[Feature] EBU R128 Real-Time Loudness Normalizer & Audiophile DSP Audio Rack

[arkalon76]

📌 Executive Summary & User Story

As an audiophile and headphone user,
I want halpradio to automatically equalize broadcast loudness across all stations and provide binaural crossfeed and vintage acoustic filters,
So that I never experience painful volume spikes between streams and can enjoy fatigue-free listening during long coding sessions.

---

🎯 Problem Statement & Community Context

Different internet radio streams broadcast at wildly inconsistent gain levels (ranging from -24 LUFS classical music to -8 LUFS hyper-compressed pop). Switching stations often causes painful volume jumps. Furthermore, listening to hard-panned stereo over headphones for hours causes ear fatigue.

---

🏗️ Detailed Functional Requirements

1. Real-Time EBU R128 Loudness Normalizer (Auto-Gain Control)

• Implement real-time integrated loudness calculation (LUFS) adhering to the EBU R128 / ITU-R BS.1770 standard.
• Apply a transparent, slow-acting automatic gain control (AGC) with smooth lookahead limiting to prevent clipping.
• Target loudness configurable in config.yaml (default: -14 LUFS, matching Spotify/YouTube standards).

2. Binaural Crossfeed Filter (Headphone Fatigue Eliminator)

• Implement Chu Moy / Bauer binaural crossfeed DSP algorithm.
• Blends a low-pass, delayed fraction of the left channel into the right channel (and vice versa) to simulate speaker acoustic crosstalk, drastically reducing headphone listening fatigue.

3. Lo-Fi / Vintage Cassette DSP Mode

• Toggleable analog acoustic effect:
  • Subtle analog tape saturation & warm harmonic warmth.
  • Mild high/low-pass roll-off and optional vinyl surface warmth.

4. 10-Band Graphic Equalizer TUI Modal (E key)

• 10 frequency bands: 32Hz, 64Hz, 125Hz, 250Hz, 500Hz, 1kHz, 2kHz, 4kHz, 8kHz, 16kHz.
• Built-in presets: Flat, Bass Boost, Vocal Clarity, Electronic, Acoustic, Deep Focus.

---

🖥️ TUI & Visual Design

┌─ 🎚️ GRAPHIC EQUALIZER & DSP RACK ──────────────────────────────────────────┐
│  Presets: [Flat]  [Bass Boost]  [*Deep Focus*]  [Lo-Fi Tape]  [Custom]       │
│                                                                             │
│  +12dB ───   ─█─   ───   ───   ───   ───   ───   ───   ───   ───            │
│   +6dB ───   ─█─   ─█─   ───   ───   ───   ───   ─█─   ─█─   ───            │
│    0dB ─█─   ─█─   ─█─   ─█─   ─█─   ─█─   ─█─   ─█─   ─█─   ─█─            │
│   -6dB ───   ───   ───   ───   ───   ───   ───   ───   ───   ───            │
│  -12dB ───   ───   ───   ───   ───   ───   ───   ───   ───   ───            │
│        32Hz  64Hz 125Hz 250Hz 500Hz  1kHz  2kHz  4kHz  8kHz 16kHz           │
│                                                                             │
│  [x] EBU R128 Normalizer (-14 LUFS)    [x] Binaural Crossfeed (Headphones)  │
│  [Tab: Select Band | j/k: Adjust dB | p: Presets | Esc: Save & Close]       │
└─────────────────────────────────────────────────────────────────────────────┘

---

🛠️ Technical Architecture & Implementation Details

• Package Location: pkg/player/dsp/normalizer.go, pkg/player/dsp/crossfeed.go, pkg/player/dsp/eq.go, pkg/ui/modals/eq_modal.go.
• Pure Go DSP Pipeline: Operates directly on the 32-bit floating point PCM audio stream buffer before piping to oto/v3 or native audio output.

---

✅ Acceptance Criteria (Definition of Done)

• EBU R128 normalizer successfully equalizes loud and quiet stations to uniform target volume without audio pumping.
• Binaural crossfeed toggles cleanly with audible natural stereo imaging on headphones.
• 10-band equalizer modal provides interactive slider controls with real-time audio response.
• DSP settings persist in ~/.config/halpradio/dsp.yaml.
• Zero measurable CPU lag or dropped audio frames during DSP processing.