Data accumulation rate

This page gives some rules for estimating the accumulation rate of data on disk for a running MCE.

Rate calculations

In general, during a regular acquisition, the data rate (bit rate) of an MCE is:

<math>S_w \times (S_{head} + N_c \times N_{RC} \times N_{rro} + S_{chk}) \times f_{DV}</math>

where:

<math>S_w</math> is the word size (=32 bits)
<math>S_{head}</math> is the size (in words) of the frame header (=43 words)
<math>N_c</math> is the number of columns per readout card (=8)
<math>N_{RC}</math> is the number of readout cards read out (see rcs_to_report_data)
<math>N_{rro}</math> is the number of rows read out (i.e. num_rows_reported)
<math>S_{chk}</math> is the size (in words) of the frame checksum (=1 word)

<math>f_{DV}</math> is the read-out data rate. When in internal-DV mode (i.e. use_dv is zero), this is:

<math>f_{DV} = \frac{50\;\mathrm{MHz}}{ N_{dr} \times N_{r} \times N_{rl} } \equiv \frac{f_{ARZ}}{N_{dr}}</math>

where:

<math>50\;\mathrm{MHz}</math> is the clock rate
<math>N_{dr}</math> is the number of samples co-added into an output frame (i.e. data_rate)
<math>N_{r}</math> is the number of rows servoed (i.e. num_rows)
<math>N_{rl}</math> is the row dwell-time in clock periods (i.e. row_len)

and

<math>f_{ARZ}</math> is the Address-return-to-zero rate, i.e. the frequency at which a single pixel is visited

A useful approximation is:

a single readout card reading 41 rows at 400 Hz is <math>\approx0.5</math> megabytes/second.

Here are some tables of accumulation rates for both kinds of subracks for various data rates and numbers of rows: