... The barrel roll is designed to protect data against bad recording tracks. Within each head group of 8 tracks, or in some modes, within a pair of head groups, the data for each ``track'' is actually recorded first on it's assigned track, then the next frame (20,000 data bits) is recorded on the next track, and the next frame on the next track and so forth. ... The options for BARREL are: . ... rolls within one group of 8 heads. roll_16 . rolls within two groups of 8 heads each. roll_auto . ...