PR4 coding is a type of partial-response bit detection that increases areal bit density over the peak detect technique. In magnetic recording, the detection of a domain reversal on the media produces a pulse or "blip." Unlike peak detection, PR4 involves sampling at points other than the peak of the blip. The read signal is a superposition of successive blips of alternating polarity.
PR4 coding allows a higher bit density on magnetic media over peak detect. Magnetic transitions on media appear as pulses when reading. PR4 allows pulses to be packed closely so that they overlap. Pulses are sampled on shoulders rather than peaks. Ideally, there are two samples per pulse and one sample of every pulse aligns with and superimposes on one sample of the adjacent pulse. The influence of neighboring pulses on samples is called intersymbol interference (ISI).
Because magnetic transitions alternate polarity (a north to south transition can only be followed by a south to north transition), the read samples have three possible states. Any pulse can be followed by either (1) no pulse; or (2) a pulse of the opposite polarity. A random data stream encoded with nonreturn to zero inverted (NRZI) coding ensures that data will be written with these constraints.
An ideal pulse produces exactly two samples. The width of the pulses depends on the speed of the magnetic media under the sensor and the height of the sensor over the media. When pulse width influences samples beyond the two desired samples, the undesirable ISI can be filtered out with a digital filter and/or analog prefilter. The sampled data stream looks chaotic for pulse widths greater than 2, but filtering renders the sample levels distinct.