ro-mono-303

Emulation of the Roland TB-303 Bass Line (1981), a monophonic bass synthesizer with a built-in step sequencer.

It runs the TB-303's original sequencer firmware on an emulated CPU, and models its voice circuit part by part from the service notes.

All clips: saw unless marked, mono, AAC 192 kb/s.

How close it is

• Accented low notes

  On both real units, accented notes low in CUT OFF lose a band around 260–330 Hz that ro-mono-303 keeps, up to 16 dB on one harmonic. Plain notes match. Nothing in the circuit as transcribed accounts for it.

  Wanted: single accented low notes at CUT OFF 0, RESONANCE 0, ENV MOD 0 and several ACCENT settings.
• The accent's sweep

  It is fitted to one unit at one setting: acidvoice.com's, with ACCENT and RESONANCE full. Its accents now peak within −70 to +30 cents of the real ones (they were 1.3–2.3 semitones short), and the climb gets halfway 2–3 ms sooner, within one period of the measurement.

  Wanted: accented runs at several ACCENT settings.
• Loud accents

  Both real units flatten their loudest accents, and the BA662 VCA chip as reverse engineered doesn't. Its output is limited to a fifth of its swing to match; why the real chip does this isn't known. There is no manufacturer data for the BA662.

  Wanted: Rohm's BA662 data sheet, or measurements of an original chip's gain against its control current.
• Against Din Sync's unit

  Brightness in the first 0.1 s of a note is about 25 % off; two takes of the unit differ by about 2 %. With the other knobs at 25 %, the resonant peak is about 250 cents high.

  Wanted: the recording sheet's single notes, from another unit.
• One calibrated unit

  acidvoice.com's unit reaches 2.05 kHz at full CUT OFF, Din Sync's 1.35 kHz, and which is more typical isn't known. CUT OFF MAX in the plugin bar covers the spread.

  Wanted: full CUT OFF and RESONANCE with no ENV MOD, from as many units as possible.
• Pitch scale

  acidvoice.com's unit's octave is wide: its low A♯ is about 25 cents flat against its high one, its WIDTH trimmer (TM5) out of trim. ro-mono-303 leaves the factory with exact octaves, and SERVICE can trim it to match a unit. How far real units usually drift isn't known.

  Wanted: single notes across all octaves.

1. 01

   Schematic and SPICE

   Each block of the voice was modelled from the schematic in the service notes, then simulated in ngspice from the same schematic and compared with the model, with typical part values and at both ends of their tolerance.

   Block	ro-mono-303	SPICE
   Slide, time constant	22.00 ms	22.04 ms
   Filter envelope, DECAY centre	530 ms	530 ms
   Accent boost, half / full ACCENT	+4.1 / +8.9 dB	+4.1 / +9.0 dB
   Note start after the gate	6.7 ms	7.0 ms
   Whole voice, note level	within 0.1–0.3 dB

2. 02

   Service notes

   All 16 adjustments and specifications in the service notes pass, among them the pitch scale (two octaves at 4.0005 : 1, specified as 4 : 1 ± 0.5 %), the filter's ring period, the envelope times, TEMPO from 40 to 300 BPM and the gate length.

3. 03

   Calibration to a real unit

   The schematic doesn't fix trimmer settings, transistor gain or potentiometer taper. These were fitted to Din Sync's TB-303 reference recordings, 400 notes recorded in 2010 from a real TB-303 at documented knob positions, with each value kept within its part's tolerance.

   Part	Schematic value	Fitted
   Filter trimmer TM3, ring period	2.0 ms	2.5 ms
   Resonance-loop transistor gain	300	385
   Filter envelope peak	10 V	9.3 V
   DECAY potentiometer	1 MΩ	860 kΩ
   Potentiometer taper	exponential	two linear segments
   VCA control offset	0 mV	−80 mV
   VCA output swing	±4.5 V	±1 V
   Accent-sweep gang of the RESONANCE pot	50 kΩ	37.5 kΩ
   Distance from the real unit	9.76	4.89

   The largest correction is the taper: with the schematic's exponential taper, the model is about an octave too bright with the controls at maximum. The last three values were fitted to how notes start and how accents peak and sweep, on Din Sync's unit and acidvoice.com's; they barely move the distance.

4. 04

   Adjusting to a unit

   Units differ. Two controls in the plugin bar correspond to differences inside the hardware:

   CUT OFF MAX:

   how high full CUT OFF reaches, 1.2 to 3 kHz. The default, 1.35 kHz, is Din Sync's unit. acidvoice.com's unit is at 2.05 kHz, the nominal parts at 2.2 kHz and a typical unit at 2.5 kHz (Robin Whittle). Roland's software reaches 1.8 kHz.

   SERVICE:

   takes the panel off. The main board and the switch board are laid out as the service notes draw them, and the six trimmers turn as on the board: TM1 TEMPO, TM2 INT CLOCK, TM3 FREQ (the filter; the fitted unit's 2.5 ms ring), TM4 TUNE, TM5 WIDTH and TM6 CV. A scope, a frequency counter and a meter sit on the service notes' check points, beside the notes' adjustment steps. FACTORY TRIM puts every trimmer back.

   

Roland's TB-303 software

Measured the same way as ro-mono-303, against the same 400 real notes: the same MIDI, the same knob positions, and its "Simple Saw" patch with no drive or delay.

Also measured in Roland's software

• Accent level: at half ACCENT, accented notes are about 11.5 dB louder. The schematic gives 4.1 dB, or 3.7–4.7 dB across part tolerances.
• Release: about 20 ms to fall silent after the gate closes. The schematic gives 14 ms, and a measured unit about 16 ms.
• Pitch: the pitch scale is about 0.2 % wide (C1 7 cents flat, C5 8 cents sharp), and TUNING covers ±1 octave where the service notes give about ±700 cents.
• Aliasing: on a 523 Hz saw, inharmonic content above 5 kHz is at −83 dB, against below −110 dB in ro-mono-303.

The ROM, d650c-133.ic8 as in MAME's tb303 set, is not included, because the program is Roland's copyrighted work. Put it in the ROM folder, where it is identified by SHA1, or add it with "Add CPU ROM…" in the plugin bar. Patterns are saved with the session in either mode.

Roland's logo and model names are trademarks, so they aren't included either, and their places on the panel are blank. The plugin installs an empty artwork file, tb303.svg, in the ROM folder for users' own artwork. About panel artwork