CD-Digital Audio
Introduction
CD-Digital Audio (CD-DA) is the standard format for audio compact discs (CDs). CD-DA is a prerecorded read-only format.
Physical characteristics of compact discs
Disc | Description |
---|---|
Playing time | 74 minutes, 33 seconds maximum |
Rotation | Counter-clockwise when viewed from readout surface |
Rotational speed | 1.2-1.4 m/sec. |
Track pitch | 1.6 µm |
Diameter | 120 mm |
Thickness | 1.2 mm |
Center hole diameter | 15 mm |
Recording area | 46 - 117 mm |
Signal area | 50 - 116 mm |
Material | Any transparent material with 1.55 refraction index. Usually polycarbonate |
Minimum pit length | 0.833 µm (1.2 m/sec.) to 0.972 µm (1.4 m/sec.) |
Maximum pit length | 3.05 µm (1.2 m/sec.) to 3.56 µm (1.4 m/sec.) |
Pit depth | Approximately 0.11 µm |
Pit width | Approximately 0.5 µm |
Number of channels | 2 channels (four channel recording possible) |
Quantization | 16-bit linear quantization |
Quantizing timing | Concurrent for all channels |
Sample rate | 44.1 kHz |
Error correction code | Cross Interleave Reed-Solomon Code (CIRC) (with 25% redundancy) |
Modulation system | EFM |
Source
Ken C. Pohlmann, The Compact Disc: A Handbook of Theory and Use. Madison, Wisconsin: A-R Editions (1989): 49.
Layers of CD-DA
Image source
Ken C. Pohlmann, The Compact Disc: A Handbook of Theory and Use. Madison, Wisconsin: A-R Editions (1989): 50.
The following table describes the process of creating a pre-recorded CD:
Image source
Replication of CDs. Image source: J.M. Eargle. The Compact Disc (CD). In Handbook of Recording Engineering. Boston: Springer (1996): 465.
Technical specifications
Specification | Description |
---|---|
Channels | 2 channel (stereo) |
Bit depth | 16-bit |
Data encoding | Linear PCM |
Sample rate | 44.1 kHz |
The sample rate of 44.1 kHz was inherited from the widespread practice of using a PCM adaptor to convert digital audio into an analogue video signal for storage on a U-matic videocassette. This was the most economical way of transferring audio from the recording studio to CD manufacturers.
Technical specifications for CD-DA are defined in the "Red Book" (IEC standard 60908:1999).
Emphasis
Some CD-DAs produced in the early 1980s are encoded with a form of signal boosting called pre-emphasis. High frequencies on the master tape are boosted slighty. On playback, the pre-emphasis process is reversed ("de-emphasis") so the output accurately represents the original input. Emphasis produces slight improvement in the signal-to-noise ratio.
Pre-emphasis is denoted by a flag in the subcode. Emphasis should be considered during forensic imaging or "ripping" files from CD-DAs so digital audio files do not play back with a distorted frequency response.
Data structure and storage
Data frame structure
Data on CD-DAs is encoded in "frames" with a defined structure that allows the CD player to distinguish the types of data on the disc. Each frame includes 24 bytes of audio data, and some additional overhead data.
Prior to EFM modulation, the information contained in one CD-DA frame includes 291 bits:
- 27 bit synchronization word
- 8-bit subcode
- 192 audio data bits
- 64 parity bits
Image source
Ken C. Pohlmann, The Compact Disc: A Handbook of Theory and Use. Madison, Wisconsin: A-R Editions (1989): 128.
Frames are assembled through a complex series of steps:
Step | Description |
---|---|
Group audio data | Six 32-bit PCM audio sampling periods are grouped into a frame. Each sampling period is derived from alternating 16-bit left and right audio channels. The sampling period is divided to produce four 8-bit audio symbols. |
Error correction encoding | Error correction encoding takes place using the Cross Interleave Reed-Solomon Code (CIRC). CIRC uses a combination of interleaving and parity bits. |
Subcode | An 8-bit subcode symbol is added to each frame. Subcode bites are designated as P,Q,R,S,T,U,V, and W. Only the P and Q bits are used for audio encoding and include infromation such as the total number of tracks on a disc, beginning and end points, and other timing information. The other six bits are available for encoding other information. |
EFM modulation | After the audio, parity, and subcode data are assembled, the bit stream is modulated using eight-to-fourteen modulation (EFM). 8-bit blocks are translated into 14-bit blocks known as "channel bits." EFM modulation helps achieve greater storage density and provides a form of error corretion because it allows for 16,384 unique bit patterns (rather than 256 patterns allowed for with 8-bit encoding). |
Merging bits | 14-bit blocks are linked by three merging bits.Two 0-bits are always used to prevent violations of the coding scheme and a third merging bit (either a 0 or 1) is added to assist with clock synchronization. |
Pit length | The signal is converted from a "non-return-to-zero" (NRZ) signal to a "non-return-to-zero-inverted" (NRZI) signal. NRZI signals simplify the pit structure of the disc. The final NRZI signal produces pits and lands between three channel bits (720 kHz) and eleven channel bits (196 kHz) long. |
Frame assembly | 24-bit synchronization word is inserted between each frame. The synchronization word delineates each frame. |
After data encoding, each frame contain 588 bits:
- 24 synchronization bits
- 336 data bits (12 x 2 x 14)
- 112 error correction bits (4 x 2 x 14)
- 14 subcode bits
- 102 merging bits (34 x 3)
Source
Adapted from Ken C. Pohlmann, The Compact Disc: A Handbook of Theory and Use. Madison, Wisconsin: A-R Editions (1989): 72-80.
CD players play 75 of these 588 bit data frames per second, thus transmitting 44,1000 samples per second.
Data storage and transmission
Note: Data on a CD-DA is written as a linear pulse-code modulation (PCM) stream. The ISO 9960 file system is not used in CD-DAs.
CD-DA data is addressed on the compact disc using a timecode system of minutes, seconds, and frames, expressed in mm:ss:ff. Timecode frames (i.e., sectors) are the smallest addressable time intervals on a CD-DA and correspond to 1/75th of a second of audio. Timecode frames are distinct from the data frames described above. Each sector can hold 98 data frames, or 2,352 bytes of audio data. The CD is played at 75 timecode frames per second, or 176,400 bytes per second (1,411,200 bit/s).
In other words, CD-DA data transfers at a bit rate of 1.4 Mbit/s.
CD-DAs hold more data per sector than CD-ROMs, but the increased storage capacity also introduces a higher error rate. As a result, traditional forensic imaging techniques that involve a single pass over the disc are not suitable for CD-DAs. Instead, software designed for CD-DA extraction is used to convert tracks into standard computer.
The volume of storage required for CD-DA data can be calculated using the bit rate. The total size in bits of a CD-DA stream can be calculated as: 2 x 16 x 44,100 x length of time in seconds
The total number of bytes can be calculated by dividing the size in bits by 8 (the number of bits in one byte): size in bytes = size in bits/8
Example: calculate storage requirements for 30 minutes of CD-DA data
CD-DA with the following characteristics:
- 2 channels
- 16 bit
- 44.1 kHz sample rate
- 30 minutes (1,800 seconds)
2 x 16 x 44,100 x 1,800 = 2,540,160,000 bits of storage
2,540,160,000 bits divided by 8 = 317,520,000 bytes
317.52 MB
Technical standards
Red Book - IEC standard 60908:1999. Audio recording - Compact disc digital audio system.