Davidian Twinline

A Davidian LV2 model of a bucket-brigade doubler, based on the circuit of a classic 1970s double-tracking pedal. The model has a BS170 boost stage, then the dry signal and a V3205 BBD delay are mixed by a Blend pot. A switch picks a short doubling delay or a longer slapback. Mono in and out, no latency on the dry path. Made with AI assistance; see Use of AI.

Build and test on Windows

build.bat
build-test\test_host.exe build\twinline.lv2\twinline.dll
build-test\lilv_check.exe build
build-test\ui_check.exe build\twinline.lv2

build.bat needs Visual Studio's C++ tools. Edit VCVARS at the top if your install is somewhere else.

• test_host.exe loads the plugin DLL directly and checks delay times, frequency response, BBD clock images, Blend and Level, block-size independence, overload and CPU use.
• lilv_check.exe checks the bundle with the lilv LV2 parser (the one most hosts use), without installing the plugin. It loads lilv and the LV2 spec files from Ardour 9, so it needs Ardour 9 installed in C:\Program Files\Ardour9. For another location, edit the two Ardour paths in tests/lilv_check.c.
• ui_check.exe hosts the GUI in a window and drives it with mouse messages.
• ui_render.exe out.bmp renders the panel to an image without opening a window.

Install

Twinline is Windows-only (64-bit) for now: the bundle's metadata names .dll files and a Win32 GUI, and there is no Linux or macOS build.

Copy the whole build\twinline.lv2 folder (twinline.dll, twinline_ui.dll and the two .ttl files) into one of the standard Windows LV2 folders: %APPDATA%\LV2 (current user) or %COMMONPROGRAMFILES%\LV2 (all users).

If your host doesn't scan that folder, add it to the host's LV2 search path or set the LV2_PATH environment variable. Close the host before copying and restart it afterwards. Replace any older copy instead of keeping two: when two bundles have the same URI, the loader uses one and ignores the other.

Look for Davidian Twinline in your host's plugin list. The plugin URI is urn:davidian:twinline.

GUI

The bundle includes a native GUI (twinline_ui.dll). Its look comes from a shared house style: src/ui/style.h (palette and text sizes), font.h (stroke font) and canvas.h (renderer). It draws with its own software renderer and puts the result in a Win32 window (ui:WindowsUI). The window layer also has X11 code, but only the Windows build has been compiled and tested.

• Drag a knob up or down to turn it. Hold Shift for fine control. Double-click resets it, and the mouse wheel works too.
• Click the upper or lower half of the Delay switch to pick Double or Slap.
• Click the footswitch to engage or bypass the pedal. Its red LED shows the state.
• The readout shows the name and value of the last control you touched.

The panel name, Twinline, is original. The panel uses none of the original unit's logo, wordmark or artwork.

Controls

Control	Range	Default	Circuit part
Level	0 to +20 dB	0 dB	C5K pot in the BS170 boost stage
Blend	0–1 (dry to wet)	0.75	A10K pot. Audio taper, about 10% wet at noon
Delay	Double / Slap	Double	SPST delay switch on the CD4047 clock
Footswitch	on/off	on	The pedal's bypass switch. Off passes the input through untouched
Enabled	on/off	on	Host bypass (lv2:enabled), so the host's bypass button works

Both bypasses fade over about 10 ms. The circuit keeps running while bypassed, so switching back in doesn't click. Flipping the Delay switch while a note is sounding bends its pitch, as changing the clock does on the real pedal.

How the model works

• Boost: the input is scaled by Level, then soft-limited with a slight asymmetry for the MOSFET stage. A 5 Hz high-pass removes the resulting DC.
• Wet path: a 100 Hz high-pass, then 4× oversampling (2× at 88.2/96 kHz, none at 176.4 kHz and up) for the BBD section.
• Pre-filter: a 7 kHz two-pole low-pass.
• BBD: a 2048-cell buffer clocked at the CD4047 rate, not the host's sample rate. Samples are taken and held at the clock edges, so the delay time, the aliasing and the clock images all follow the real clock. A soft limit before the BBD stands in for its limited voltage swing.
• Post-filter: a Sallen-Key low-pass (7 kHz, Q 0.73) and a 4.8 kHz one-pole, then a Butterworth filter back down to the host rate.
• Mix: dry and wet are mixed by the Blend taper, then crossfaded with the clean input for bypass.

