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Principle of Data Format Conversion The data is recorded at nominal speed (4Gsp/sec) in a RAM and played back by 16 different correlators that operate at 1/16 th the speed (250 Msp/sec). The 16 correlations are obtained on consecutive time windows and are added together. On the borders of the time windows a few samples Are not relevant to the same epoch and should not be correlated. This induces a small loss.

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Basic sequence Every 4 nsec : One block of 16 samples is written at the WP address. The WP address is incremented by 1 One block of 16 samples is read at one of the 16 RP addresses. The sector number is increased by 1. The downloaded block is pushed into the corresponding correlator via a 16-sample parallel-to-serial converter.

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A few assertions It takes the same time (16x4nsec) for the correlator to load and process the 16 samples from the P/S converter as for the Read Pointer to poll the 16 sectors. It takes the same time for the Write Pointer to cover the full RAM space as for one Read Pointer to read one sector.

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The geometrical delay It is the time spent by one sample in the RAM between its writing and its processing. For convenience all the Sector 0 players (one per antenna) synchronously start at RAM address 0 at the beginning of a Walsh period The write pointer of each antenna is offset by the corresponding delay value. One RAM address bit represents 16 samples, or 4 nsec. 12 microseconds (4 kilometres) use 3000 RAM locations.

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Blanking When one Read Pointer reaches the end of one sector, it jumps back to the beginning. The correlator shift register is still full with old samples that would not be correlated with the new ones. Integration must be blanked while the correlator pipe gets purged. The loss of time is (correlator length/sector length) e.g. 1k/128k

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Overwriting When the Write Pointer has completed a full turn, it starts overwriting the old data. It is important that the data on one sector is not overwritten before the full sector has ended being read. This does not occur as long as the geometrical delay does not exceed 1/16 th the reading time of one sector. Next is a timing diagram where all this can be seen

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Next comes an animation that may facilitate the understanding. For this the RAM space has been deployed linearly although its nature is rather circular. One cursor hit represents 16 cycles of 4 nanoseconds.

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Sector F Sector 0 Sector 1 Write pointer Antenna X Antenna Y Write pointerDelay X Delay Y

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Sector F Sector 0 Sector 1 Antenna X Antenna Y Write pointer is 16x faster

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Sector F Sector 0 Sector 1 Antenna X Antenna Y

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Sector F Sector 0 Sector 1 Antenna X Antenna Y

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Sector F Sector 0 Sector 1 Antenna X Antenna Y Delay X Delay Y

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Sector F Sector 0 Sector 1 Antenna X Antenna Y

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Sector F Sector 0 Sector 1 Write pointer Antenna X Antenna Y Write pointerDelay X Delay Y

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