Current Status of the Gigabit VLBI Recorder System
Junichi Nakajima (nakaji(AT)nict.go.jp)
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
REC/PLAY BACK data conversion between VLBI bit stream and HDTV flame.
Data independent recorder UTC synchronization including rotary-head phase
control.
Time line command could be defined for event procedure control.
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.
Photo 1.
Photo 2.
Photo 3.