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BARREL

Barrel roll is not used on the disk based system so can be ignored in essentially all cases.

BARREL sets the mode of the barrel roll. 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. After 8 or 16 frames, depending on mode, the track is back on it's original assigned head. With this happening, if a recorder track is bad, some of all channels in the roll group are lost, but no channel is lost completely. Thus there is just a drop in sensitivity rather than a distortion of the information being measured. Very nearly all users should take the default of roll_auto. The options for BARREL are:

Barrel roll should be ok for PCFS systems (VEX files). In Dec 2000 the definition of barrel rolling has been updated to reflect the discovery that the current (VLBA) practice is time reversed with respect to the documentation.

Barrel roll is turned off for disk systems.

roll_off

turns off the barrel roll.

roll_8

rolls within one group of 8 heads.

roll_16

rolls within two groups of 8 heads each.

roll_auto

tells the on-line system to pick the best roll it can do.

One case where the user may need to set the roll is when the data will be correlated with 2048 point FFT's (1024 point output spectra). This cannot be done with a 16 track roll and so one of the lesser rolls should be forced.

• Argument: A string of up to 9 characters.
• Options: One of the 4 options listed above.
• Default: roll_auto
• Usage: Defaults to previous station.
• Example: BARREL=roll_8