Data Acquisition  From receivers to correlator  Data Acquisition System  Recorder  Station Unit – DAS2  Correlator  Key Features:  Digital, KISS recording  Longitudinal, X-track/pass/head, mux, barrel-roll

Samplers/ADCs  Sample at Nyquist rate: at twice maximum frequency in the band.  Digitise samples to 1-bit or 2-bit precision  Accept 36% or 13% loss of sensitivity.  (Be prepared to accept more losses when data are resampled after phase rotation)

Data Multiplexing  Until VLBA, one channel = one track  Can multiplex (mux) bitstreams N to 1 (fan in)  Or demux 1 to N bitstreams (fan out)  For faster or slower playback with  Increased efficiency or  Enhanced spectral resolution respectively.  NOTE: N = 1, 2 or 4, maybe.

Barrel-Rolling  Tracks can be swapped repetitively  Principle is one of shared misery  All suffer, not just one  But have to keep “track” of tracks and  Keep inverse process time aligned:  Use AUX data field and  Swap between data frames

Data Modulation  Reproduction is via Faraday’s Law of magnetic induction – no magneto-resistive replay heads.  Frequency response is poor at low frequencies.  Nearly all ‘1’s and nearly all ‘0’s are bad.  Optionally, can modulate data in each frame with a fixed pseudo-random sequence. Makes long runs of constant data unlikely.  This aids the recovery of the clock from bit stream.

Data Frames – Each Track (64!)  22,500 bits per frame, frame rates 800, 400,.. Per second.  64-bit AUX data (e.g. track label)  36-bit SYNC pattern  64-bit time code and CRC  bit astronomical data  2496-bit odd parity (1 for 8) except over SYNC  NOTE: parity changes rate from 16 to 18Mbps  Manchester encoded for clock/data recovery.

Station Unit  Philosophy: SSIK  Has to be undo correctly all that has been done -as if data had not been recorded. Ultimate aim: to produce channel bitstreams as if they were coming from the samplers/ADCs.  Has to do some extra jobs as well and has to interface gracefully to the correlator.

SU Modules  CkRM: clock and data recovery from bitstream  TRM: track recovery module – reform frames  CRM: channel-recovery module (demux, barrel)  Phase calibration  DM: delay module provides signal delays  SUIM: SU interface module provides sampler statistics, pulsar gating, correlator headers with delay step and phase information for correlator, and reserialises the data

Reference  See “Mark IIIA/IV/VLBA Tape Formats, Recording Modes and Compatibility” by A.R. Whitney.  ftp://dopey.haystack.edu/pub/mark4/memos/ pdf  Next talk: VLBI Correlators