1 Upgrade components

This involves replacing the old Mark III formatter with a new, plug-compatible Mark IV formatter. It supports all the familiar Mark IIIA formats, most of the ``standard'' VLBA formats (with the exception that it always formats in data replacement mode), and a set of new Mark IV formats. Some highlights of its capabilities include:

• Has 32 ``video-signal'' input channels to accommodate the (possibly) upgraded 16 video converters. Each video channel has two-bit A/D converter samplers (``sign'' bit, ``magnitude'' bit) providing a total of 64 bit streams.
• Has fan-out/fan-in capabilities: can multiplex one input bit stream (sign or magnitude of an input video signal) onto multiple recorder tracks and multiple bit streams onto a single recorder track.
• Can drive two head stacks for 64 simultaneous output tracks. (Can be further upgraded to drive 128 simultaneous output tracks, i.e. four head stacks.)
• Can output at 16 Mbits/second per track (18 Mbits/s with parity bits included.)
• Can manage the write-enable functionality previously found in tape controller.

This consists of:

• a new head assembly module
• one Mark IV read module
• two Mark IV write modules
• one power distributor module
• new I/O board for the tape recorder.

The head assembly module replaces the old read and write interface boxes (behind the tape recorder reel panel) with new head interface and preamplifier boards fitted onto a new ``septum divider.'' The two head stacks of Mark IIIA are reused and placed side-by-side in the upper head block position.

The read and write modules complement the head assembly to form the complete R/W electronics. The power distributor module is required to create supply voltages for these modules from the existing supply voltages. I/O board has to be swapped because the old one cannot control new read/write modules.

Reasons for upgrading R/W electronics:

• old electronics supports only 28 write and 14 read tracks
• old electronics cannot write at 16 Mbits/s per track
• old electronics cannot write using two head stacks simultaneously.

Note that the new I/O board does not support selection of enabled write tracks. This is handled by the Mark IV formatter. Similarly, note that reading from tape at 16 Mbits/s is not supported by the Haystack design of the Mark IV read module.

The R/W subsystem can be further upgraded to accommodate four head stacks. This requires:

• upgrading the formatter to have 128 output track drivers
• two additional narrow-track head stacks
• additional mechanics ie. the lower head block assembly
• another head assembly module for the lower head block position
• two additional Mark IV write modules.

The existing Mark III video converters are retrofitted to allow bandwidths of 8 and 16 Mhz. This is done at the expense of the two existing bandwidths whichp were seldom used. The correspondence between old and new bandwidth codes is illustrated in the following table (deduced from NASA FS8.2x source code, ``.../fslb/ivced.f''.)

Bandwidth          Mark IIIA    Mark IV
code in MAT
----------------------------------------
0                  External     External
1                  0.125        0.125  (*
2                  0.25         new 16
3                  0.5          0.5    (*
4                  1            new 8
5                  2            2
6                  4            4

*) These filters are not necessarily available in every existing
MkIII video converter.

The filter change involves:

• replacing the existing filter boards in each VC (two boards)
• changing the narrow-band filters into wider ones (requires acquiring four passive filters per VC: 2x (8 and 16 MHz))
• changing all precision capacitors on the single-sideband mixer board.

Eventually, one might want to acquire two additional video converters to feed all the 32 input channels of the Mark IV formatter.

This modification involves changing the so-called ``precision plate'' of the recorder into a new, higher-precision one. Also, some other smaller recorder parts (e.g. idle roller) are replaced.

The purpose of this upgrade is to allow the use of thin tape, the 13 μm tape variant that allows for 18000 feet of tape on standard 14 inch reels. This upgrade also improves tracking with ordinary thick tape.

Upgrading the tape path is not mandatory if Mark IV recordings are being made with thick tape only.

The current decoder cannot decode at rates above 8 Mbits/s. Currently, there are no plans to retrofit this---instead, recordings are going to be played back at half speed for checking purposes.

The data buffer cannot feed the system computing phase calibration information faster than at 9600 baud. This will be compensated by replacing the UART chip and its clock generator with new ones which allow communication at 115200 baud. This should allow reading the complete contents of the 128 kilobyte buffer in under 13 seconds.

The MAT commands for the tape transport and the formatter change as the result of this upgrade. Therefore, the field system must ``talk'' to these devices in a new way. This support is included in the NASA FS8.2x and thus it should be able to control Mark IV hardware in a Mark IIIA compatible fashion.