K-4 VLBI Data-Acquisition System

Hitoshi Kiuchi (kiuchi(AT)nict.go.jp)

Communications Research Laboratory
4-2-1 Nukui-kita, Koganei, Tokyo 184-8795, Japan

Abstract: We have developed a new VLBI system based on the conventional K-4 system. The system was designed to achieve automated operation throughout the entire process, it is operated one operator for observation arrangement and correlation processing. The KSP system has two data transfer systems, one is a tape-based system and the other is a real-time system. The new acquisition system is designed to interface both systems, the two systems are possible to be operated simultaneously. The new acquisition system has multi-bit quantization sampling modes. The total data rate is up to 256 Mbps, and up to 16 video channels can be selected. Filterings are carried out in the digital filter. The digital filter is used because it results in good phase characteristics. This system is specially designed for the Key Stone Project, and an input interface unit is incorporated in the VSOP.

1. The New K-4 System

The KSP data-acquisition system (Figure 1), a high-end version of the K-4 system [Kiuchi et al., 1997], has a maximum recording rate of 256 Mbps and is fully automatic. The different features of the systems are given in Table 1. The data-acquisition system consists of a reference distributor, IF distributor, local oscillator, video converter, input interface, output interface, data recorder, and digital mass storage system (automatic tape changer). The local oscillator synthesizes the local frequency signal for the video converter (Figure 2). This video converter converts windows in the IF-signal (500-1000 MHz) input into video signals. The frequency conversion is done by a image rejection mixer using single-sideband conversion. The output interface unit is used at a correlation processing station for tape-based VLBI.

Figure 1. Block diagrams of the new K-4 (observing station configuration).

Figure 2. Block diagram of the new K-4 video converter and local oscillator.

Table 1. The different features of K-4 systems and other VLBI systems.

1.1. Input Interface Unit

The input interface unit samples the video signal from the video converter (16-channel max.), and sends the digital data (256 Mbps max.) to the data recorder and/or the ATM transmitter (for a real-time VLBI system: in this issue) together with the time data. A block diagram of the input interface unit is shown in Figure 3. The acquisition system has one-bit and two-bit sampling modes for VLBI, and also has 4- and 8-bit sampling modes for general-purpose data acquisition. The anti-aliasing filtering is done in analog (32 MHz), and after sampling, the 16-MHz, 8-MHz, etc. filtering is done by the digital filter. The advantages of using the digital filter are that it can obtain good phase characteristics and can reduce coherence loss for wide bandwidths. Suitable coefficients of the digital filter can be selected for each observation; for example, the digital filter is used as a bandpass filter for the line observation. The coefficients of the basically lowpass filter needed for Nyquist sampling are provided in the read-only memory (ROM). The output rate of data to the recorder or the ATM transmitter can be selected from five rates ranging from 16 to 256 Mbps. Time-code insertion can be at uniformly bit-space or at uniformly time-interval, or no time-code can be selected. In real-time VLBI, an uniformly bit spaced time code is used, as required by the ATM side. The suitable quantization threshold level of the A/D converter is adjustable. The configuration for one- and two-bit sampling is shown in Table 2. This input interface can be interfaced in analog to the Mark-III system with no modifications, using some BNC cables. This input interface is incorporated in the VSOP (VLBI Space Observatory Program).

Figure 3. Block diagram of the new K-4 input interface.

Table 2. New K-4 data configuration * = Mark-III mode, ** = VLBA mode, vs = VSOP mode.

1.2. Data Recorder

We adopted a rotary-head type recorder that uses a cassette tape, which uses the ANSI (American National Standards Institute) X3.175-1990 ID-1 format. The data recorder's error rates during recording and replaying can be read through a host computer. Helical-scan recording is used to record high-rate digital signals. With a large cassette, the K-4 recorder provides up to 770 Gbits of data-storage capacity. The recording time is 200 min. (large cassette, 16-mm thick tape), with a 64-Mbps recording rate. Recording and playback are possible at different data rates: 256, 128, 64, 32, and 16 Mbps, making the data recorder suitable for many different applications. The playback heads are placed so that the recorded data are immediately played back during recording. This read-after-write facility makes it possible to monitor the error conditions of recording in real time. The bit error rate after correction was better than 1x10-10. The data recorder employs a built-in diagnostic system, which is designed to detect operation errors or hardware faults. Error messages or warning information is fed to the host computer via the remote control interface, and to the front panel display. The periodicity of the time code is undesired for spectrum analysis. Only the sampled raw data are desired. The K-4 recorder has helical data tracks, two longitudinal annotation tracks and a control track (Figure 4). The VLBI data are recorded on the helical data tracks. A set of four helical data tracks has one track set ID number, which is a sequential number as a tape counter. The track set ID numbers are recorded on the control track, and can be read at any tape speed, even when fast forwarding or rewinding. There is an obvious relationship between the track set ID and the time code, and it is possible to manage the time code under the track set ID and time code block. The time data are written over the data train as the time code block in pre-observation. After the time-code block, the data timing is checked by track set ID, which means that the output data are only raw data digitized during observation. A data format fully compatible with the conventional K-4 system is also provided.

Figure 4. Tape format and data format.

1.3. Digital Mass Storage System (Automatic Tape Changer)

In KSP, we adopted an automatic tape changer as a digital mass-storage system (Figure 5). The system accommodates one tape drive and 24 tapes, or two tape drives and 16 tapes. The mass-storage capability is up to 2.3 TB. A barcode reader is built into the cassette-handling system to identify individual cassettes within the mass- storage system. Information from the barcode reader is available to the host controller via the remote control interface, and is written on the log which is utilized for correlation processing.

Figure 5. Digital Mass Storage System (Automatic Tape Changer).

2.1. View of the data acquisition system

A photograph of the data acquisition system is shown Figure 5. From left side, an ATM transmitter, an weather monitoring rack, an antenna control rack, a receiver control and monitoring rack, a backend rack, a digital mass-storage system. Three IF signals are received by IF receivers in the receiver control and monitoring rack. The IF signals send to an IF distributor in the backend rack. The two sets of video converters (500 -- 1000 MHz : 8 ch/unit) converts IF signal to video signal. And the other video converter (100 -- 500 MHz : 16 ch/unit) is prepared as a conventional K-4. The video signals are send to the input interface (black panel), and the digital data is send to the digital mass-storage system. The data is recorded on the magnetic tapes, or by-passed to the ATM transmitter.

Figure 6. View of the data-acquisition system.

2.2. View of the correlation processing room

A photograph in Figure 6 shows the view of the correlation processing room. We have two correlation processing systems, one is tape-based system (front) and the other is real-time system (left side). In the tape-based system, left side are digital mass-storage systems, and right side are correlation processor racks. In KSP, there are four sets of digital mass-storage systems, and 6-baseline correlators. The output interfaces are upper side, the correlators are lower two units in each correlation processor rack, and other parts are blank panels. It is possible to extend 10-baseline correlation system with these three racks. The tapes are loaded to or unloaded from the data recorder (lower side) automatically by digital mass-storage systems.

Figure 7. View of the correlation processing system.


Kiuchi, H., J. Amagai, S. Hama, and M. Imae, K-4 VLBI data-acquisition system, Publ. Astron. Soc. Japan, 49, 699--708, 1997.

Updated on June 2, 1997. Return to CONTENTS