B.5 Head Positions and Track Assignments
This appendix describes the default track and head assignments used on the VLBA for
projects scheduled using SCHED. These are gory details that most users should not need
to be concerned about. Just let SCHED use its defaults and the observations should be
ok.
The VLBA uses 14 head positions. Both the positions and the order in which they are used are the
same for all projects. The head position for each index number are in the following table in
microns.
HEAD POSITIONS
--------------------------------------------
Forward Passes Reverse Passes
Index Offset Index Offset
--------------------------------------------
1 -319 2 31
3 -271 4 79
5 -223 6 127
7 -175 8 175
9 -127 10 223
11 -79 12 271
13 -31 14 319
-------------------------------------------
All forward passes are to be written in the negative offset positions and all reverse passes are to be
written in the positive offset passes. This ensures that nearly all adjacent tracks are in the same
direction and minimizes the required guard bands.
The following tables give the head index position and the and the set of heads that are used for each
pass for experiments using 1, 2, 4, and 8 passes per head position: Number: 1 2 3 4 5 6 7 8 9 10 11 12 13
14
HEAD INDEX and HEAD SET for each PASS
---------------------------------------------------------------
For 1 Pass Per Head Position:
Passes 1 - 14
Head Index 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Head Set 1 1 1 1 1 1 1 1 1 1 1 1 1 1
---------------------------------------------------------------
For 2 Passes Per Head Position:
Passes 1 - 14
Head Index 1 2 1 2 3 4 3 4 5 6 5 6 7 8
Head Set 1 1 2 2 1 1 2 2 1 1 2 2 1 1
Passes 15 - 28
Head Index 7 8 9 10 9 10 11 12 11 12 13 14 13 14
Head Set 2 2 1 1 2 2 1 1 2 2 1 1 2 2
---------------------------------------------------------------
For 4 Passes Per Head Position
Passes 1 - 14
Head Index 1 2 1 2 1 2 1 2 3 4 3 4 3 4
Head Set 1 1 2 2 3 3 4 4 1 1 2 2 3 3
Passes 15 - 28
Head Index 3 4 5 6 5 6 5 6 5 6 7 8 7 8
Head Set 4 4 1 1 2 2 3 3 4 4 1 1 2 2
Passes 29 - 42
Head Index 7 8 7 8 9 10 9 10 9 10 9 10 11 12
Head Set 3 3 4 4 1 1 2 2 3 3 4 4 1 1
Passes 43 - 56
Head Index 11 12 11 12 11 12 13 14 13 14 13 14 13 14
Head Set 2 2 3 3 4 4 1 1 2 2 3 3 4 4
---------------------------------------------------------------
For 8 Passes Per Head Position
Passes 1 - 14
Head Index 1 2 1 2 1 2 1 2 1 2 1 2 1 2
Head Set 1 1 2 2 3 3 4 4 5 5 6 6 7 7
Passes 15 - 28
Head Index 1 2 3 4 3 4 3 4 3 4 3 4 3 4
Head Set 8 8 1 1 2 2 3 3 4 4 5 5 6 6
Passes 29 - 42
Head Index 3 4 3 4 5 6 5 6 5 6 5 6 5 6
Head Set 7 7 8 8 1 1 2 2 3 3 4 4 5 5
Passes 43 - 56
Head Index 5 6 5 6 5 6 7 8 7 8 7 8 7 8
Head Set 6 6 7 7 8 8 1 1 2 2 3 3 4 4
Passes 57 - 70
Head Index 7 8 7 8 7 8 7 8 9 10 9 10 9 10
Head Set 5 5 6 6 7 7 8 8 1 1 2 2 3 3
Passes 71 - 84
Head Index 9 10 9 10 9 10 9 10 9 10 11 12 11 12
Head Set 4 4 5 5 6 6 7 7 8 8 1 1 2 2
Passes 85 - 98
Head Index 11 12 11 12 11 12 11 12 11 12 11 12 13 14
Head Set 3 3 4 4 5 5 6 6 7 7 8 8 1 1
Passes 99 - 112
Head Index 13 14 13 14 13 14 13 14 13 14 13 14 13 14
Head Set 2 2 3 3 4 4 5 5 6 6 7 7 8 8
---------------------------------------------------------------
The following give the lists of the first track (head) for each channel for experiments using all heads in a
pass. Channels using multiple tracks because of fan out and/or 2 bit mode use the listed track plus
successive even or odd tracks. For the multiple pass per head position modes, the listed
tracks are divided into groups and successive passes (at a head position) use the successive
groups.
FIRST HEAD per CHANNEL
---------------------------------------------------------------
Tracks Per Channel: 8
Modes: (VLBA1:4, 2 bit)
First Heads: 2 18 3 19
---------------------------------------------------------------
Tracks Per Channel: 4
Modes: (VLBA1:4, 1 bit), (VLBA1:2, 2 bit)
First Heads: 2 10 18 26 3 11 19 27
---------------------------------------------------------------
Tracks Per Channel: 2
Modes: (VLBA1:2, 1 bit), (VLBA1:1, 2 bit)
First Heads: 2 6 10 14 18 22 26 30
3 7 11 15 19 23 27 31
---------------------------------------------------------------
Tracks Per Channel: 1
Modes: (VLBA1:1, 1 bit)
First Heads: 2 4 6 8 10 12 14 16
18 20 22 24 26 28 30 32
3 5 7 9 11 13 15 17
19 21 23 25 27 29 31 33
---------------------------------------------------------------
Mark~III Mode B with channels in ascending frequency order.
Sidebands should alternate.
First Heads: 18 4 20 6 22 8 24 10 26 12 28 14 30 16 32 2
19 5 21 7 23 9 25 11 27 13 29 15 31 17 33 3
---------------------------------------------------------------
Mark~III mode E with channels in ascending frequency order.
First Heads: 4 6 8 10 12 14 16 2
18 20 22 24 26 28 30 32
5 7 9 11 13 15 17 3
19 21 23 25 27 29 31 33
---------------------------------------------------------------
Mark III mode A is not normally used with VLBA systems because the gain of only 2 tracks over mode
A is not worth the extra tape usage. When it is used, the assignments are special because a decision has
to be made about what subset of tracks to use.
Perhaps an example will help clarify all of this. Consider an experiment that uses 2 passes per head
position and 4 tracks per channel. There will be 4 channels and the following pattern is used for the
first few passes.
Pass Head Dir Heads.
pos Chan 1 Chan 2 Chan 3 Chan 4
1 -319 For | 2 4 6 8 | 10 12 14 16 | 18 20 22 24 | 26 28 30 32 |
2 31 Rev | 2 4 6 8 | 10 12 14 16 | 18 20 22 24 | 26 28 30 32 |
3 -319 For | 3 5 7 9 | 11 13 15 17 | 19 21 23 25 | 27 29 31 33 |
4 31 Rev | 3 5 7 9 | 11 13 15 17 | 19 21 23 25 | 27 29 31 33 |
5 -271 For | 2 4 6 8 | 10 12 14 16 | 18 20 22 24 | 26 28 30 32 |
6 79 Rev | 2 4 6 8 | 10 12 14 16 | 18 20 22 24 | 26 28 30 32 |
7 -271 For | 3 5 7 9 | 11 13 15 17 | 19 21 23 25 | 27 29 31 33 |
8 79 Rev | 3 5 7 9 | 11 13 15 17 | 19 21 23 25 | 27 29 31 33 |
etc.
Note that when there is both fan out and 2 bits per sample, the “sign” bits are fanned out over the first
n tracks (eg 4 for VLBA1:4), and then the “magnitude” bits are fanned out over the next n
tracks.