diff --git a/src/compress.rs b/src/compress.rs
new file mode 100644
index 0000000..04ca8d9
--- /dev/null
+++ b/src/compress.rs
@@ -0,0 +1,39 @@
+//! Delta encoding for archived downlink frames. Live frames stay raw.
+
+/// Encode a channel as first sample plus deltas, quantised to `step`.
+pub fn delta_encode(samples: &[f64], step: f64) -> Vec<i32> {
+    let mut out = Vec::with_capacity(samples.len());
+    let mut prev = 0.0;
+    for (i, &s) in samples.iter().enumerate() {
+        let q = (s / step).round();
+        out.push(if i == 0 { q as i32 } else { (q - prev) as i32 });
+        prev = q;
+    }
+    out
+}
+
+pub fn delta_decode(deltas: &[i32], step: f64) -> Vec<f64> {
+    let mut acc = 0i64;
+    deltas
+        .iter()
+        .map(|&d| {
+            acc += d as i64;
+            acc as f64 * step
+        })
+        .collect()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn round_trips_within_a_step() {
+        let samples = [4.01, 4.02, 4.05, 3.98];
+        let step = 0.01;
+        let decoded = delta_decode(&delta_encode(&samples, step), step);
+        for (a, b) in samples.iter().zip(decoded) {
+            assert!((a - b).abs() <= step / 2.0 + 1e-9);
+        }
+    }
+}
diff --git a/src/main.rs b/src/main.rs
index e447aed..7a7f9b7 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,6 +1,7 @@
 //! Kestrel launch sequencer: walks the terminal count, honours holds, and
 //! hands off to the flight computer at T-0.
 
+mod compress;
 mod sequencer;
 mod telemetry;