Abstract: A magnetic tape includes plural diagonal tracks, each storing a preamble sequence close to user data such that a transducer for reading the track reads the preamble sequence immediately before reading the user data. The preamble sequence is such that when the track is read by the transducer the preamble sequence has a power sequence with a power spectrum substantially equivalent to the power spectrum of the user data. An AGC circuit responds to the transducer.

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: A magnetic tape includes plural diagonal tracks, each storing a preamble sequence close to user data such that a transducer for reading the track reads the preamble sequence immediately before reading the user data. The preamble sequence is such that when the track is read by the transducer the preamble sequence has a power sequence with a power spectrum substantially equivalent to the power spectrum of the user data. An AGC circuit responds to the transducer.

Abstract: A method of encoding a digital data stream for writing to a digital storage (DDS) format tape data storage medium comprises writing a byte stream to a preamble region, immediately preceding a user data region. The byte stream has a power spectrum substantially similar to a power spectrum of the user data region. An automatic gain control circuit responds to the preamble data stream in a substantially similar fashion to its response to the user data stream.

Abstract: A method of encoding a digital data stream for writing to a digital storage (DDS) format tape data storage medium, comprises writing a byte stream to a preamble region, immediately preceding a user data region, said byte stream having a characteristic of having a power spectrum (900) substantially similar to a power spectrum of said user data region. An automatic gain control circuit (504) responds to the preamble data stream in a substantially similar fashion to its response to the user data stream. The disclosure may overcome a problem in read channels detecting prior art DDS-1 to DDS-4 format preamble regions in that an automatic gain control circuit of the read channel responds differently to a cyclical preamble region in the known DDS formats, compared with its response to user data, giving rise to increased bit error rate in a sequence detector device of the read channel.

Abstract: An automatic gain control 20 is adapted to compensate to equalise the gain for signals of different characteristics (e.g. the preamble region and the main data region of a signal retrieved from a tape). The signals are amplified by a VGA 32, the output of which is monitored by a peak detector 44, whose output is compared at amplifier 48 with a respective target value for the particular signal region from a associated counter or register 60, 62, 65, to control the gain control signal applied to the VGA 32 by a gain control 50. The output of the comparator amplifier 48 is monitored before and after the transition between the first and second signal regions and the reference values in the appropriate register 60, 62 incremented or decremented in accordance with any step in the output of the amplifier 48, substantially to equalise the gain before and after the transition.

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.