Synthesizer Basics: Oscillators, Filters, ADSR and LFOs

Begin with an initialized patch: one saw oscillator, low-pass filter open, resonance low, effects off and LFO amounts at zero. Use the amplifier envelope with a short attack, full sustain and about 300 ms release. Keep output low. Hold A, lower cutoff until the tone darkens, then release the key and listen for the tail.

This lesson follows a basic subtractive synthesizer: oscillator, filter, amplifier, plus the controls that move them. You will build a pluck and a sustained sound, then play the same phrase through both. You need only one playable synth with those controls; the printable examples also work as a lesson away from a screen.

One note: pitch, amplitude and timbre

Pitch describes how high or low the note sounds. Amplitude describes the size of the signal; changing it usually changes loudness, but amplitude and perceived loudness are not interchangeable. Timbre is the sound’s character: a smooth tone and a buzzy tone can share a pitch. Spectrum and the way a sound begins and fades both contribute to timbre, as UNSW’s acoustics lesson explains.

Read the solid path first: the oscillator supplies the tone, the filter changes its frequency balance, and the amplifier shapes its amplitude. Then follow the dashed controls. An envelope or LFO can change a destination without being another audible layer in this simple patch. This is a teaching path, not a wiring diagram for every synth; instruments may provide multiple oscillators, different routing and separate envelopes.

Start with one oscillator, no effects and no LFO, pitch-envelope or filter-envelope modulation. Keep your listening level comfortable. Play the same note for the same length each time: change octave for pitch, output level for amplitude, then waveform for harmonic balance. Write a concrete observation such as “the start is slower” rather than “this one is better.”

Moog’s introduction describes subtractive synthesis as shaping a harmonically rich source with a filter. This is one useful approach. FM, sample-based synthesis, physical modeling and wavetable methods involve other ways of making or organizing sound; the Ableton instrument reference documents several of them. You do not need to master those methods before making this comparison.

Oscillators: a repeating shape becomes a tone

An oscillator repeats a waveform. Frequency tells you how many cycles occur each second, in hertz, abbreviated Hz. In our tuning, A3 is 220 Hz and A4 is 440 Hz. Play the opening octave slowly, using the MIDI numbers below the notes to identify the register. The piano keyboard helps you find the repeated letter names.

A3–A4 goes up an octave; A4–A3 comes back down. Each half note lasts two quarter-note beats, so the rhythm stays fixed while the register changes. The lower A sits below the treble staff and the upper A sits within it. The numbers are MIDI identifiers, not fingerings. Keep the oscillator waveform unchanged so you can hear the pitch comparison before testing its harmonic ingredients.

The fundamental is the repetition frequency of this periodic tone. Harmonic 1 is the fundamental; harmonic 2 is twice that frequency, harmonic 3 is three times it, and so on. A spectrum shows the strengths of these frequency components.

A sine contains only its fundamental frequency. An ideal saw has every harmonic, with amplitudes proportional to one divided by harmonic number. An ideal square wave spends half its cycle high and half low: this is 50% duty cycle. It has odd harmonics only, with amplitudes proportional to one divided by harmonic number. An ideal symmetric triangle also has only odd harmonics, but their amplitudes fall as one divided by harmonic number squared. Carnegie Mellon’s waveform chapter gives these recipes.

Read across a row to connect shape with ingredients. The left plots scale the ideal peak to one; the bars on the right instead use harmonic 1 as their amplitude reference. They are not power plots. The dashed signals stop at harmonic number 6, so they cannot reproduce the ideal square, saw or triangle exactly. Triangle terms alternate signs; its bar heights show magnitudes. Neither matching plot height nor matching fundamental amplitude guarantees equal perceived loudness.

Listen to Synth Waveforms and Their Harmonics. Its controlled examples use only harmonic slots one through six at the same A3 fundamental. Compare sine with saw first, then square with triangle. The waveform names suggest useful starting points, not promises that every synth’s oscillator or every acoustic instrument has exactly these spectra.

