We show how to compute the error probability of frequency-shift keying with limiter-discriminator-integrator detection in the presence of narrowband filters in the transmitter and receiver when the symbols are pseudoternary, i.e. they are generated from binary symbols using duobinary, modified duobinary and dicode rules. We show numerical results assuming a Butterworth filter in the receiver with 2, 3 and 4 poles. We optimise the sampling time, frequency deviation, filter bandwidth and threshold setting, and the conclusion is that the duobinary is the best partial response system. However, when compared with an uncoded, binary system the duobinary system is better only when the bandwidth is very small.