From: Jon Hagen
Date: 5-31-01


I. Introduction

Our recently acquired VLBI terminal (data acquisition system) is referred to as a VLBA MARK IV or VLBA4 terminal, part VLBA terminal (the data acquisition equipment at each of the VLBA sites) and part MARK IV vlbi terminal (successor to the MARK III system). The VLBA4 is supported by the Field System (the ever-evolving universal control program used in VLBI) and is expected to become a popular configuration,The two citations below show that VLBA4 is already a recognized designation. Note: "VLBA4" is also the name of one of the VLBA recording modes:"VLBA1,VLBA2, VLBA3, VLBA4", but here, VLBA4 is synonymous with VLBA MARK IV.


The following paragraphs from an Australian VLBI memo,


Gives the type of Data Acquisition Rack present. This is mainly to identify the type of formatter, which will let the program know about the capabilities available. Valid types are: VLBA, MKIV, VLBA4, MKIII, S2, K4, VSOP and NONE. The main non-obvious option above is VLBA4, which is a VLBA DAR but with a Mark IV formatter installed. This will have VLBA BBC's and IF switching, but Mark IV formatting characteristics.


Gives the type of tape recorder(s) present Valid options are: VLBA, MKIV, VLBA4, MKIII, S2, K4, VSOP and NONE. The VLBA4 option is for VLBA recorders which have been modified for 16 Mbps per track operation. They are usually associated with MKIV or VLBA4 DARs.

And this paragraph is taken from the attached memo: "VLBI Data Acquisition and Recorder Systems; A Summary and Comparison", by William T. Petrachenko:



There are several VLBI Data Acquisition (DAS's) and recorder systems currently in use. In the Mk3 family there are the Mk3a, Mk4, VLBA geodetic VLBA (VLBAg) and VLBA4. In the K4 family there are the K4-type1, k4-type2 and k4-VSOP. The S2 family......

II. VLBA4 Terminal as a Member of the Mk3 Family

The VLBA4 Terminal is classified by Petrochenko as a member of the Mk3 Family, which he describes as follows:
The Mk3 family of systems is characterized by its use of a large number of baseband converters (BBC's) and a multi-track longitudinal tape drive. [Note: the term BBC and video converter (VC) refer to the same thing.] In geodesy, a large number of BBC's are used to synthesize the wide effective IF bandwidths required to achieve high precision in the determination of the delay observable. Physically, the Mk3 systems are packaged in two 6' high racks. One rack holds the DAS and the other holds the recorder. All DAS modules are packaged in VLA-style bins and the rack is permanently wired using NIM connectors so that no cables need to be connected or disconnected when modules are exchanged. The recorder uses a modified Metrum 9600 transport equipped with a 36-head narrow track headstack which is capable of recording up to 16 passes on a tape with all heads enabled. Data are stored on 1'' wide tape wound on 14'' diameter open reels. There are 8 BBC's in the VLBA system and 14 in each of the Mk3a, Mk4, VLBAg and VLBA4 systems.


III. Description of our VLBA4 Terminal

Our VLBA4 system is packaged in two racks, each 6' 7½" high. We have 8 VLBA BBC's, with wiring in place to increase this number to14. The modifed Metrum 96(C) recorder is equipped with 72 heads (2 stacks of 36). In each stack, only the inner 32 heads are used. The Mark4 formatter can output 64 data streams at rates up to 16 Mbs. Therefore our maximum data recording rate is 64 x 16Mbs = 1024 Mbs. Note: the maximum recording rate can be upgraded to 2048 Mbs by installing two additional head stacks in the recorder and an expansion board in the formatter.
The head width is 38µ and the pitch is 698.5µ. The overall headstack width is therefore 24485.5µ (35 x 689.5µ +38µ) and the allowed range of motion about a centered position is +/- 457.25µ ( ½ x [25400-24485.5] ). Each headstack has its own Inchworm positioning motor. Track spacing is 48µ, so the 698.5µ pitch allows 14 passes be written on a tape if a single head is used or 7 passes when both heads are used, for a total of 448 tracks. (14 x 32 = 7 x 64 = 448). Note: a proposed upgrade would sqeeze in 16 passes, for a total of 512 tracks.

At the maximum data rate, each of the 64 data streams from the formatter transfers 16Mbps of data. With parity information added, the maximum bit rate to each head is 18Mbps. This requires that the tape speed be 18E6 bps/56.25E3 bits per inch = 320 inches/sec.

Data is written on the tape with a maximum longitudinal density of 56.25 kb/inch. The maximum number of bits stored on an 18,000ft reel of tape is therefore 448 tracks x 56.25E3 bits/track/inch x 18000 ft x 12 inches/ft = 5.44e12 bits = 5,443 Gb or 680 GBytes.

Our Mark 4 formatter has 32 analog input ports, enough for USB and LSB from 16 BBCs. The system uses 16 of these inputs for the USB and LSB signals from our 8 BBCs. When and if the additional 6 BBCs are installed, we will use another 12 inputs for a total of 28 out of 32. The formatter contains a 2-bit digitizer for each one of its 32 analog input ports. There are 64 output lines to the two head stacks.

In the formatter, the 64 output streams from the digitizers are not simply hardwired to 64 heads. At low data rates, data from more than one source can be written on a track. At high data rates, data from one source can be spread across several tracks.

(See MARKIV MEMO #230.2, "Mark IIIA/IV/VLBA Tape Formats, Recording Modes and Compatibiliity")

IV. Original Documentation

Our VLBA4 terminal came with the following documentation:


1. Signatron Tape Drive Documentation: one volume, a 3" thick three-ring binder with no title page or table of contents. This volume contains copies of Haystack circuit diagrams, mechanical drawings, parts lists, and timing diagrams, but no text. The first section is "Dry Air Kit"

2. Signatron Data Acquisition Rack (DAR) Documentation: a 4" thick three-ring binder, again with no title page or table of contents, or text. This volume contains copies of Haystack schematics for the formatter, decoder, BBCs, etc. At the end is a Signatron-generated wiring list for the rack.

3. Metrum Technical Manual - "Maintenance Instructions", Volume 1 of 2 and Volume 2 of 2, each in a metal back binder, 3" thick.

4. Metrum Tehnical Manual "Operators Instructions for Model 96(C), Black plastic spine,   1" thick.

V.Supplementary Documentation



5. Metrum Recorder Documentation V1.2.4 CD ROM Disk compiled by Michael Wunderlich of Bonn. Contains Haystack schematics and memos, text from Interferometrics manuals, etc. Very useful.

6. VLBA Technical Report No. 14, Rev. B Operation of the VLBA Tape recorder at the VLA and the VLBA Sites, C. Janes and G. Peck, Nov 1993.