The printable waveform reference lets you compare the graphs while listening. Its first section keeps the time axes identical; its second section separates the harmonic recipes; its third gives frequencies for an A3 fundamental. Try the two saw clips labeled before upper-harmonic attenuation and upper harmonics reduced to 20%. Predict what will change before listening, then describe the result without assuming the clips have identical loudness.

Harmonics are components, not extra key presses

For a 220 Hz fundamental, harmonics one through six are 220, 440, 660, 880, 1100 and 1320 Hz. They combine inside one periodic tone. Playing several keys creates separate note events instead. UNSW’s harmonic-fusion demonstration makes this distinction audible: components can fuse into the character of one note rather than sound like a chord.

These notes are a listening aid for the first four harmonic frequencies of A3. Harmonics 1, 2 and 4 coincide with the stated A tuning; E5 only approximates harmonic 3. The third harmonic is 660 Hz, while equal-tempered E5 is about 659.255 Hz. Play the notes separately to locate the register, then return to one oscillator note. Four printed attacks should not be mistaken for the internal components of one sustained sound.

The third harmonic lies a fifth above the second: 660 divided by 440 is exactly 1.5. A keyboard’s equal-tempered fifth is slightly different. Use Intervals: the 5th for the musical distance and Hearing intervals for recognition, while keeping the frequency calculation separate from the keyboard approximation.

MIDI numbers, octaves and tuning

A MIDI note number identifies a key; it is not an audio recording. The MIDI Association’s message summary distinguishes Note On, Note Off and velocity data. A receiving instrument determines the resulting sound. The same note events can therefore drive our pluck or our sustained patch without carrying either patch’s complete timbre.

Our convention is C4 = middle C = MIDI 60. MIDI Notes, Octaves and Frequency prints a numbered keyboard, an A-octave frequency ladder and four number exercises. Software octave names can differ; compare note numbers when labels disagree. The piano-roll reading guide covers that separate display skill.

Count movements between notes, not the number of printed endpoints: MIDI 60 through 72 contains thirteen notes and twelve semitone moves. Every sounded note here is an eighth; the remaining rests finish bar 2. Sharps provide one ascending spelling, rather than a rule for all chromatic music. Notice that E–F and B–C need no black key between them. Consecutive MIDI numbers still describe one semitone at those white-key boundaries.

A semitone is one move to the adjacent key, black or white; Half steps & whole steps shows the distances. The Chromatic Scale adds right-hand fingering for a one-octave C-to-C exercise. In twelve-tone equal temperament, each upward semitone multiplies frequency by the twelfth root of two. Twelve such moves double it; they do not add a fixed number of hertz.

Assuming A4 = 440 Hz, twelve-tone equal temperament and no retuning or transposition, frequency equals 440 multiplied by two raised to the power of “MIDI number minus 69, divided by 12.” Thus A2, A3, A4 and A5 are 110, 220, 440 and 880 Hz. UNSW’s note-frequency reference explains the calculation; other tuning settings change the results.

A4–A5 applies the same octave rule one register higher: MIDI 69–81 and 440–880 Hz. Each half note lasts two beats at this tempo.

The 6th, the 7th & the Octave puts the octave among musical intervals. Octave Signs: 8va & 8vb explains reading an octave away from written pitch; an octave bracket changes how notation is read, whereas a synth’s octave control changes its pitch setting. Check both before deciding that a MIDI part is in the wrong register.

Check your octave arithmetic before opening the synth: starting at MIDI 57, an octave up gives 69 and an octave down gives 45. Starting at 69, two octaves down gives 45 again. Now reverse the question: 880 Hz to 220 Hz is two successive halvings, so it spans two octaves downward. This checks the same relationship through numbers and frequency rather than relying on the software’s octave label.

Filters: change the balance, not the note name

A low-pass filter attenuates higher frequencies relative to lower ones. A high-pass filter does the reverse. Cutoff identifies the transition region; it is not a wall beyond which every component vanishes. Apple’s filter-slope explanation describes gradual attenuation. Start with a saw, because its upper harmonics give you something clear to reduce.

