# Audio Spectrum

FFT bars over a synthesised arpeggio

```{raw} html
<a class="example-run-button" href="../demos/features_audio_spectrum.html">▶ Run in browser</a>


```

**Tags:** `audio` `spectrum` `fft` `synthesis`

An AudioSynth-baked arpeggio loops on an AudioPlayer while a log-spaced bar
spectrum dances in time with it. Each frame the demo reads
get_playback_position(), slices a 2048-sample window out of the clip's own
PCM buffer (backend_data), and runs np.fft.rfft over it. The engine has no
audio output tap, so this analyses the source buffer the example itself
generated, synchronised by playback position; it cannot analyse file-backed
streams (their backend_data is None).


## What it demonstrates
- AudioSynth voices (oscillators + ADSR envelopes + a LowPass drone) baked
  into an AudioClip via bake() / AudioClip.from_pcm.
- backend_data as a float32 interleaved numpy buffer, indexed at
  int(get_playback_position() * sample_rate) frames during playback.
- A windowed rfft per frame, folded into log-spaced bands with peak-hold
  caps, drawn with the immediate-mode 2D renderer.

Controls:
  SPACE - Pause / resume
  ESC - Quit

Run: uv run python examples/features/audio/spectrum.py
Headless self-check: uv run python examples/features/audio/spectrum.py --test

## Source

```{literalinclude} ../../examples/features/audio/spectrum.py
:language: python
:linenos:
```