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install @audio/filter-biquad\n```\n\n```js\nimport { highpass, lowpass, bandpass, notch, allpass } from '@audio/filter-biquad'\n```\n\nHz/Q-parameterized wrappers over the shared [`@audio/biquad`](https://github.com/audiojs/filter/tree/main/packages/biquad) kernel — one call per filter shape, coefficients recomputed only when `fc`/`Q`/`fs`/`order` actually change, state cached on the `params` object.\n\n```js\nlowpass(buffer, { fc: 2000, fs: 44100 })                // 2nd-order (default) — no registration needed\nhighpass(buffer, { fc: 80, fs: 44100 })                  // rumble filter\nbandpass(buffer, { fc: 1000, Q: 5, fs: 44100 })          // narrow band\nnotch(buffer, { fc: 50, Q: 30, fs: 44100 })              // remove 50 Hz mains hum\nallpass.first(buffer, { a: 0.5 })                        // 1st-order, coefficient a\nallpass.second(buffer, { fc: 1000, Q: 1, fs: 44100 })    // 2nd-order, center fc, quality Q\n```\n\n`lowpass`/`highpass` default to a 2nd-order RBJ biquad ($-12\\,\\text{dB/oct}$). For 4th-order-and-up Butterworth cascades, register a designer once — kept out of the default import so the common 2nd-order case stays dependency-free:\n\n```js\nimport butterworth from 'digital-filter/iir/butterworth.js'\n\nlowpass.useButterworth(butterworth)                       // once, before any order > 2 call\nlowpass(buffer, { fc: 2000, order: 4, fs: 44100 })        // 4th-order Butterworth, -24 dB/oct\n```\n\n| Function | Param | Default | |\n|---|---|---|---|\n| `lowpass`, `highpass` | `fc` | — | required, Hz |\n| | `order` | `2` | `2` = RBJ biquad; `4+` = Butterworth (needs `useButterworth`) |\n| | `Q` | `0.707` | quality factor (1/√2, maximally flat) |\n| | `fs` | `44100` | sample rate, Hz |\n| `bandpass` | `fc` | — | required, center Hz |\n| | `Q` | `0.707` | narrower band = higher Q (differs from the raw `@audio/biquad` kernel's default of `1`) |\n| | `fs` | `44100` | sample rate, Hz |\n| `notch` | `fc` | — | required, null frequency Hz |\n| | `Q` | `30` | notch width — lower (e.g. `5`) is wider |\n| | `fs` | `44100` | sample rate, Hz |\n| `allpass.first` | `a` | — | required, pole at z = −a |\n| `allpass.second` | `fc`, `Q`, `fs` | — / `0.707` / `44100` | RBJ cookbook allpass |\n\nPass the same `params` object on every call to persist state across blocks:\n\n```js\nlet params = { fc: 1000, fs: 44100 }\nfor (let buf of stream) lowpass(buf, params)\n```\n\n**Use when:** building an audio-facing biquad filter (Hz/Q, not raw SOS coefficients); bypass this and use `@audio/biquad` directly when you already have coefficients or need cascade/analysis primitives.\n\n---\n\nPart of [@audio/filter](https://github.com/audiojs/filter) — the filter family umbrella. This README is generated from the umbrella docs.\n\nMIT © [audiojs](https://github.com/audiojs)\n","readmeFilename":"README.md"}