Gain describes the output relative to the input: 0 dB means unchanged amplitude, negative dB means attenuation, and positive dB means a boost. A two-pole low-pass approaches a 12 dB-per-octave roll-off well above cutoff; a four-pole version approaches 24 dB per octave. Q controls resonance in this model; your synth may label that control Resonance instead.

This is an analog two-pole low-pass magnitude model, not a measurement of a commercial synth. Both curves use the same 660 Hz cutoff parameter; Q is the model’s resonance setting, and raising it creates a peak near that region. The six panels apply only the Q = 0.707 gain to the saw’s harmonic coefficients, preserving nonzero upper partials. The frequency axis is logarithmic, so equal horizontal distances represent equal ratios. Phase and the filter’s transient behavior are outside this plot.

Resonance emphasizes a region near cutoff; it can make a sweep more pronounced. Moog describes that control in its introduction. Begin with resonance low, hold A3, and move only cutoff. Then repeat with more resonance and a restrained output level. Listen for the difference between removing brightness and emphasizing a narrower frequency region.

Keep pitch fixed during this test. If the tone becomes duller, check whether you actually hear a lower fundamental or simply fewer strong upper components. A filter sweep can change timbre dramatically without moving the played key. A sine offers less harmonic variety to subtract than a saw in this simple comparison.

Choose the sound change first, then the relevant control. To compare register, move A3 to A4 while leaving the waveform alone. To compare harmonic balance, hold the note and change cutoff. To compare the beginning and ending, change attack or release separately. For repeated motion, identify what the LFO controls before adjusting its speed. These categories guide listening; they do not require a sound to have only one audible change.

Use the six coefficient panels to make a prediction: the input saw’s second harmonic is half the first, but after this filter its second harmonic has lost more of its amplitude than the first. The filter changes the relationship between ingredients. It does not simply turn the entire sound down by one common multiplier. Compare that result with lowering the amplifier’s output while leaving the filter fixed.

ADSR: separate the key’s gate from the tail

An amplitude envelope shapes level through a note. Attack rises toward the peak; decay moves from the peak toward sustain; sustain is the held level; release moves toward silence after note-off. Apple’s ADSR explanation defines those four controls. Sustain is a level, not a duration. Holding a key longer can extend the plateau without changing that level.

First play two short key presses separated by rests, then one whole-bar press. At this tempo a quarter-note gate is 500 ms and a whole-note gate is 2000 ms. The notation describes the performance input; it does not draw the sounding amplitude envelope. If the synth keeps sounding into a rest, listen to its release setting before changing the rhythm. A long tail does not mean the key was held for longer.

The gate is the interval between note-on and note-off. In Draw an ADSR Envelope, the worked note has a 1000 ms gate: two quarter beats at 120 BPM. Attack lasts 100 ms, decay 200 ms, sustain is 50%, and release lasts 300 ms. Milliseconds are thousandths of a second. Those parameters describe an ideal linear amplitude model, not a universal knob calibration.

Trace the breakpoints in order: start, peak, sustain arrival, note-off, silence. The sustain plateau begins at 300 ms and lasts 700 ms, because the key comes up at 1000 ms. Release then adds 300 ms, giving 1300 ms from note-on to silence. The vertical percentages refer to amplitude within this model, not perceived loudness. Straight segments make the arithmetic visible; real synths can use different curves and stage behaviors.

Now keep the gate fixed and change sustain to 25%. The plateau drops; it does not become shorter. Restore 50% and lengthen release: the tail changes after note-off. Real patches may loop envelopes or release before attack or decay finishes. Our drawings keep note-off after those stages so you can read the basic relationships without guessing a device’s early-release behavior.

The ADSR exercise asks you to draw two specified envelopes, then read missing values from a third graph. Its note-offs remain fixed, so the changing starts and tails can be compared directly. Cover the inverted key before plotting. The listening examples provide completed versions under “Check yourself”; MIDI downloads supply the note timing, while the audio demonstrates the stated envelope. The coordinates give a more precise check than guessing a sound adjective.

Solve a second envelope without the worksheet. Keep attack at 100 ms and decay at 200 ms, hold until 1500 ms, and choose a 400 ms release. Sustain still begins at 300 ms. Its plateau now lasts 1200 ms, and silence arrives at 1900 ms. Extending the key-down interval changed the plateau’s duration; changing release added a different amount after the key came up.

