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Each 50 ms data block consists of 64,000 time samples for each signal path presented at the input (number of tiles x 2 polarizations). The 256T correlator configuration supports up to 512 signal paths. Input data is transferred to GPU memory and promoted from 8-bit integers to 32-bit floats. The 64,000 time samples of each path are partitioned into 10 blocks of 6,400 samples (5 ms), each of which is FFT’d on the GPU using “cuFFT”, resulting in 10 time samples on each of 6,400 ultrafine channels of resolution 200 Hz.
Fractional delays are then applied The design supports a future option to apply delays to each signal path to point the telescope to a specified correlation pointing centre. The required delay values are generated within the Delay Engine and . When that mode becomes available, it will involve integer sample delays being applied at the time the sub-observation data is assempled, and the residual fractional delays being applied within the FX engine. The required fractional delay values for each signal path (on 5 ms resolution) are passed to the FX Engine via the prepended metadata block written to the input ring buffer. Delays for the start and end of every 50 ms data block are provided, from which the FX Engine linearly interpolates the delay to be applied to each 5 ms sub-block. Delays are applied by multiplying the frequency-domain samples of each sub-block by a phase gradient, whose complex gain values are taken from a pre-computed look-up table to increase speed.
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