Feedback

Eating Its Own Tail

A feedback loop is a signal that hears itself. The output of a system routes back to its own input, where it gets processed again, and again, and again. This simple idea — output becomes input — is the engine behind half of audio: delay repeats, filter resonance, compressor behavior, oscillator sustain, and the howl of a microphone too close to a speaker. Understand feedback and you understand the hidden connection between effects that seem unrelated.

Positive Feedback

Output amplifies itself.

In positive feedback, the output is added back to the input. Each pass through the loop increases the signal. If the loop gain is above 1.0, the signal grows exponentially — this is runaway feedback, the screech of a mic too close to a speaker. Below 1.0, each repetition is smaller than the last, and the signal decays. At exactly 1.0, it sustains forever.

The boundary between growth and decay is the story. Below unity gain, positive feedback creates sustain and resonance — things ring out, echoes repeat, notes hold. Above unity, it's chaos and buildup. Every delay pedal's feedback knob is controlling this exact threshold: how close to the edge of self-oscillation do you want to live?

Signal Flow

animated · watch the signal circulate

Output Over Time

Parameters

loop gain0.85×

damping0.15

input0.50

Key insight: Watch what happens as gain approaches 1.0. Below it, the signal decays (each loop iteration is smaller). At 1.0, it sustains indefinitely. Above it, it grows until something clips. That boundary — unity gain — is the most important number in audio.

The Connections

Every concept in Tone Science is connected through feedback:

Delay

The feedback knob is a positive feedback loop with a time delay in the path. More feedback = more repeats.

Reverb

Algorithmic reverbs are networks of delay lines with feedback. The decay time IS the feedback amount.

Saturation

Tube amp feedback (negative) reduces distortion. Remove the feedback and the amp distorts more — some guitarists prefer this.

Compression

Feedback compressors measure the output to control the input. The sidechain IS the feedback path.

Modulation

Flanger feedback creates resonant peaks in the comb filter. More feedback = more metallic resonance.

Filters

Resonance (Q) is internal feedback. Every resonant filter is a feedback loop operating at audio frequencies.

The deeper pattern

Feedback is the oldest idea in cybernetics — the study of systems that regulate themselves. Norbert Wiener defined it in 1948: a system where the output influences the input. A thermostat is negative feedback. A microphone howl is positive feedback. A compressor sidechain is negative feedback applied to dynamics. A resonant filter is positive feedback applied to frequency.

In audio, feedback is the mechanism that turns simple components into complex behavior. A delay line without feedback is a single echo. With feedback, it’s a cascading decay that can sustain indefinitely. A filter without feedback is a gentle slope. With feedback, it rings, resonates, and eventually sings on its own. The feedback path is where simple becomes rich, where predictable becomes alive.

This is why feedback is the right place to end Tone Science. It’s the thread that connects every piece in the series. Harmonics are what you hear. Delay is when you hear it. Reverb is where you hear it. Saturation is how it’s shaped. Compression is how it’s controlled. Modulation is how it moves. Stereo is where it lives in space. But feedback — feedback is why any of it comes alive.

The loop is the life.