Up: The S2 baseband processing
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.
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:
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
- RF-to-IF down-conversion;
- IF-to-baseband conversion, generating either "real" or "complex"
- 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.
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
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)
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
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.
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
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
Up: The S2 baseband processing
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