Values read from the layout

Item	Value	From
Clock, Slap	22.7 kHz → 90.1 ms	CD4047, f = 1/(4.4 RC), R = 100K, C = 100p
Clock, Double	65.0 kHz → 31.5 ms	47K + 6K8 switched in parallel with the 100K
BBD length	4096 stages = 2048 clock periods	V3205
Post-filter	7.0 kHz, Q 0.73	Sallen-Key, 33K/33K with 1n/470p
Output RC	4.8 kHz	33K and 1n
Pre-filter	7.2 kHz	4K7 and 4n7 (taken as a matching 7 kHz stage)

Measured from the built plugin, the onset of a burst arrives after 32.8 ms (Double) and 91.4 ms (Slap). That includes the filters' group delay. The wet signal is about 0.2 dB down at 1 kHz, 5 dB down at 5 kHz and 11–12 dB down at 7 kHz. With a 5 kHz tone on Slap, the BBD clock image is about 36 dB below the tone. CPU use is about 0.5% of one core at 48 kHz.

Estimates

The layout doesn't show these, so they are guesses. Tune them by ear:

• Boost range and clipping: 0 to +20 dB, with the boost limiting around +8 dBFS. This assumes 0 dBFS = 1 V. Constants are at the top of src/twinline.c.
• BBD headroom: a soft limit at about 1.2 V.
• Wet high-pass: 100 Hz.
• Blend direction: clockwise is assumed to be more wet.
• The clock reading itself: the timing parts were traced from a small image, so the two delay times could be off if the switch wiring differs from how it was traced.

Simplifications

• No BBD hiss or bias trimmer (TR1). The V3205 here has no compander, so the real pedal is noisier than the model.
• The op amp filters are ideal, apart from the soft limits above.
• The LED indicator isn't modelled. The GUI's footswitch LED shows the bypass state instead.

Code layout

File	Contents
src/twinline.c	DSP and LV2 entry points
src/ports.h	Port numbers shared by the plugin and the GUI
src/lv2_abi.h	The LV2 C interface, so no SDK is needed
src/ui/panel.h	GUI layout, drawing and mouse handling, with no platform code
src/ui/style.h, canvas.h, font.h	House style, software renderer and stroke font
src/ui/ui_lv2.c	LV2 UI entry points and the Win32 and X11 window layers
bundle/	LV2 metadata
tests/test_host.c	DSP checks on the built DLL
tests/lilv_check.c	Checks the bundle with the lilv LV2 parser
tests/ui_check.c	Hosts the built GUI and drives it with mouse messages
tests/ui_render.c	Renders the panel to a .bmp file

Use of AI

This project was made with AI assistance. The code, tests, LV2 metadata and this README were written by Claude (Anthropic's Claude Opus 5.5, through Claude Code), directed and reviewed by the author.

• The author supplied the circuit layout, the GUI house style, the names and the licensing choices. They also decided what to build and changed parts of the result themselves.
• The AI read component values off the layout image and worked out the clock rates and filter corners. It also wrote the DSP model, adapted the GUI to this panel, and wrote the test programs and documentation.
• Checking: the results are checked by the automated tests described above (delay times, frequency response, Ardour's LV2 parser, and the GUI driven by simulated mouse input). The values listed under Estimates are the AI's guesses and haven't been checked against a real pedal.

License

Copyright (C) 2026 Davidian

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. It is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See LICENSE for the full text.

Third-party licences

Component	Licence	How it's used
LV2 C interface (lv2/core/lv2.h, lv2/ui/ui.h)	ISC	Its declarations are re-stated in src/lv2_abi.h, the UI ABI block of src/ui/ui_lv2.c and three test programs, so no LV2 SDK is needed. Copyright and permission notice in LICENSE-LV2.
LV2 vocabularies (lv2core, ui, units)	ISC	The .ttl files refer to them by URI. Nothing is copied.
lilv	ISC	tests/lilv_check.c loads the copy that ships with Ardour 9 at run time. Not included in this repository.
Ardour 9	GPL-2.0-or-later	Used only as a test host and as the source of lilv for lilv_check. No Ardour code is included.
Microsoft C runtime	Visual Studio licence	build.bat links it statically (/MT) into the DLLs you build. It counts as a system library under the GPL, so this is allowed. No binaries are committed to this repository.

Every source file starts with an SPDX licence line saying which of these applies.