Tools

Spectral

FFT spectrogram - see harmonics, sidebands, and filter sweeps in the frequency domain.

What is a Spectral?

A spectrum analyzer shows you what an oscilloscope cannot. Where the scope draws the waveform in time (voltage on the y-axis, time on the x-axis), the spectrum analyzer draws the same signal in frequency (loudness on the y-axis, frequency on the x-axis). Same data, different lens. The scope tells you what the wave *looks like*; the analyzer tells you what the wave *is made of*.

Every periodic signal can be decomposed into a sum of pure sine waves at integer multiples of its fundamental frequency. A sawtooth at 100Hz is mathematically identical to a sine at 100Hz plus a sine at 200Hz at half-amplitude plus a sine at 300Hz at one-third-amplitude plus a sine at 400Hz at one-quarter-amplitude, and so on forever. This is Fourier's theorem, and it is the foundation of digital audio. The spectrum analyzer performs the decomposition (via the Fast Fourier Transform, the FFT) and shows the result.

Why care? Because what makes a sound *interesting* is its frequency content, not its waveform shape. Two saws at the same pitch sound the same; the same saw through a low-pass filter sounds completely different - and on a scope it just looks like a slightly smoother saw. On a spectrum analyzer the difference is dramatic: the upper harmonics (the high bins) drop visibly as you lower the cutoff. FM synthesis adds sidebands you can count by eye. Distortion adds a forest of high-order harmonics. Noise is a flat plateau across the full spectrum. The frequency domain is where these stories are visible.

Spectrum analyzers became practical for synthesists with the FFT - an algorithm published by Cooley and Tukey in 1965 that reduced the cost of frequency analysis from O(N²) to O(N log N). A modern computer can FFT a 2048-sample window every millisecond without breaking a sweat, which is why a real-time spectrogram is now a small accessory module rather than a piece of lab equipment.

Our Spectral

Webrack's Spectrum Analyzer is 16 HP, runs a 2048-point FFT with a Hann window and 50% hop - a new spectrum roughly every 23ms. That gives roughly 21Hz per bin - enough resolution to clearly resolve bass-region detail and still see harmonics into the high treble. The display is a scrolling spectrogram with a perceptual color map (blue = quiet through red/yellow = loud).

Analysis is sample-accurate and stays smooth even when the rest of the rack is busy. The FFT is a textbook radix-2 Cooley-Tukey butterfly with bit-reversal permutation, tuned to never glitch. Output magnitudes are clamped to a -120dB floor so quiet signals don't smear the visible range.

Four knobs frame the view: Gain (-20 to +40 dB) shifts display brightness; Lo and Hi set the frequency range; Speed controls how fast the spectrogram scrolls (1-8x). The single THRU output passes the signal unchanged for chaining downstream. There is no audio output beyond pass-through - this is a measurement instrument, not a sound source.

In a patch

The simplest pairing is Scope + Spectrum Analyzer side by side. Patch your final VCA (or whatever you want to inspect) into both, and the two displays show complementary truths: the scope shows the *waveform shape*, the analyzer shows the *frequency content*. Run a VCO saw through a Filter and watch the analyzer's harmonic comb collapse as you lower cutoff - the harmonics literally disappear in real time.

Use it to *understand* synthesis techniques. Patch an FM Operator carrier and modulator and watch sidebands appear at multiples of the modulator frequency on either side of the carrier. Try a Wavetable Oscillator and watch the spectrum reshape as you scan through the table. Try Plaits and see why each model has its own characteristic spectrum.

Use it to debug. If your patch sounds harsh, the analyzer often shows you why: aliasing as a forest of unrelated high-frequency spikes, DC bleed as a non-zero 0Hz bin, hum as a thin spike at 50/60Hz. Pair the analyzer's THRU output with downstream effects to monitor without breaking the chain.

Inputs

IN (audio) — Audio input to analyze. Any audio signal works - oscillators, filtered sounds, effects, final mixes, anything you want to see the spectrum of.

Outputs

THRU (audio) — Pass-through output. The input signal is copied here unchanged. Use this to chain the analyzer into a signal path without breaking the audio flow.

Controls

Gain — Display gain from -20dB to +40dB. Adjusts the brightness of the spectrogram without affecting the audio. Turn up for quiet signals, turn down if the display is too bright.
Lo Range — Low frequency bound of the display (20Hz to 2000Hz). Set this to zoom into the frequency range you care about. For bass-heavy patches, try 20-500Hz.
Hi Range — High frequency bound of the display (1000Hz to 22050Hz). Lower this to zoom into the low-mid range, or keep it at maximum to see the full spectrum.
Speed — Scroll speed of the spectrogram (1x to 8x). Faster speeds show more temporal detail but the history scrolls off screen quicker. Slower speeds let you see longer patterns.

Inspired by

A small in-rack spectrum analyzer in the lineage of standalone FFT tools and the spectrum modes of multi-utility Eurorack instruments. Hann window, 50% hop, color-mapped magnitude in dB.

Mordax DATA spectrum mode
classic studio FFT analyzers
iZotope/SPAN-style frequency displays

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