Playback data from an S2-PT looks exactly like sampler data, except it has some bit errors (specified at 1e-4). This is unlike most other VLBI recorders such as the Mark III and Mark IV which overwrite user data with their format information and send formatted data across their output interface.

The S2 records data on 8 SVHS tapes in parallel at a max. rate of 128 Mbits/sec.

The user data rate for one S2 tape is 16 Mbits/sec, which gets increased to 17.44 Mbits/sec internally to allow format insertion (9% overhead). The S2 tape format is structured into frames and lines (called blocks in the S2) just like the video signal normally recorded by a VCR. There are 64 frames per second (up from the usual 60) and 250 blocks per frame. There are 6 additional non-data "sync" blocks at the start of each frame, as well as another 6 blocks worth of "dead" time (called clock-runup).

Each of the 250 data blocks contains a 5-byte header and 125 user data bytes. The header includes a 16-bit sync word, block ID byte (0-249), and two bytes of auxiliary data, although only one of these bytes is actually ever used. All of the auxiliary data bytes from each frame (250 in total) are assembled together to form an auxiliary data record. Each auxiliary data record is filled with specific information at record time, and each frame's record is usually the same as the last frame. The auxiliary information includes the UTC time stamp, tape ID (label), system serial numbers, observing mode, user-supplied text fields, and other useful information. The auxiliary records use a 16-bit CCITT CRC for error detection (actually two of them).

email: rgonzale@naic.edu