Current Status of the Gigabit VLBI Recorder System

Junichi Nakajima (nakaji(AT)

Kashima Space Research Center
Communications Research Laboratory
893-1 Hirai, Kashima, Ibaraki 314, Japan

1. Introduction

In the end of 1994, joint Japanese VLBI development group has decided to employ TOSHIBA GBR-1000 recorder for the next generation VLBI system. The project started from 1995 and we have completed the first 1-Giga-bit cassette type data recorder in 1996. Prof.Chikada, Prof.Kawaguchi, Dr.Miyoshi (NAO), Mr.Hama, Mr.Kaneko,Mr.Kiuchi and Nakajima (CRL) are the contributed persons in this recorder project.

2. Gigabit Recorder System Function Test

The next generation recording system consists of two major parts. One is VLBI data interface unit (DRA-1000) and another is Gigabit Recorder itself (GBR-1000). VLBI data from any kind of A/D samplers are connected to the VLBI data interface (DRA-1000). These data are transferred to the recorder(GBR-1000) according to its HDTV data recording procedure. The random data including radio star noise are recorded as a part of uncompressed HDTV flame format. The 1024 Mbps cassette recorder now available in the market needed slight speed up modification. It has been completed before summer. Currently we are debugging the VLBI data interface unit which is designed for VLBI and other scientific data recording. The three major features of this DRA-1000 are The gigabit recorder UTC sync. recording test was carried out with recorder company TOSHIBA, YEM (Yamashita Engineering Manufacture), NAO and CRL staff in August. Two GBR-1000 units are prepared. In this experiment test ramp generators are used instead of noise from radio telescope. At defined certain UTC 1 PPS, the generated ramp pattern switched to the other. We are investigating the position of data change point on the tape and confirming whether the recording is accomplished with correct UTC flame in the tape. The DRA-1000, VLBI interface is handling the 1024 Mbps data with 32 MHz clock 32 parallel signal lines. Thus we need 31.25 ns digital accuracy for recording/play back logical sequence. This Gigabit Recorder does not depend on inserted time code as well as K3 (Mk-IIIa), this function test is most important step though the other longtitudal system does not need. Gigabit recorder refer (or generate) its tape UTC control track both in longtitudal and helical tracks. Here I introduce the UTC sync. functional test with photo. This will let you understand the next generation recorder system concept. Photo 1 shows the test bench. GPIB command PC, Ramp data generator, DRA-1000, digital oscilloscope and GBR-1000 are prepared for function test. Photo 2 is DRA-1000, the VLBI interface is disassembled under the test. The interface receive GPIB commands from observation computer or correlation process control computer. This interface convert the VLBI sampled bit stream and GBR-1000 HDTV format in both direction. Photo 3 shows the change of recorded ramp patterns on the tape. In this case the assigned REC start UTC is located in the middle of the certain GBR-1000 HDTV flame. Since the GBR-1000 is produced for the high-definition television originally. The data is visualized by PC multi-scan monitor during both in observation and correlation processing. This is one of great advantage. We will be able to check the recording condition through the monitor. For example, an AD sampler is saturated and send biased data, or an AD sampler is malfunction and no data, apparently the monitor screen will show corresponding single color. This feature is useful when non-skilled operator must diagnose the system in each VLBI station. In actual radio astronomical observation, the data should be seen as random pattern(like TV sand storm) from the assigned recording start point.

In addition all kind of time line command are examined. The "time line command" is a recorder command with defined execute time. When we carry out the observation, we need several REC/STOP commands before a tape run comes to the end. These sequence are programmed in DRA-1000 CPU intelligent interface as time line events. When we correlate the data, also the same play back sequence are programmed several recorders in advance. This will reduce communications between recorders and controller.

3. Future schedule

Sampler and recorder interface test is planned in October. Samplers are already completed. We will carry out 1 Gbps domestic VLBI experiment as soon as possible to obtain test correlation data for 1 Gbps VLBI correlator. All equipment for 1 Gbps VLBI hardware will be available by the middle of 1997.

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Updated on October 28, 1996. Return to CONTENTS