A tie does not mean a new attack

Ties vs Slurs distinguishes one sustained pitch from separately played connected notes. For our simple gate interpretation, tied half notes make one four-beat press. Two repeated half notes make two attacks. If a synth unexpectedly reattacks a tied passage, inspect the note events; if connected notes behave differently, inspect its envelope retrigger and legato settings.

Hold through the tie in bar 1: there is one attack, not another on beat 3. In bar 2, release after two beats and count through the half rest. At quarter = 120 these gates last 2000 ms and 1000 ms respectively. The two examples use the same pitch but different key durations. Compare them with one release setting so you can distinguish a held note from a tail continuing after the key is up.

LFOs: choose what moves, then how fast

An LFO, or low-frequency oscillator, provides repeated modulation. Route it to pitch for vibrato, amplitude for tremolo, or cutoff for changing brightness. Rate sets repetition speed; depth sets the excursion in the destination’s units. The Ableton manual documents oscillator, amplifier and filter modulation. An LFO is a control source here, rather than another note you play.

A cent is one hundredth of an equal-tempered semitone. Here the pitch moves 20 cents above and below the played note: one fifth of a semitone in each direction.

The repeating shape and rate stay fixed while the destination changes. The pitch row produces a vibrato comparison; the amplitude row produces a tremolo comparison; the cutoff row changes frequency balance. The cutoff excursion is symmetric in musical distance around 660 Hz, not in added hertz. These are control trajectories, not plots of the final sound wave. A depth number is meaningful only together with its destination, units and instrument’s mapping.

Use a small depth first so the note remains easy to follow. Then double rate while keeping depth unchanged: movement becomes faster without necessarily traveling farther. Some instruments offer free-running Hz and tempo-synced divisions. Rather than assume a universal frequency boundary for all LFOs, read your instrument’s range and synchronization description.

At 120 quarter-note BPM, one cycle per quarter lasts 500 ms, or 2 Hz; one per eighth lasts 250 ms, or 4 Hz. Tap the references below with Tempo & the Metronome, then hold one note while the LFO repeats. You need not press a key at every modulation cycle.

Tap bar 1 to feel quarter-note cycles and bar 2 to feel eighth-note cycles. Then hold one A while the modulation continues at either rate. The repeated notes on the staff demonstrate timing, not the only way to trigger a patch. Some LFOs restart with a note and others continue independently, so check the trigger mode before expecting a new key press to begin at the same modulation phase.

Make a pluck and a sustained sound

For the hands-on comparison, use one saw oscillator and keep its filter unchanged: choose a two-pole low-pass if available, set resonance low and begin near 1320 Hz. Turn off LFOs and effects. The graphs use a six-harmonic approximation for calculation; your synth's ordinary saw is sufficient for this listening exercise. Use the two amplitude-envelope settings below.

Both recipes use an ideal linear amplitude envelope. The pluck reaches silence at 190 ms if the gate lasts at least that long; keeping its key down cannot create a nonzero sustain plateau. The sustained recipe reaches its 70% level at 500 ms, so hold beyond that point to hear a plateau. Keep source, filter and input phrase fixed for the comparison. Then vary one parameter, rather than changing the entire patch and losing the cause of the difference.

For the comparison, use polyphonic mode with enough voices for release tails, and retrigger the amplitude envelope for each new note. Turn off legato envelope sharing. If your synth is monophonic, test each note separately first; the overlap between successive tails will differ.

Play this original A-minor phrase with the pluck. Bar 1 is A–C–E, bar 2 D–C–B–A, bar 3 C–D–E–G, and bar 4 E–A. The numbers below are degrees relative to A, with lowered third and seventh compared with A major. Every quarter gate exceeds the pluck’s attack-plus-decay time, so the note can fade before its written value ends.

Keep the written note values while using the pluck envelope. Its zero sustain level lets the sound decay during a held key, so do not shorten every note merely because you hear a short tone. The half notes in bars 1 and 4 make that especially clear. Read the degree labels as pitch relationships, not finger numbers. The phrase is an original teaching example, not a transcription or a claim about any genre’s standard melody.

