Your Mother Saves Data on EightTrack Tapes 4371

I had wondered about this as well.

Here is the justification given in the ansi x3.22-1973 9trk 800bpi spec:

a2.4.3.1 the track numbering is logically buttigned 1 through 9, starting at the reference edge of the tape a2.4.3.2 the bits buttignments were selected for maximum reliability and preformance according to the following criteria: (1) it is more probable that a '1' bit will be misinterpreted as '0' than the converse (2) errors defined in (1) are more likely to coccur near the edges of the tape than near the center (3) certain portions of the code will be used more often than others. most frequently used will be the numerics (0-9) and next in order will be the alpha characters. a2.4.3.3 based on these criteria the following bit buttignment evolved: (1) bit z was buttigned to track 7. this bit will be used in non-ascii code applications where the binary weight of 2^7 on track 7 is required. compatibility with other 9-track systems, when recording binary information is enhanced by this placement of bit z. (2) the parity bit is considered critical especially in double numeric packing since it is possible to generate a configuration that consists of the parity bit only; namely 'zero,' 'zero'. it is anticipated that this configuration will happen frequently; therefore P was placed near the center of the tape on track 4. (3) bits 5 and 6 are critical because '1' bitsa re always present in the numerics. therefore,b^5 and b^6 were placed adjacent to P on tracks 3 and 5, respectively. (4) since bit 7 is a '1' in all alpha characters, it is considered next in live and placed adjacent to b^6 on track 6. (5) bits 3 and 4 are considered the least critical for the criteria above and therefore were placed on the two edge tracks 1 and 9. (6) bits 1 and 2 are each '0 or '1' with equal probability and were therefore buttigned the two remaining tracks, 2 and 9.