Deterministic mock audio sources

Linear: AUT-100

Three sources that emit timestamped [AudioChunk]s with byte-exact reproducible samples — no microphone, no GStreamer. Every test that wants "this is what audio looks like" without a live device uses one of these.

Left to right: SineWaveSource(440 Hz, A=0.7) — five cycles fitting the panel; SilenceSource — flat zero everywhere; StepPulseSource(120) — a single 1.0 spike at frame 120, silent otherwise. Rendered against a white backdrop per the CLAUDE.md asset-choice rule (audio shape needs light backing to read).

api

The three shapes

Each source advances an internal frame counter on next_chunk(frames) and stamps PTS via [MediaTime::from_sample]. Two successive next_chunk(48_000) calls on a 48 kHz source produce chunks with pts = 0 s and pts = 1 s — no rounding drift.

Why three shapes

The histogram quantizer in M-MEDIA.8 has three correctness assertions:

1. Silence → zero bars. Trivial; SilenceSource is the input.
2. Sine wave → stable RMS. A pure sinusoid at amplitude A has RMS = A/√2 ≈ 0.7071·A. Constant across buckets. SineWaveSource is the input; the test asserts (observed - A/√2).abs() < tolerance.
3. Pulse → expected peak. A single 1.0 spike at frame K should show up as peak == 1.0 exactly in the bucket containing frame K, and peak < epsilon everywhere else. StepPulseSource is the input.

These are the same three properties M-MEDIA.4 tests today, and the same three M-MEDIA.8 will assert against the histogram output. Mock sources let the entire audio-visualization stack be TDD'd before any real audio infrastructure exists.

Quick start

#![allow(unused)]
fn main() {
use media::audio::AudioFormat;
use media::mock_audio::{SineWaveSource, SilenceSource, StepPulseSource};

let fmt = AudioFormat::mono_f32(48_000);

// Sine wave at 440 Hz, amplitude 0.5. RMS ≈ 0.5 / √2.
let mut sine = SineWaveSource::new(fmt, 440.0, 0.5);
let chunk = sine.next_chunk(48_000); // 1 s
assert!((chunk.rms() - 0.5 / 2.0_f32.sqrt()).abs() < 0.01);

// 100 ms of silence.
let mut hush = SilenceSource::new(fmt);
assert!(hush.next_chunk(4_800).peak() < f32::EPSILON);

// Single spike at frame 7.
let mut pulse = StepPulseSource::new(fmt, 7);
let chunk = pulse.next_chunk(16);
assert!((chunk.samples()[7] - 1.0).abs() < f32::EPSILON);
}