A row of frequency bars under the track progress, on by default and
toggleable with the client spectrum config option. Because the audio
plays on the server and clients may be remote, the spectrum is produced
server-side, not captured locally: audio-player taps its own output
into a lock-free ring on the audio thread (one store per sample, no
locks), crabidy-server runs a Hann + realfft over 2048 samples at 20fps,
folds it into log-spaced bars, and broadcasts them as a new SpectrumFrame
on the update stream. The task idles when nothing is playing or no
client is listening. The TUI renders block-glyph bars in the now-playing
pane; the web client renders the same bins as CSS bars.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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