Subcode

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When writing data to a CD ( audio CD , CD-ROM , ...), in addition to the pure user data and the parity bytes of the CIRC, a so-called subcode is embedded which contains information on how the data can be read out by CD players. Depending on the CD standard, such data include e.g. B. tables of contents, playing times, title marks, index marks, ISRC , text (title names, song texts), MIDI data or even graphics.

On a CD, 14 of 588 channel bits per frame, i.e. 8/588 = 1.36% of the data carrier, are reserved for the subcode. Exactly 1 data byte per frame with 24 data bytes of user data, i.e. 98 bytes of subcode per sector. Each byte of this data stream is divided into 8 different channels, which are designated with the capital letters P, Q, R, S, T, U, V and W. CD players simply ignore the channels they don't support.

Subcode channels

  • P-channel: start marker of a title (P = 1 during the pause between the titles and P = 0 during the title).
  • Q channel: Includes all information for controlling the playback process: time information, both absolute from the beginning of the CD and relative to the start mark of the track, track number, index marks and ISRC ; In the lead-in area, the Q channel contains the table of contents ( TOC = T able O f C ontents) (72 bits / sector)

Structure of Q channel

S0-Bit + S1-Bit + ControlBits (4) + Adr.Bits (4) + Data bits (72) + CRC-Bits (16) 

- S0,S1 Bit: Synchronisation für die Sub-Code Demodulation
- Contr.Bits: Pre-Emphasis (aufnahmeseitige Höhenanhebung); Anzahl Audiokanäle (Bit 1 ist dabei das erste Bit im Datenstrom und somit das höchstwertige Bit im Nibble) 
  ~Bit 1: 0 für 2 Kanal und 1 für 4 Kanäle
  ~Bit 2: 1 für Datentracks
  ~Bit 3: 0 für Kopierschutz und 1 für Digitale Kopie erlaubt
  ~Bit 4: 0 für Pre-Emphasis aus und 1 für ein
- Adr.Bits: Angabe wie Datenbits zu interpretieren sind
  ~Mod 1 (0001): Angabe über Anzahl der Musikstücke, Indizierung und Laufzeit
  ~Mod 2 (0010): Angabe über Katalogisierung der CD
  ~Mod 3 (0011): Identifizierung (z. B. ISRC)
- CRC zur Fehlerkorrektur
  • RW channels: MIDI, character graphics, CD-Text data, ... (CD formats CD + G and CD-Text )

literature

  • Helmut Hofmüller: Handbook of home recording. 1st edition, Elektor-Verlag, Aachen, 1998, ISBN 3-89576-044-7