Word synchronization for servo read signals in tape drives

A tape drive, comprising:

• a read/write head for reading and writing data on magnetic tape;
• a servo reader associated with the read/write head for reading a PPM-encoded servo pattern on the magnetic tape to produce a servo pattern read signal;
• and
• a servo controller for processing the servo pattern read signal to produce positional information for controlling relative positioning of the tape and read/write head;
• wherein the servo controller includes:
• a soft output detector for processing the PPM-encoded servo pattern read signal to produce a series of soft output samples corresponding to respective bits encoded in the servo pattern;
• and
• a sync word detector for producing block correlation values for respective positions of a sliding L-sample block in the series of soft output samples by calculating, at each block position, bit correlation values indicating correlation between respective samples and corresponding bits of the sync word, and summing each bit correlation value minus a predetermined function of the corresponding sample value;
• said sync word detector being adapted to detect a sync word at the block position with the maximum block correlation value in an (N+L−1)-sample sequence of the series of soft output samples.

A method for detecting L-bit sync words occurring at N-bit intervals in a PPM-encoded servo pattern read signal read from magnetic tape in a tape drive, the method comprising the steps of:

• processing the PPM-encoded servo pattern read signal to produce a series of soft output samples corresponding to respective bits encoded in the servo pattern;
• calculating bit correlation values at each block position indicating correlation between respective samples and corresponding bits of the sync word;
• summing each bit correlation value minus a predetermined function of the corresponding sample value to produce block correlation values for respective positions of a sliding L-sample block in the series of soft output samples;
• and
• detecting a sync word at the block position with the maximum block correlation value in an (N+L−1)-sample sequence of the series of soft output samples.