A lowpass with a continuous slope and a flat level: the Kawai K5's DDF #127

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opened 2026-10-05 23:46:13 +00:00 by bcox · 0 comments

bcox commented

2026-10-05 23:46:13 +00:00

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bcox asked on 2026-10-05 for a ticket for each engine gap in his moregaps.md feature table that no existing ticket covers. Engine claims are from reading the tree at f6aa8eba on main. Instrument claims are from that day's research, read as page images of the K5's owner's manual by a research agent and not re-checked; the sources are listed in that document. Nothing here is built, measured or listened to.

Summary

The K5's DDF, its one filter, is a lowpass with a cutoff, a slope and a "flat level", no resonance, under a six-segment envelope. Slope is continuous rather than a pole count, and flat level is the level the response settles to above the cutoff rather than falling forever: a lowpass that becomes a shelf as the flat level rises.

Here a slot's slope is a pole count, 1 or 2 on the SVF and 1 to 4 on the ladder (dsp/va_filter.h:396-397), and nothing above the cutoff survives but what a source puts on the post-filter bus.

What is reachable, as read

Flat level: a source on the post-filter bus goes around the row at the same amplitude (dsp/va_oscillator.h:95-99), so a lowpassed copy plus a dry copy at a level is a lowpass with a floor. It costs a second oscillator or a mix wave on the other bus, and the floor's level is a lane.
Slope: not continuous. The ladder's pole mixing reaches 6, 12, 18 and 24 dB per octave as integers.

The work, as a sketch

A floor lane on the filter slot: the dry input mixed back after the row, in the slot rather than through the buses, so one source gets the shelf without spending a second.
A continuous slope as a crossfade between adjacent pole taps of the ladder (LadderTaps, dsp/ladder.h:304-321, already sums taps to make the pole counts) -- a slope lane from 1 to 4 poles rather than an integer. The SVF's one-to-two-pole case is the same crossfade with two taps.

Open

Whether the K5's slope is a pole-count sweep or something else; the manual gives a number range and no curve.
Whether a floor belongs on every slot or on the EQ layer kind proposed in #122.

bcox asked on 2026-10-05 for a ticket for each engine gap in his `moregaps.md` feature table that no existing ticket covers. Engine claims are from reading the tree at `f6aa8eba` on `main`. Instrument claims are from that day's research, read as page images of the K5's owner's manual by a research agent and not re-checked; the sources are listed in that document. **Nothing here is built, measured or listened to.** ## Summary The K5's DDF, its one filter, is a lowpass with a cutoff, a slope and a "flat level", no resonance, under a six-segment envelope. Slope is continuous rather than a pole count, and flat level is the level the response settles to above the cutoff rather than falling forever: a lowpass that becomes a shelf as the flat level rises. Here a slot's slope is a pole count, 1 or 2 on the SVF and 1 to 4 on the ladder (`dsp/va_filter.h:396-397`), and nothing above the cutoff survives but what a source puts on the post-filter bus. ## What is reachable, as read - Flat level: a source on the post-filter bus goes around the row at the same amplitude (`dsp/va_oscillator.h:95-99`), so a lowpassed copy plus a dry copy at a level is a lowpass with a floor. It costs a second oscillator or a mix wave on the other bus, and the floor's level is a lane. - Slope: not continuous. The ladder's pole mixing reaches 6, 12, 18 and 24 dB per octave as integers. ## The work, as a sketch - A `floor` lane on the filter slot: the dry input mixed back after the row, in the slot rather than through the buses, so one source gets the shelf without spending a second. - A continuous slope as a crossfade between adjacent pole taps of the ladder (`LadderTaps`, `dsp/ladder.h:304-321`, already sums taps to make the pole counts) -- a slope lane from 1 to 4 poles rather than an integer. The SVF's one-to-two-pole case is the same crossfade with two taps. ## Open - Whether the K5's slope is a pole-count sweep or something else; the manual gives a number range and no curve. - Whether a floor belongs on every slot or on the EQ layer kind proposed in #122.