"Road Grader"

1. "Road Grader" • Joshua Sopher & David Nygren, LBNL • March 20, 2005 • IceCube Collaboration Meeting

2. Historical perspective • Original notion in PDD: • Most pulses are simple SPE-like waveforms • “Recognize” SPE pulses & process waveforms • Report derived Q & time for these pulses • Don’t process all other complex waveforms • No zero-suppression, report raw waveform • Algorithmic implementation: unpleasant Two processing methods: bad idea

3. “Road Grader” Algorithm • Perspective: “Simple is Good” • Road grader scrapes up all good data: • zero-suppression + data compression • Samples near baseline & below threshold are unimportant for timing & charge • All fADC & ATWD waveforms treated identically • Very few parameters to meddle with • (and lose track of!)  Stability of data guaranteed

4. Project Goals • Suppress and compress data to meet the data rate requirement for DOM-to-surface data transmission: < 20 kbytes/s/DOM • Realize compressor in firmware to minimize processing time. • Efficient operation within DAQ FPGA design • CPU to be used for state control, message management, etc, not for data processing

5. Technical description • Waveforms are similar to fax scan lines: • Run-length encoding, followed by Huffman encoding • Suppression replaces baseline data with zeroes • Run-length encoding counts the repetitions of same valued data • Huffman “lite” encoding replaces “zero” bytes with a “zero” bit

6. Suppression • ATWD and fADC data words are 10 bits wide. • Data below a threshold is replaced by zeros, and data above a threshold is left unchanged • This produces a large run length of zero valued data, for a typical single-pulse waveform

7. ATWD pre-pulse behavior • Baseline noise is small, ± 2 counts peak-to-peak. • Occasional pre-pulse baseline “shift”: -3 counts • Threshold is set 4 counts above the baseline. • Maximum threshold: 8 - 9 counts • Typical SPE: 200 counts at peak. • Pulse samples with amplitudes above 8 -9 counts (~4% of an SPE) are never suppressed.

8. Threshold impact • Threshold causes not more than one sample of uncompressed data to be lost. • There will be virtually no loss of useable waveforms due to compression. • Pulse (non-zero) data will be identical to uncompressed data. • Reconstructed pulse has negligible errors.

9. Run length encoding • Zero-suppressed data is run-length encoded. • Run-length is zero for non-repeated data. • Run-length encoding produces number pairs: data followed by the number of repetitions. • Pre-pulse: 0 0 0 0 0  0,4 • Pulse: 43 89 22  43,0 89,0 22,0

10. Huffmann encoding • A zero-valued 10-bit word is replaced by a 1-bit wide “zero flag”. • A non-zero flag bit is added to non-zero data, forming a 11-bit word. • Decompression of data requires an additional flag bit  12 bit words.

11. Compressed data • The compression ratio depends on the sampling rate, the pulse width, and waveform complexity. • Compressed data is 12 bits wide for both repeated zeros & non-repeated non-zero data values. • For a pulse 8 samples wide, with leading and following zeroes, compressed data = 12 + (8 x 12) + 12 = 120 bits = 15 bytes • For a pulse 4 samples wide, with leading and following zeroes, compressed data = 12 + (4 x 12) + 12 = 72 bits = 9 bytes

12. Data compression ratio • For a 8 samples wide ATWD pulse: the compression ratio = 128 x 10/120 = 10 • For a 4 samples wide fADC pulse: the compression ratio = 256 x10/72 = 35 • Every hit also has an 8-byte header that includes the coarse time-stamp ( 32 bits) + various hit descriptor bits ( 32 bits)

13. Basic rates • String 21 measured PMT rate = <750 Hz> • LC tag rate (nearest neighbor only) = ~15 Hz • Non-tag rate (mainly SPE) = ~735 Hz • Data rate requirement < 20,000 bytes/s/DOM • This keeps data flow below danger zone: • Network occupancy >50% not allowed

14. Data flow rate - “Hard” LC • Mode: HardLocal Coincidence • LC tag present: Header + ATWD + fADC data • LC tag absent: no data at all! Hit discarded! • Data rate = (header + fadc + atwd) x tag rate = (8 + 15 + 9) x 15 Hz = 480 bytes/s • Compression is not really needed…but, • All isolated hit data is lost

15. Data flow rate - “Soft” LC • Operating mode: Soft Local Coincidence • LC tag present: Header + ATWD + fADC data • LC tag absent: Header only, no ATWD, no fADC data • Tagged data rate = (header + (fadc + atwd)) x tag rate = (8 + 15 + 9) bytes x 15 Hz = 480 bytes/s • Non-tagged rate = 8 x 735 Hz = 5880 bytes/s • Sum = 6360 bytes/s  • Zero-suppression & run-length encoding needed

16. Data rates - “Flabby” LC • Mode: Flabby Local Coincidence • LC tag present: Header + ATWD + fADC data • LC tag absent: Header only + fADC data, no ATWD • Tagged data rate = (header + fadc + atwd) x tag rate = (8 + 15 + 9) bytes x 15 Hz = 480 bytes/s • Non-tagged data rate = (8 + (1 + .2) x 9) bytes x 735 Hz = 13,818 bytes/s • Sum = 14,298 bytes/s (reasonable margin)

17. Possible issues • ATWD baseline may need monitoring • Baseline drift, if any, needs to be tracked • Easy to imagine auto-tracking capability • ATWD pulses over-sampled @ 300 MHz • Typical: ~11 samples/pulse • Why is this? Pulses are wider than expected • Delay line + amps + ATWD driver affect r • PMT gain is probably higher than we need • Pulse tail adds many samples, little information

18. Summary • “Road grader” is conceptually simple. • Reconstructed pulse fidelity is excellent. • Compression ratio meets project goals. • Implementation is pretty well-tested. • Incorporated in the new FPGA for DAQ. • ATWD issues may need some attention. • No obvious flaws preventing utilization.