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ALMA Data Rate Concerns
- ALMA-70.00.00.00-004-A-SPE: ALMA Export Data Format: 2004-04-14: "Each actual cross correlation spectral data element is stored as a couple of signed integers of size B_cross (=2 or 4) bytes (real part then imaginary part)"
- ALMA Memo 501: Estimating the Data Rate (showing 4 byte/visibility format)
- Data Rates for the ALMA Archive and Control System (Scott, Myers & Momose 2002) 2001-12-06 version
- Project book chapter on Computing from 2000 (showing 6 and 60 MB/s limits)
- "The ALMA Archive" (Conference proceeding) (quoting the 60 MB/sec datarate)
- "ALMA Correlator Real-Time Data Processor" (Conference proceeding) (quoting 1 GB/sec correlator output rate)
Comparison of ALMA and SMA correlator capacities for maximum uniform resolution observations
| Telescope | Polarization mode | Max channels | Bandwidth | Resolution | Band 3 (90GHz) | Band 6 (230GHz) | Band 7 (345GHz) |
|---|---|---|---|---|---|---|---|
| ALMA | single | 8192*4 | 4*(2 GHz) = 8 | 244 kHz | 0.82 km/s | 0.32 km/s | 0.21 km/s |
| dual | 4096*4 | 488 kHz | 1.64 km/s | 0.64 km/s | 0.43 km/s | ||
| cross | 2048*4 | 976 kHz | 3.28 km/s | 1.28 km/s | 0.86 km/s | ||
| SMA | single | 6144*2 | 2*(2 GHz) = 4 | 320 kHz | N/A | 0.43 km/s | 0.28 km/s |
| dual | 3072*2 | 2*(2 GHz) = 4 | 640 kHz | N/A | 0.56 km/s | ||
| 2 bands | 3072*2 | 2*2*(2 GHz) = 8 | 0.86 km/s |
- The peak data rate quoted in the project book = 60 MB/sec
- This implies a peak data rate per baseline with 50 antennas = (60 MB/sec) / 1225 baselines = 49.0 kB/sec
- For 64 antennas, it would be (60MB/sec)/2016 kB = 30 kB/sec
- With 50 antennas, if I want 8192 channels * 4 quadrants * 4 bytes (i.e. 4 bytes per complex number visibility), then I need 131kB per baseline per integration. So I am limited to T=(1225*0.131MB)/(60MB/sec) = 2.67 second integrations.
- With 64 antennas, this number becomes 5.0 seconds.
- But the minimum dump time is listed as 0.256 seconds, so this implies that the maximum supported data rate needs to be 626 MB/sec when the data rate is upgraded in the future.
- For 64 antennas, this number becomes 1.03 GB/sec
- With the current data rate limit, if I want to use 0.512 sec dumptimes, today I would be limited to 6283 total channels.
- If I spread these evenly across 8 GHz, then I would have 1.3 MHz channels.
- If I require 0.3 km/s resolution at 230 GHz (0.23MHz), then I could only cover 1.4 GHz at once.
- If I require 0.3 km/s resolution at 90 GHz (0.09MHz), then I could only cover 0.55 GHz at once.
Data rate calculations for OTF interferometry
- datarate_new.xls: 2 spectral lines and highest uniform correlator resolution
| I | Attachment | Action | Size | Date | Who | Comment |
|---|---|---|---|---|---|---|
 xls |
datarate_new.xls | manage | 98 K | 2009-10-15 - 11:06 | ToddHunter | Data rate calculations for OTF inteferometry of 2 lines; and highest uniform correlator resolution |
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