4-10
Menu Operations
500300 Rev. U
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- The read and write densities will not be changed if a tape with no ID
burst is loaded.
For more information on the NO-ID DEN parameter, refer to Section 3.6.1.6.
4.3.2.20
SEISMIC: (Default = NO)
The SEISMIC configuration item was implemented in the 341X tape drives to
provide a alternative reading algorithm for “Seismic” tapes. The term Seismic Tape
refers to tapes recorded during the process of oil exploration, whereby the signals
from several geophones were recorded onto digital tape for later computer analysis.
800 CPI Seismic tapes were recorded during the early years of 9–Track tape
technology. Seismic tapes were typically recorded in the field, or at sea, under
adverse conditions. Sometimes poorly–maintained equipment was used, which
produced low–quality tapes. Reading Seismic tapes today, three decades after they
were recorded, presents a special challenge to any tape drive. Therefore, Qualstar
developed the SEISMIC option for reading difficult–to–read 800 CPI tapes.
Most 800 CPI tapes will be correctly read with the SEISMIC option OFF (normal
mode). However, the SEISMIC option may be used with any 800 CPI tape that
cannot be read error free in the normal mode (SEISMIC option OFF). The SEISMIC
option is referred to in the manual in Table 4-2.
Three algorithm changes are made when the SEISMIC option is set to ON:
1. Dynamic Character Gate Timing is enabled
2. CRC and LRC error reporting is disabled
3. Read thresholds are set to higher levels
All three of these alternatives are associated with abnormally recorded tape
characteristics.
1. A poorly–maintained drive might write irregularly spaced characters. With
Dynamic Character Gate Timing enabled, the read gate timing is varied, according
to the spacing between the current character and the previous character. This
timing change may allow tapes having closely spaced or widely spaced characters to
be read correctly. However, enabling Dynamic Character Gate Timing reduces the
read reliability for tapes with constant character spacing. A data block in which a
character misses the Character Gate will be flagged by with a Hard Error (HER).
2. Some older tapes are missing the Cyclic Redundancy Character (CRC), or the
Longitudinal Redundancy Character (LRC) that are normally found at the end of
each data block. Normally, this omission will always cause a Hard Error (HER)
indication, even though the block's data content may actually be correct. When the
SEISMIC option is enabled, the 341X tape drive will not set an HER indication for a
CRC or LRC error. However, a block data parity error will still be reported as an
HER. The CRC and LRC characters will be output if they exist on tape.
3. Tapes can be recorded over previously–recorded media. An erase head on the
recording drive is supposed to erase all remnants of previously recorded data, so
