Apparatus for modulating data having a basic data length of m bits, to
a variable-length code (d, k; m, n; r) having a basic code length of n bits. A
sync signal is added to a received train of codes after a minimum run. The sync
signal has a pattern that is repeated twice continuously. The minimum run is repeated
no more than six times; and the sync signal exhibits seven types of sync signal
IDs, with any adjacent two bits of a sync signal ID being spaced apart by a distance
of 2.