Method and apparatus for timing recovery in digital data communications

Abstract

In apparatus for receiving digital data signals in a digital communication system, especially an Integrated Services Digital Network, timing recovery problems associated with erratic zero crossing when multiple terminals transmit to a common receiver are mitigated by detecting the rate of change of the digital data signal between two predetermined positions and controlling sampling of the digital data signal in dependence upon the difference between such rate of change and a reference. The rate of change conveniently corresponds to the trailing 'corner' of a framing pulse, i.e. the point at which the signal commences the transition from one polarity to the other. The 'corner' is much more stable, i.e. less susceptible to variation due to multiple terminal effects, than the zero-crossing which follows it, since the zero crossing may vary relative to the 'corner'. The transition may be determined by sampling the digital data signal at two discrete positions and determining when the two resulting samples differ in amplitude by a predetermined amount. One of the two pulses, namely the one with the larger amplitude, may then be used for data recovery purposes.