Abstract: A digital data storage (DDS) system for reading DDS tapes employs a partial response maximum likelihood detection system which utilises redundancy in the 8-10 DC free modulation encoding to reduce low frequency noise. The system incorporates a time-varying trellis decoder which embodies some of the PR1 rules together with the rules regarding the charge state or the digital sum variation (DSV) implicit in 8-10 modulation coding. The decoder operates to reject low frequency noise such as that caused by crosstalk noise between adjacent tracks on the tape. The trellis topography has been considerably simplified by adopting a two step six state trellis which operates on bit pairs and in which the states relate to the current DSV value, and sign of the previous bit.

Abstract: There is disclosed a medium cartridge comprising: a casing; a data storage medium for storing data; and at least one shock sensor for sensing a shock condition experienced by said medium cartridge.

Abstract: An adaptive filter 24 for filtering a signal comprising first streams interspersed with second streams (for example the A and B tracks of a signal retrieved from a tape storage device) comprises respective stores for storing initial filter coefficients for each type of stream. The corresponding coefficients are loaded for the beginning of each stream. The stored coefficients for each stream may be a standard set, or they may be from the adaption process of one or more previous streams of that type. The coefficients may be taken part way through the adaption process, in a minimum error portion of the stream.

Abstract: An adaptive (FFE) filter 24 adaptively filters a signal and updates a plurality of coefficients stored on capacitors 69 to minimize the error between the filter output at 72 and a set of notional reference values supplied by a reference generator 74. The stored values of one or more coefficients are monitored and if the monitored coefficient or coefficients pass respective thresholds or leave respective ranges, indicating the onset of maladaption a default set of coefficients is forced.

Abstract: A phase lock loop for recovering a bit clock from a signal containing variations in the bit rate includes mans for resetting the frequency of said phase lock loop to a preset value if the loop loses its lock or if adverse conditions are detected. The loop may be reset if the frequency passes a preset limit or if other apparatus responsive to signal quality or signal to noise ratio detects adverse conditions. In one embodiment the signal is also subjected to automatic gain control and to adaptive filtering, and the frequency may be reset if the A.G.C. and/or the adaptive filter coefficients pass preset thresholds.

Abstract: A Digital Data Storage (DDS) data storage mechanism for reading DDS tapes containing data at either 61 kbpi or 122 kbpi has two circuits for recovering data from signals induced in a magnetic head by magnetic patterns on the tape. One circuit incorporates a clipping amplifier, a phase-locked loop (PLL), and a latch to sample the two-level signal obtained at 61 kbpi, at times defined by clock pulses from the PLL. The second circuit incorporates a PR-1 partial-response maximum-likelihood (PRML) detector to decode the three-level signal obtained at 122 kbpi. The cosine frequency response characteristic of the PR-1 channel facilitates use of the same rotary head drum and magnetic head gap for reading tapes at either bit density.