In this phrase the pluck is already silent before note-off, so its release setting has no audible tail to shape. Test release with the sustained recipe, or release a pluck key before its 190 ms decay has finished.

Now play exactly the same notes with the sustained recipe. Quarter notes reach the sustain level at note-off; half notes allow a plateau. The 400 ms release may overlap the next note when voices can ring independently. If the starts seem missing on a short figure, shorten attack before increasing volume. If gaps disappear, shorten release before rewriting the rhythm.

The score has not changed; the amplitude recipe has. With a 500 ms quarter-note gate, attack plus decay occupies the whole gate in this model. A 1000 ms half-note gate leaves another 500 ms at sustain before release begins. Listen especially to the E5 half note in bar 1 and the two half notes in bar 4. If your synth shares or retriggers envelopes differently, use these timings as the comparison goal and consult its manual.

To make a note begin bright and then darken, keep the amplitude sustained and assign a separate envelope to low-pass cutoff. Start with a low cutoff, choose a positive envelope amount, a short attack and a short decay, and set that envelope's sustain low. Each attack now opens the filter before it closes again. Reduce the envelope amount to hear the difference from an amplitude pluck.

Next, transpose the entire phrase down one octave while keeping its rhythm. The A-minor degree pattern stays the same; its frequencies move down by a factor of two. Listen again with the same filter setting. Then decide whether to move cutoff too. Keeping a fixed cutoff and keeping the same relationship between cutoff and note frequency are different experiments, so label which one you are doing.

Give the phrase a musical shape

Sound design does not replace performance. The Dynamic Ladder compares relative loud and soft; it supplies no universal conversion to velocity, amplitude percentage or decibels. Fermata, Tenuto & Accent gives a strong-start comparison, while Phrasing connects notes across a musical sentence. Choose how your patch should respond before recording those gestures.

In bar 1, emphasize the first attack. In bar 2, connect the notes, grow toward C5 and ease back to A. The slur marks connection; the gradual loudness shape is the playing instruction in the panel, not a printed hairpin. Test whether your patch responds to your intended gesture. Velocity may affect level, brightness or another assigned destination, so a stronger key strike is not a guaranteed fixed increase in loudness.

End by explaining one choice: “I shortened release so the rests in bar 1 of the short-gate example became audible,” or “I reduced cutoff while keeping A fixed.” If setup is the obstacle, the home-production guide covers connecting your tools. A Tissimo piano teacher can check the phrase’s timing and articulation while you compare the patches in a live lesson.

Free worksheets for this lesson

• Synth Waveforms and Their Harmonics — ideal shapes, finite harmonic recipes and listening comparisons.
• MIDI Notes, Octaves and Frequency — numbered keyboard, frequency ladder and number exercises.
• Draw an ADSR Envelope — worked graph, plotting tasks, listening and coordinate key.
• The piano keyboard — locate letters through black-key groups.
• Half steps & whole steps — measure adjacent-key distances.
• The Chromatic Scale — practice consecutive semitones.
• The 6th, the 7th & the Octave — compare wide musical intervals.
• Intervals: the 5th — recognize the five-letter distance.
• Hearing intervals — connect distances with listening cues.
• Octave Signs: 8va & 8vb — read octave displacement brackets.
• The Dynamic Ladder — compare relative dynamic levels.
• Ties vs Slurs — distinguish holding from connecting attacks.
• Fermata, Tenuto & Accent — compare duration and strong-start gestures.
• Phrasing — shape a connected musical sentence.
• Tempo & the Metronome — keep a steady comparison pulse.

Sources

• Spectrum, harmonics and timbre — UNSW Physics
• Basic waveform shapes — Carnegie Mellon University
• Note names, MIDI numbers and frequencies — UNSW Physics
• Summary of MIDI 1.0 Messages — The MIDI Association
• Synthesis 101: An Introduction to Subtractive Synthesis — Moog Support
• Live Instrument Reference — Ableton
• Attack, decay, sustain, and release — Apple
• Filter slope — Apple