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ECEE - 443

ECEE - 443. Compact Disc (CD) Coding – Signal Processing and Error Correction Jessica Lau. Need for Error Correction. Signal incentive to small defects on bottom surface of CD such as dust Larger defects = Wiping of data Example: scratches on disc Data on CDs are spread all around

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ECEE - 443

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  1. ECEE - 443 Compact Disc (CD) Coding – Signal Processing and Error Correction Jessica Lau

  2. Need for Error Correction • Signal incentive to small defects on bottom surface of CD such as dust • Larger defects = Wiping of data Example: scratches on disc • Data on CDs are spread all around • Missing bits of data can be reconstructed using error correction codes. • Minimize the effect of error on Disc

  3. Generalized CD Data Path Outline of a main data channel in a CD Components: • CIRC Encoder • Sub Channel / Sub Code • EFM Encoder

  4. Audio Data Break-down • Block 98 frames or 2352 bytes / Block Read : 75 blocks/sec • Frame  36 bytes / Frame  24 data + 8 parity + 3 sync + 1 subcode bytes • Blocks /CD (74mins)  74x60x75 = 333,000 blocks

  5. C1 Encoder Interleaving Stage C2 Encoder Input Data Data Data Parity Parity CIRC Encoder • CIRC – Cross Interleave Reed-Soloman Code  Parity blocks are generated in addition to data and encoded onto the disc  Interleaving stage break the data up so it is not contiguous on Disc, i.e. long burst error associated form scratches or larger scale of defects

  6. CIRC – Parity Generation • Parity bits  additional bits that represent whether the total number of ‘1s’ is even or odd • Used to carry error detection and correction information • Example: 1 Parity (all bits)  1101 0000  1101 0000 1 4 Parity(1st 4, 2nd 4, 1256, 2367 bits)  1101 0000 1001 Error detected  1101 0001  by checking parity bit, error detected in 2nd parity bit, hence error in 8th bit

  7. CIRC – Interleaving Technique • Process of scrambling, breaking /damaging and unscrambling frame of data • Example U N I V E R S I T Y O F W A S H I N G T O N Defects  ----- R S I T Y O F W A S H I N G T O N  hard to reconstruct first word 1. Scrambling  O N S T H U G R F S I I O T W N N V E I Y A 2. Damaging  U G R F S I I O T W N N V E I Y A 3. Unscrambling  U N I V E R I Y O F W A S I G T N 4. Interpolation  Guessing of the missing letters

  8. CIRC Process – 3 Stages • Stage 1 – C1 Process • 32 bytes= 28 data + 4 ECC bytes • Detect a bad C1 frame and fix only a single error • A bad C1 frame - at least 2 error and location unknown • Stage 2 – De/Interleaving Process • Only 1 out of 28 bytes from C1 frame is taken into C2 due to data scattering on Disc • Each byte is delayed by 1 frame until last byte is used • Stage 3 – C2 Process • 28 bytes from 28 different C1 frames, including 4 parity bytes • Number and locations of errors known • Fix up to 4 errors at a time

  9. Interpolation • Used when error correcting code fails • Used to fill missing bits of data by guessing • Only valid for audio CD • Example: CD audio tracks 400, 525, 650, 825, 1100  Defect  400, 525, 650, ? , 1100  using interpolation, estimate the half value = 875  825 Vs. 875 - close enough for audio signal, human ear unable to detect such mistake.

  10. Eight to Fourteen (EFM) Modulation • Applied to each byte in the data stream before the data is put on CD • Minimize the number of 0 to 1 and 1-0 transitions -- thus avoiding small pits • Binary bits translated into EFM codes first • Digital 10  0000 1010 EFM 10  1001 0001 0000 00 • Rules: no more than ten and no less than two consecutive zeroes in the data stream on the disc. This also means that there can be no consecutive ones.

  11. EFM Modulation -Example (a) EFM coded data (b) NRZ coded data (Non return to zero) require twice the pits on CD high BW (c) NRZI coded data (Non Return to Zero Inverted) half pits half BW Advantages: Reduce overall BW

  12. Subcode • Additional subcode byte is added to the frame after CIRC encoded. • Each bit in this byte represents a subcode channel labeled P,Q,R,S,T,U,V and W • In practice only the P and Q subcode channels are used. • P subcode  simple music separator flag indicates audio silence between tracks.  in leadout, it alternates at 2 Hz to signal to the player that the disc has ended.

  13. Subcode • Q subcode • Contains the table of contents for the disc. • During normal playing time, the Q subcode denotes the current time on the disc in minutes, seconds and frames • Denotes the disc UPCIEAN number and for each track, the International Standard Recording Code (ISRC). • UPCIEAN - The Universal Product Code (UPC)/ European Article Number (EAN) bar code

  14. Some Facts – ECC & CD Player • Parity & Interleaving Combination • Detect and correct a burst error up to 4000 bad bits • Physical defect of 2.7 mm long • Interpolation • Detects and corrects a burst error up to 13700 bad bits • Physical defect of 8.5 mm long

  15. Trade-offs Data Capacity Vs. Redundancy • Data CDs – 10%capacity for error detection and correction codes (ERC) Example: 2324 bytes of data  only using 2048 bytes • Audio CDs – A quarter of the incoming bytes used for ERC Example: 24 out of 32 bytes of incoming data are used, 8 bytes for ERC  74mins of music is actually 98mins

  16. Q & A

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