Abstract: In visual display apparatus in which characters are displayed by modulation of a TV type raster on a cathode ray tube, the normally uniform spacing of the sweep lines of the raster is selectively modified to produce groups of lines having decreased spacing therebetween. The groups of lines are modulated to display rows of characters on the screen of the cathode ray tube. The spacing of the remaining lines between the groups is at the same time increased to provide a wider separation between the rows of characters thereby improving the aesthetic appearance of the display. Video modulation, synchronizing and control signals are obtained from a composite input signal. The control signals are utilized to operate a sweep modification circuit.

Abstract: In a magnetic-disc data storage device using exchangeable discs for the recording of data on circular tracks, a reference track is provided on the disc and during interruptions in the transfer of data, a read-write head is moved to the reference track and the error in its expected position, due to dimensional changes in the disc or the storage device or due to misalignment of the head, is measured. In subsequent data tracks access the position of the head is corrected by error measured so that the head is correctly aligned with the selected data track. Measurement of the error is carried out more frequently when the error is changing rapidly.

Abstract: The positioning of the head positioning mechanism of a magnetic disc file is controlled by a servo system arrangement which involves separately monitoring the temperature conditions in the vicinity of the center track of a disc file and the head positioning mechanism; producing related electrical signals which are used in conjunction with signals defining the address of a required track of a disc and a reference voltage to obtain a temperture corrected signal which is used to produce a head positioning control signal.