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docs/media/audio/audio-analysis-spectrum/src/analyzer/spectrum-analyzer.md

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+# `SpectrumAnalyzer` — magnitude spectrum + scalar features
+
+## Purpose
+
+`SpectrumAnalyzer` is the workhorse analysis op of this package: it slices an `AudioDocument` into overlapping frames, windows each frame, runs the real FFT, and reduces the per-bin magnitudes into a fixed-length `spectrum` plus three scalar features — centroid, rolloff, flux. The default 128-bin output is the standard size used by visualisers and feature pipelines.
+
+The class implements `analyze` (single-shot whole document), `analyzeStream` (per-frame async generator suitable for real-time visualisation), and `analyzeBlock` (one already-windowed block via a `BlockContext`).
+
+## Features
+
+- Slices `doc` into `frameSize`/`hopSize` overlapping frames (defaults `2048`/`512`).
+- Hann window by default; Hamming or Blackman selectable via `window`.
+- Log-spaced bucket-mean reduction to the configured `bins` (default `128`).
+- Centroid, 85% rolloff, and positive-flux scalars per result.
+- `analyzeStream` yields per-frame results; the final frame's vectors match `analyze`.
+- `analyzeBlock` runs a single FFT-sized block with `BlockContext.sampleRate`.
+
+## Usage: Basic
+
+```ts
+import { SpectrumAnalyzer } from "@suaveplan/audio-analysis-spectrum/analyzer/spectrum-analyzer";
+
+const analyzer = new SpectrumAnalyzer();
+const result = await analyzer.analyze(doc);
+console.log(result.centroid, result.rolloff, result.flux, result.spectrum);
+```
+
+## Usage: Advanced
+
+```ts
+const analyzer = new SpectrumAnalyzer();
+const ctrl = new AbortController();
+setTimeout(() => ctrl.abort(), 50);
+try {
+  for await (const frame of analyzer.analyzeStream(doc, {
+    frameSize: 4096,
+    hopSize: 1024,
+    bins: 256,
+    window: "blackman",
+    signal: ctrl.signal,
+  })) {
+    updateVisualiser(frame.spectrum);
+  }
+} catch (err) {
+  // AUDIO_ANALYSIS_SPECTRUM.ABORTED expected on early cancel.
+}
+
+const block = analyzer.analyzeBlock([new Float32Array(2048)], {
+  sampleRate: 48_000,
+  startSample: 0,
+});
+```
+
+## API Reference
+
+- `name`, `description`, `analyze(doc, opts?)`, `analyzeStream(doc, opts?)`, `analyzeBlock(block, ctx)`.
+- `opts` includes `bins`, `frameSize`, `hopSize`, `window`, `signal`; defaults from the Zod schema.
+- Result extends `AnalysisResult` with `op: "spectrum-analyzer"`, `spectrum`, `centroid`, `rolloff`, `flux`.
+
+## Implementation Notes
+
+The DSP pipeline lives in the `dsp` module so `SpectrumAnalyzer` stays an orchestrator. `analyze` retains only the final frame's `spectrum`/`magnitude` and the previous frame's `magnitude` so flux is well-defined. Each frame revalidates the abort signal between iterations, satisfying the mid-scan abort scenario. `analyzeBlock` does not consult an `AbortSignal` — block calls are bounded-time and the call site owns its own cancellation.