Table of Contents
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| 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 | |
| Sampling frequencySample rate | 44.1 kHzChannel bit rate | 4.3218 Mbps |
| Data bit rate | 2.0338 Mbps | |
| Data-to-channel bit ratio | 8:17 | |
| Error correction code | Cross Interleave Reed-Solomon Code (CIRC) (with 25% redundancy) | |
| Modulation system: | EFM |
| Info | ||
|---|---|---|
| ||
Ken C. Pohlmann, The Compact Disc: A Handbook of Theory and Use. Madison, Wisconsin: A-R Editions (1989): 49. |
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| Info | ||
|---|---|---|
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Replication of CDs. Image source: J.M. Eargle. The Compact Disc (CD). In Handbook of Recording Engineering. Boston: Springer (1996): 465. |
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Technical specifications
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| 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.
CD-DA audio has a bit rate of 1.4 Mbit/s.
Technical specifications for CD-DA are defined in the "Red Book" (IEC standard 60908:1999).
Emphasis
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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
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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:
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| Info | ||
|---|---|---|
| ||
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.
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| Tip | ||
|---|---|---|
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CD-DA with the following characteristics:
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 |
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.
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Technical standards
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Red Book - IEC standard 60908:1999. Audio recording - Compact disc digital audio system.