2 S2 baseband processing system overview

  An overview of the S2 baseband processing system is shown in schematic form in Fig. 1. The S2 recorder and computer/workstation are standard available items; in particular the S2 recorder is produced and available from ISTS (Cannon et al. 1997). The S2-TCI system configuration is a master/slave configuration with the computer controlling the S2 recorder and data transfer. The data transfer is implemented via direct memory access (DMA), with S2 playback data transferred to computer RAM. Typically, data are either processed immediately from RAM or sent to disk for later processing from disk; alternately, as described later, data may be sent directly to a more capable (super) computer. The hardware interface between S2 and computer is via a custom-designed Computer Interface Card (CIC) residing within the S2, and a commercially-available DMA Card (DMAC) residing within the computer. This interface consists of 16 data lines, a data transfer clock with a DMA transfer rate of 16 MBytes/s or 8 MWords/s where a word is defined as 16 bits, and a "SYNC" signal designating start-of-transfer, all sourced by the S2. The software monitor/control interface between S2 and computer is via the S2 Recorder Control Link (RCL) which uses standard ethernet network communication protocols. The S2-TCI monitor/control program, dubbed 2tci, has been designed to provide simple, automated data transfer from S2 recorder to computer.

  

Figure 1: S2 Baseband Processing System (S2-BPS) Overview. Baseband data are recorded on the S2 at up to 128 Mbits/s. S2 tapes are shipped to the processing center, where data are transferred to computer via the S2 Tape-to-Computer Interface (S2-TCI) at up to 16 MBytes/s, with subsequent processing in software. In this figure, only the "local" computer is shown; the optional "remote link" shown provides the mechanism for data transfer to a more powerful computer for more efficient processing

2.1 Observatory data acquisition system

  The function of a radio telescope data acquisition system is to generate digitized baseband voltage signals, representing the incoming electric field, suitable for recording onto videotape. The basic elements are typically:

• RF-to-IF down-conversion;
• IF-to-baseband conversion, generating either "real" or "complex" baseband signals;
• Baseband low-pass (anti-aliasing) filtering;
• Digitization, or analog-to-digital (A/D) conversion, consisting of sampling and quantization steps. Sampling of a "real" signal of baseband bandwidth B MHz occurs at the Nyquist rate of 2B Msamples/s; sampling of a "complex" signal, i.e. of two signals I and Q, each of bandwidth B MHz, occurs at B Msamples/s for each of I and Q. Quantization: typically 1- or 2-bits per sample is used, although higher-level quantization is often used;
• High-speed synchronization of digitized samples to UTC;
• Digitized baseband signal recording.

These functional steps of a generic baseband acquisition system are displayed in Fig. 2, where the S2 recorder is shown as baseband signal recorder.

  

Figure 2: S2-BPS - Baseband Acquisition System. Pulsar baseband data are recorded on an S2 Record Terminal (S2-RT) at up to 128 Mbits/s (16 MBytes/s). For simplicity, two IF input channels and "real"-signal baseband generation at 2-bit quantization are shown. Data samples recorded to S2 tape are time-tagged synchronized to the observatory UTC time-reference "SYNC" (1 Hz) signal

2.2 S2 recorder

  Developed originally for VLBI applications, the S2 recorder is based on the use of commercial VHS tape transports (VCR's), modified for use in digital high density, high data rate radio astronomical applications (Wietfeldt et al. 1996b). There are two forms of S2 recorder: an S2 Record Terminal (S2-RT) and an S2 Playback Terminal (S2-PT). These machines are identical except for the number of data Decoders; an RT has only one Decoder for real-time diagnostics during recording, whereas a PT has eight Decoders for full bandwidth data playback capability.

At the radio telescope, astronomical signals from the data acquisition system signal digitizer(s) enter the S2-RT User Interface Card (UIC), which provides the input/record interface hardware at the telescope. The data are passed from the UIC to the S2 Formatter and to videotape. The primary role of the Formatter is to insert UTC time-stamps onto videotape to identify data samples recovered from tape on playback. The Formatter overhead information, which includes sync words, UTC and auxiliary user information such as observing mode, raises the effective data rate to 17.44 Mbits/s/transport to implement the S2 "transparent" or "non data replacement" data format. The advantage of this data format type is that none of the input data are overwritten or lost, a feature particularly important in the pulsar application. At the processing center, an S2-PT reproduces the digitized data exactly, apart from tape errors, as it existed at the radio telescope digitizer(s) output. This is accomplished via eight Decoders (one per transport) in the S2-PT. Each Decoder recovers the recorded data and time-stamps off tape, and passes its 16 Mbits/s deformatted data to the UIC, which reproduces the original digitized data and the record timing. The recovered UTC is available over the S2 Recorder Control Link (RCL) every second, synchronous to the S2 1 Hz ("SYNC") reference marker at the UIC output.

At full bandwidth, a single S2 utilizing eight VHS transports with currently available super-long-play (SLP) thin tape video cassettes records at a maximum aggregate rate of 128 Mbits/s (16 MBytes/s) for an unattended operating time of up to 8.5 hours and total data capacity of up to 4 Tbits or 500 GBytes. The unattended operating time may be extended beyond 8.5 hours for narrower recording bandwidths. Numerous channelizations are supported by the S2. Up to 16 digital bit-streams at up to 32 Mbits/s/bit-stream may be recorded, such that the 128 Mbits/s aggregate data limit is not exceeded. The S2 has been designed as an "intelligent" machine, requiring simple initial configuration and subsequent automated operation. It may be operated remotely using standard ethernet network communication (TCP/IP telnet and socket) protocols.

2.3 S2 tape-to-computer interface (S2-TCI)

  The role of the S2 Tape-to-Computer Interface (S2-TCI) is to transfer data recorded on S2 tapes to computer. The S2-TCI is shown schematically in Fig. 3. The S2-TCI consists fundamentally of an S2-PT and a computer. Two additional cards are required in addition to the standard S2 and computer systems: a Computer Interface Card (CIC) within the S2; and a DMA Card (DMAC) within the computer. The roles of these cards are described below. The interface between S2 CIC and computer DMAC consists of 16 data lines, data transfer clock (typically 8 MHz, such that 16-bit data transfers yield the 128 Mbits/s or 16 MBytes/s maximum rate), and a SYNC signal designating start-of-transfer, all sourced by the S2. We note that the critical task of maintaining the precise timing relationship of recorded and reproduced data vs. UTC is the responsibility of the S2 recorder, and requires no special user attention.

  

Figure 3: S2-BPS - Processing Center S2-TCI System Overview. Pulsar baseband data recorded on an S2 Record Terminal (S2-RT) at 128 Mbits/s (16 MBytes/s) are played back on an S2 Playback Terminal (S2-PT). Data are transferred to computer RAM via DMA at up to 16 MBytes/s and to a "local" output device, typically RAM/disk; the optional "remote link" shown provides the mechanism for data transfer to a more powerful "remote" computer for data processing