Although the linear tape recording method is simple and efficient, it was not suited to recording streaming data, such as broadcast video signals once higher capacities beyond open reel tapes were required. The time taken to change tracks, although not a big issue with modern drives containing large data buffering would have resulted in possible loss of video frames.
The first helical scan recording equipment was developed for video recording of analogue signals, whereby the tape is moved slowly and the read/write drum spins at high speed. This recording technique was extended to allow digital data to be stored, and continues to be developed.
The original concept saw the data stored perpendicular to the media which required guard bands, but it was soon discovered that the accuracy and recording density could be increased significantly by laying the data down diagonally. By using two record heads with a different azimuth, it is possible to dispense with recording guard bands, allowing the recording density to be increased, even slightly overlapping data tracks.
The read/write drum also incorporates a pair of read heads, which can immediately read the data written to the tape in order to check its veracity. This allows for immediate error correction if the tape drive firmware was programmed for it, allowing another copy of the data to be rewritten if required.
Recorded Block Locations
Helical scan tapes also include a control track, which allows a fast scan of the tape to be performed to locate tape marks, the end of data marker and any block number required. It is only since memory in cartridge was introduced, that data about the number of blocks written to the tape and the positions of data blocks and tape marks can be stored.
In order to avoid increasing excess wear on the tape while a particular position is being located, the tape is pulled away from the read/write drum during the process. The tape is placed back in contact with the spinning read/write drum once the approximate block position has been located, with the final positioning done by reading the data tracks.
Additional Data Stored
If the tape contains memory in cartridge the end-of-data, and the block locations on the tape can be stored. All tapes will record the following information in a system area at the front of the media:
• Age of the cartridge
• Number of times cartridge has been loaded
• Count of errors encountered
• Calibration information
• Initialisation data