Systems and techniques relating to interpreting signals on a noisy channel with polarity uncertainty. A signal processor, such as a read channel transceiver device usable in a magnetic recording system, includes a detector operable to find a data pattern that indicates control information in a read signal and to determine a signal polarity of the read signal by determining Euclidean distances between a sampled sequence from the read signal and multiple possible sequences corresponding to preamble-shifted and polarity-reversed versions of the data pattern. The read signal is obtained from a partial response channel, such as in a storage device, and the data pattern can be a servo mark selected based on Euclidean distances between the servo mark and preamble-shifted and polarity-reversed versions of the servo mark generated according to a target channel and an encoding scheme specified for the storage medium.