Read channel
The circuitry that reconstructs bits from the analogue voltage the read head delivers — the hardest step between the magnetic surface and clean data.
The read head does not deliver bits but a noisy analogue waveform, which the read channel filters, clocks and decodes. Early drives used peak detection — a data separator hunting for voltage peaks — but as bits were packed tighter the peaks melted together. The 1990 answer was PRML (partial response, maximum likelihood): sample the signal and compute the most probable bit sequence instead of chasing peaks. IBM's Corsair (0663) was first. Later variants — EPRML and noise-predictive ML — modelled the noise itself and pushed density further. The read channel is therefore as much signal processing as magnetism.