Partial-response (PR) signalling allows a better handling of intersymbol interference and leads to a more efficient utilization of the bandwidth of a given channel. In PR systems a controlled amount of interference can be allowed. As the intersymbol interference then is known, the receiver can take it into account. PR signalling in communications permits transmission at the Nyquist rate, and provides an attractive trade-off between error probability and available bandwidth. PR signalling is extensively used in data transmission systems, such as Pulse Code Modulation (PCM) systems, and will find wide application in recording systems.
A general description of partial-response signalling principles is given by P. Kabal and S. Pasupathy in "Partial-Response Signaling", IEEE Transactions on
Communications, Vol.COM-23, No. 9, Sept. 1975, pp. 921-934.
The maximum-likelihood sequence detection (MLSD) technique for the recovery of the transmitted data sequence in receivers of transmission or recording systems using PR signalling has been described by several authors, e.g. G. D. Forney in "Maximum-Likelihood Sequence Estimation of Digital Sequences in the Presence of Intersymbol Interference", IEEE Transactions on Information Theory, Vol.IT-18, No. 3, May 1972, pp. 363-378, and R. W. Wood and D. A. Petersen in "Viterbi Detection of Class IV Partial Response on a Magnetic Recording Channel", IEEE Transactions on Communications, Vol. COM-14 (1986), pp. 454-461.
An important task in a digital data receiver is the provision of the correct timing phase for sampling the signal furnished by a transmission or recording channel. Initially, the timing phase bears no relation to the timing of the received signal. The system must yet be brought into synchronism. For fast synchronization, a known training sequence is transmitted/recorded prior to the actual data sequence. A large phase correction may be necessary for initial adjustment--"acquisition" --of the timing phase. Once the timing phase has been acquired, small corrections are necessary for compensating--"--tracking" --small differences between the rate of the signal received and the frequency of the free-running sampling clock of the receiver. The receiver clock is usually provided by a variable frequency oscillator (VFO).
A problem may occur during timing-phase acquisition when the initial sampling phase occurs at the point halfway between the desired sampling times. Then, the mechanism correcting the phase may reverse its direction of adjustment several times and the timing phase remains in the vicinity of this unstable equilibrium point for an extended period of time. Although this hangup effect occurs rarely, the length of the training sequence must be chosen such that the system may still synchronize in this situation. The hangup effect thus poses a major problem when fast and reliable synchronization is needed.
The article "Timing Recovery in Digital Synchronous Receivers" by K. H. Mueller and M. Mueller, published in the IEEE Transactions on Communications, Vol.COM-24 (1976), pp. 516-531, describes methods for adjusting the timing phase in synchronous digital data receivers for systems free of intersymbol interference. It does not mention the hangup problem or present methods to overcome it. This problem is described, for example, in the paper "Hangup in Phase-Lock Loops" by F. M. Gardner, IEEE Transactions on Communications, Vol.COM-25 (1977), pp. 1210-1214. However, the method proposed to overcome hangups has the drawback of requiring two phase detectors in quadrature, thus increasing the receiver complexity.