THE S2 BASEBAND PROCESSING SYSTEM FOR PHASE-COHERENT PULSAR APPLICATIONS

The S2 baseband processing system (S2-BPS) is designed for a variety o f pulsar measurements such as ultrafast signal fluctuations and pulse timing. Its key features are: the S2 recorder; the S2 Tape-to-Computer Interface (S2-TCI) which provides an efficient high data rate transfe r from S2 tapes to computer; and the tightly coupled control interface to the computer which enables automated data transfer and processing. 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 applications, A single S2 rec order ''tape-set'' of eight SVHS tapes provides up to 500 GBytes of da ta storage, and an unattended operating time of up to 8.5 hours at the maximum data rate of 128 Mbits/s or 16 MBytes/s, corresponding typica lly to 16 MHz bandwidth in two circular polarizations at 2-bit quantiz ation, which generates a data rate of up to 1 GByte/minute. With 16 MH z individual record channels, a single S2 yields a sample time resolut ion of 31 ns, For more challenging applications, multiple S2's may be used for increased bandwidth and higher time resolutions. In this pape r, we present an overview of the S2 baseband processing system and pla ns for future implementations of the system, and demonstrate first res ults of Vela pulsar recordings at Parkes in September 1995. We pay spe cific attention to the S2-TCI, a system designed to transfer S2 record ed data from tape to computer memory (RAM) via direct memory access (D MA). Computer data may be processed locally or sent to a more powerful ''remote'' (super) computer for processing.