How to Program an Acid Bassline: 303 Accents and Slides

What makes a bassline sound acid?

An acid bassline is a looping sixteenth-note pattern played through a squelchy, resonant low-pass filter, where accented notes bark louder and brighter and slid notes glide in pitch. The sound comes from the Roland TB-303 and its quirks, not from any particular melody.

Roland released the TB-303 in 1981 as a bass machine meant to replace a bass guitar, and it could only produce a sawtooth or a square wave from one oscillator^[1]. It sold poorly, then Phuture's "Acid Tracks" in 1987 turned its twisting filter into the foundation of acid house^[1]. The genre's sound still comes from the same recipe: high resonance, low cutoff, and programmed accent, slide and octave changes that make a simple pattern move^[2].

This page is about programming the pattern and understanding accent and slide so they work in any DAW or emulation. For a full track built around the line, see how to make acid house.

Which 303 controls shape the acid sound?

Five knobs do almost all the work: cutoff, resonance, envelope mod, decay and accent. Roland's own TB-03 reissue puts exactly those five up front as its hands-on sound controls^[3], and nearly every emulation copies the layout.

On the original, the filter envelope decay ran from about 200 ms to 2 seconds on normal notes, while accented notes were locked to 200 ms^[4]. That fixed short decay is why accents sound punchy no matter where the Decay knob sits.

What does accent do on a 303?

Accent makes a step louder, shortens its filter envelope and pushes the filter higher, so the note snaps out of the line. It is a per-step on/off switch, and the Accent knob sets how strong the effect is.

The part most emulations try hardest to copy is the accent sweep circuit. Its capacitor does not fully discharge between steps, so when accents follow each other the filter peaks climb higher on every note^[5]. With resonance turned up, that circuit produces the distinctive upward "wow" at the start of accented notes^[4].

How many accents per bar?

Start with three to five accents in 16 steps. Put one on or near the downbeat so the loop has an anchor, and place the rest on off-beat sixteenths. Two or three accents in a row give you the climbing filter effect; accenting every step flattens the line because nothing stands out anymore.

How does slide work on a 303?

Slide holds a note into the next one without retriggering the envelopes and glides the pitch between them. On a stock TB-303 the glide takes about 60 ms, a fixed time rather than a knob^[4].

The timing is unusual. A slide programmed on one step keeps its gate high into the following note, and the pitch glide itself happens during that next note^[6]. Because the envelope does not restart, a slid note has no new filter pluck. Slides therefore feel smooth and vocal, while accents feel percussive.

Recreating slide in a regular synth

Set the synth to mono with legato on, turn glide or portamento on in legato-only mode and set a short constant glide time around 50-80 ms. Now only overlapping notes glide, which is exactly how 303 emulations read slides from MIDI. The portamento and glide explainer covers constant-time versus constant-rate glide in more detail.

How to program an acid bassline in any DAW

Draw a one-bar loop of sixteenth notes, then encode accents as high velocity and slides as overlapping notes. Most 303 emulations follow that convention. FL Studio's Transistor Bass treats velocity of 100 or more as an accent and accepts slide notes or overlapping notes for slides^[7], and the free JC303 sequencer records velocity 100 and above as accent and a second note struck while one is held as a slide^[8].

1. Set tempo and grid
   Pick 120-130 BPM for acid house or 130-140 BPM for acid techno. Set the piano roll snap to 1/16 and make a one-bar loop.
2. Choose a small note set
   Use the root in two octaves plus two or three other notes, for example the minor third, fifth and flat seventh. Acid lines rarely need more.
3. Fill about 10-12 of the 16 steps
   Leave four to six rests. Gaps give the filter envelope room to breathe and make the groove.
4. Add octave jumps
   Move two or three steps up an octave. Octave leaps with resonance up are the fastest route to the classic squelch.
5. Mark accents with velocity
   Set normal steps to around 80 and accented steps to 110-127. Aim for three to five accents per bar.
6. Create slides by overlapping
   Extend two or three notes so they overlap the next note slightly. Slide into octave jumps for the elastic, rubber-band sound.
7. Loop and perform the filter
   Record cutoff, resonance and env mod automation over 16-32 bars instead of drawing it. Hand-played knob moves are the acid sound.

A 16-step starter pattern in C minor

Read each group of four as one beat. A means accent, S means slide into the next step, and a dash is a rest.

Beat 1: C2 A, C2, -, C3 S
Beat 2: D#2, -, C2, C2 A
Beat 3: G2 S, A#2, -, C2
Beat 4: C3 A, C2, F2 S, G2

Once it loops, change one thing every eight bars: move an accent, shift a slide, or swap a rest for a note. Small edits keep the line hypnotic without making it static.

Which free 303 emulations can you use?

JC303 is the strongest free option right now, and Open303 is the older open-source engine it is built on. Any subtractive synth with a resonant filter, mono legato and glide can also get close.

If you want an official or commercial model instead, compare Roland Cloud's TB-303 with D16 Phoscyon 2. FL Studio users should know that Transistor Bass is a trial in FL Studio unless purchased separately or included through the All Plugins Edition^[7].

How should you process an acid bassline?

Distortion and a tempo-synced delay are the two classic companions. Roland even built overdrive and delay into the TB-03, effects the original never had^[3].

• Distortion Put a pedal-style overdrive or waveshaper right after the synth. Distortion flattens the level jumps between accented and normal steps, so ride resonance lower when the drive is high. Any plugin from the free saturation roundup works.
• Delay A 3/16 or 1/8 dotted delay at 15-25% wet makes the line sound busier than it is. High-pass the delay returns around 300 Hz so the repeats do not smear the low end.
• EQ High-pass the 303 around 40-60 Hz if a separate sub or the kick owns the lowest octave, and tame the resonance peak with a dynamic EQ band if it spikes.
• Compression A gentle compressor after distortion evens out peaks when resonance and accent stack up. Sidechain it lightly to the kick if the two fight.
• Reverb Keep it short or on a send. A long wash over a fast acid line turns the pattern to mush.

Automation matters more than any plugin. Open the cutoff slowly over a breakdown, drop resonance into the drop, then spike env mod for the last eight bars. The drums in a techno beat can stay simple when the 303 does this much work.