06 — ECG signal processing

flowchart LR
    RAW["raw recording\ndrifty, unaligned"] --> BASE["median filter\nsubtract slow drift"]
    BASE --> WIN["R-peak windowing\ncut aligned windows"]
    WIN --> NORM["z-score normalize\nkill volume differences"]
    NORM --> CLEAN["clean fixed-size beats"]

Step	Problem	Fix	Concrete value
Sampling	continuous voltage, model needs numbers	store 360 numbers/s	0.6 s window ≈ 216 numbers
Baseline wander	breathing drifts the trace up/down	median filter tracks and subtracts drift	keeps sharp QRS
Windowing	beats start anywhere	center every window on its R peak	same alignment always
Normalization	loud vs quiet recordings	subtract mean, divide by spread	shapes, not volume

Out: clean, aligned, same-size beat windows — the input to everything downstream.

Next: 07-scalograms.md