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The output visibility channels are "centre symmetric" in terms of how the way their boundaries are aligned within the coarse channel bandwidth. The centre output fine channel is centred symmetrically on the centre of the coarse channel (as was the case with the legacy correlator/fine PFB). Remaining output channels extend above and below the centre channel symmetrically. In cases where there is an odd number of output channels across the coarse channel, there are full-width channels at the lowest and highest ends of the coarse channel bandwidth. In cases where there is an even number of output channels across the coarse channel, there are half-width channels at the lowest and highest ends of the coarse channel bandwidth. See: MWA Fine Channel Centre Frequencies
The channel averaging process involves the summation of the ultrafine channel complex visibility values for all the ultrafine channels comprising each output visibility fine channel, i.e. . That is, each output visibility channel is formed by the sum of fscrunch complex values. For example, for 10 kHz output visibility channels, the complex visibilities of 50 ultrafine channels are summed to produce each output channel. Note that for the centre output fine channel, the centre (DC) ultrafine channel is excluded from the summation to remove any DC component present in the coarse channel, and the output value is re-scaled accordingly to maintain a consistent output magnitude as with other channels. Note that only Only 200 Hz of bandwidth is lost in this process, rather than a complete output channel (10 kHz in as was the case with the legacy correlator where an entire 10 kHz was lost).
During this the channel averaging process, each visibility can have has a multiplicative weight applied , which in future can be based on a data occupancy metric that takes to normalise the magnitude of output visibilities to a desired absolute scale. This normalisation factor is automatically adjusted according to the selected integration time and fscrunch factor such that output visibility values remain at a consistent magnitude. In future, this normalisation factor will be able to be optionally combined with additional baseline-specific weighting factors based on the input data occupancy, i.e. taking account of any input data blocks that were missing due to lost UDP packets or pre-correlation RFI excision (a potential future enhancement). The centre (DC) ultrafine channel is excluded when averaging and the centre output channel values are re-scaled accordingly. The weighting factors applied to each baseline (which are common to all fine channels of that baseline) are placed in the output ring buffer along with the visibility values themselves. This allows downstream processes to track what weightings were applied. At present no visibility weightings are applied and the output weights values are all set to 1.0.
The averaged output channel data is then transferred back to host memory where it is re-ordered before writing to the output ring buffer.
Visibility Re-ordering
xGPU utilizes a particuar data ordering that is optimized for execution speed. The visibility set is re-ordered by the CPU into a more intuitive triangular order by the CPU: [time][baseline][channel][polarization]. The See: Add Link Here
The re-ordered visibility sets (one per integration time) are then written to the output ring buffer.
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