Tandberg Data
Track, Tape format and Encoding Specification
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6-8
Tandberg SLR Reference Manual
6.4.2. Data Encoding; VR
2
(Variable Rate and
Randomization) RLL Rules
VR
2
RLL rules are
used for the ALRF-1
and ALRF-6 tape
formats
Prior to the recording of the data on the tape, the randomized data and control information
(plus the CRC-bytes and the seed) will be transformed into an encoded bit pattern according
to the following rules:
The VR
2
RLL encoder processes serial data beginning with the MSB of the randomizer seed,
and ending with the LSB of the last CRC byte. The user bits are identical to the channel bits,
except that a few additional bits are inserted. While processing the data the encoder
determines if the VR
2
RLL constraints are being maintained. At the point a VR
2
RLL con-
straint is violated the algorithm inserts one bit into the recorded data stream. The VR
2
RLL
constraints are defined in Section 6.4.2.2.
The variable rate-encoding does not insert bits after the LSB of the last CRC byte or during
the preamble.
6.4.2.1
Encoding
The inserted bit is a function of the previous two recorded bits. This VR
2
RLL insertion is
shown pictorially below. The logical NOT operation is represented by the symbol ! , n is the
recorded bit at time t and n+1 is the recorded bit at time t+1 cycle.
One bit Insertion (Preamble Pattern)
The second to last bit is inserted.
Bit Position
1 2 3 4 5 .............
n n+1 n n+2 n+3.............
|
Inserted Bit
One bit Insertion (Phase, Amplitude, Null)
The second to last bit (inverted) is inserted.
Bit Position
1 2 3 4 5 .............
n n+1 !n n+2 n+3.............
|
Inserted Bit
6.4.2.2
VR
2
RLL Constraints
The variable rate encoding algorithm monitors the incoming data stream for four different
conditions. If one of the conditions is violated a single bit is inserted into the data stream. The
purpose of the encoding algorithm is to ensure the encoded data contains adequate tracking
and amplitude information for reliable channel decoding and a preamble pattern is prevented
from occurring during the randomizer seed field, data field and CRC field. The four conditions
are described below. Each constraint violation inserts one bit but it is conceivable that
consecutive insertions may occur when a constraint violation continues, following a single bit
insertion.