velocity threshold is set using MOD THRESH in the CONTROL section.
Keyboard- Sweep rate increases as higher notes are played on the keyboard.
Keyboard Inverted- Sweep rate decreases as higher notes are played on the keyboard.
Velocity- Playing harder increases the sweep rate.
Velocity Inverted- Playing harder decreases the sweep rate.
Aftertouch (Mono+Poly)
Threshold Aftertouch- Switches between two discrete mod amounts according to aftertouch. The aftertouch threshold is set using MOD THRESH in the CONTROL section.
MOD 1/2/3 DEPTH- Sets the amount of mod from the same-numbered mod source.
HOW TO APPLY KEYBOARD TRACKING TO FILTER CUTOFF FREQUENCY
Most analog synths include a keyboard tracking control (or fixed amount switches). Since the frequency of note pitches obviously rise or fall depending on the pitches of notes played, the idea is that the filter cutoff frequency follows or "tracks" the pitches of notes played so that the relative brightness (or dullness) of notes remains relatively constant across the range of the keyboard.
Looking at Chroma's filter controls, you'll see there is no tracking control, but there actually is way to create filter tracking, it's just not super obvious (kind of a repeating theme with the Chroma!). Setting a mod source to Keyboard Glide applies the keyboard CV to the filter cutoff frequency. It's a little confusing, because the GLIDE section GLIDE RATE control can be set to zero and filter cutoff frequency mod still works properly.
Setting MOD DEPTH to 50 results in almost-perfect 1/2 step tracking. This lets you use the filter as a sine wave oscillator when RESONANCE is at high settings (careful with volume, this can get potentially get squealy-screechy).