|Home | Audio Magazine | Stereo Review magazine | Good Sound | Troubleshooting
Departments | Features | ADs | Equipment | Music/Recordings | History
It is the compact disc (CD) that has introduced most people to digital audio reproduction. Table 1 is a comparison of the LP and CD systems, showing that CD is far superior to LP in each aspect of dynamic range, distortion, frequency response, and wow and flutter specifications. In particular, CD exhibits a remarkably wide dynamic range (90 dB) throughout the entire audible frequency spectrum. In contrast, the dynamic range of the LP is 70 dB at best. Harmonic distortion of CD reproduction is less than 0.01%, which is less than one-hundredth of that of LP. Wow and flutter are simply too minute to be measured in a CD system. This is because, in playback, digital data are first stored in an RAM and then released in perfect, uniform sequence determined by a reference clock of quartz precision.
With a mechanical system like that of the LP, the stylus must be in physical contact with the disc. Therefore, both the stylus and the disc will eventually wear out, causing serious deterioration of sound quality. With the CD's optical system, however, lack of contact between the disc and the pick-up means that there is no sonic deterioration no matter how many times the disc is played.
Mechanical (and, for that matter, variable capacitance) systems are easily affected by dust and scratches, as signals are impressed directly on the disc surface. A compact disc, however, is covered with a protective layer (the laser optical pick-up is focused underneath this) so that the effect of dust and scratches is minimized. Furthermore, a powerful error-correction system, which can correct even large burst errors, makes the effect of even severe disc damage insignificant in practice.
Main parameters of the CD compared to the LP are shown in Table 2. FIG. 2 compares CD and LP disc sizes. FIG. 3 compares track pitch and groove dimensions of a CD with an LP; 60 tracks of the CD would fit into one track of an LP.
FIG. 4 gives an overview of the optical discs available. The CD single is the digital equivalent of a 45 rpm single. It can contain about 20 minutes of music and is fully compatible with any CD player. A CD Video (CDV) contains 20 minutes of digital audio which can be played back on an ordinary CD player and 6 minutes of video with digital audio. To play back the video part you need a Video Disc Player or a Multi Disc Player (MDP).
Multi Disc Players are capable of playing both Compact Discs and Laser Discs (LDs). Optical discs containing video signals can be distinguished from discs containing only digital audio by their colour. CDVs and LDs have a gold shine, while CDs and CD singles have a silver shine.
Recording and read-out system on a CD
The data on a compact disc are recorded on the master by using a laser beam photographically to produce pits in the disc surface, in a clockwise spiral track starting at the centre of the disc.
The length of the pits and the distance between them from the recorded information are as shown in FIG. 5.
In fact, on the user disc, the pits are actually bumps. These can be identified by focusing a laser beam onto the disc surface: if there is no bump on the surface, most of the light that falls on the surface (which is highly reflective) will return in the same direction. If there is a bump present, however, the light will be scattered and only a small portion will return in the original direction (FIG. 6). The disc has a 1-mm-thick protective trans parent layer over the signal layer, i.e., the pits. More important, the spot size of the laser beam is about 1 mm in diameter at the surface of the disc, but is as small as 1.7 µm across at the signal layer. This means that a dust particle or a scratch on the disc surface is literally out of focus to the sensing mechanism. FIG. 7 illustrates this. Obviously, control of focus must be extremely accurate.
Before being recorded, the digital audio signal (which is a 16-bit signal) must be extended with several additional items of data.
• error correction data;
• control data (time, titles, lyrics, graphics and information about the recording format or emphasis);
• synchronization signals, used to detect the beginning of each data block;
• merging bits: added between each data symbol to reduce the DC component of the output signal.
The audio signal normally consists of two channels of audio, quantized with a 16-bit linear quantization system at a sampling frequency of 44.1 kHz. During recording, pre-emphasis (slight boost of the higher frequencies) may be applied. Pre-emphasis standards agreed for the compact disc format are 50 and 15 µs (or 3183 and 10 610 Hz).
Consequently, the player must in this case apply a similar de emphasis to the decoded signal to obtain a flat frequency response (FIG. 8).
A specific control code recorded along with the audio signal on the compact disc is used to inform the player whether pre emphasis is used, and so the player switches in the corresponding de-emphasis circuit to suit.
Alternatively, audio information on the CD may comprise four music channels instead of two; this is also identified by a control code to allow automatic switching of players equipped with a four-channel playback facility. Although, on launching CD, there were no immediate plans for four-channel discs or players, the possibility for later distribution was already provided in the standard.
Additional information on the CD
Before the start of the music program, a 'lead-in' signal is recorded on the CD. When a CD is inserted, most players immediately read this lead-in signal, which contains a 'table of contents' (TOC). The TOC contains information on the contents of the disc, such as the starting point of each selection or track, number of selections, duration of each selection. This information can be displayed on the player's control panel, and/or used during program search operation.
At the end of the program, a lead-out signal is similarly recorded which informs the player that playback is complete. Furthermore, music start flags between selections inform the player that a new selection follows.
Selections recorded on the disc can be numbered from 1 through 99. In each track, up to 99 indexes can be given, which may separate specific sections of the selection. Playing time is also encoded on the disc in minutes, seconds and 1/75ths of a second; before each selection, this time is counted down.
There is further space available to encode other information, such as titles, performer names, lyrics and even graphic information, which may all be displayed, for instance, on a TC screen during playback.
Compact disc production
Compact disc 'cutting'
FIG. 9 is a block diagram comparing CD digital audio recording and playback systems with analog LP systems.
The two systems are quite similar and, in fact, overlap can occur at record production stage. However, where LP masters are mechanically cut, CD masters are 'cut' in an electro-optical photo graphic process: no 'cutting' of the disc surface actually takes place.
The compact disc production process follows seven main stages, illustrated in FIG. 10:
1. A glass plate is polished for optimum smoothness.
2. A photo-resistive coating is applied to its surface. The roughness of the glass surface and the thickness of the coating determines the depth of the pits on the compact disc.
3. The photo-resistive coating is then exposed to a laser beam, the intensity of which is modulated with digitized audio information.
4. The photo-resistive layer is developed and the pits of information are revealed.
5. The surface is silvered to protect the pits.
6. The surface is plated with nickel to make a metal master.
7. The metal master is then used to make mother plates. These mothers are in turn used to make further metal masters, or stampers.
Compact disc stamping
The stamping process, although named after the analogous stage in LP record production, is, in fact, an injection moulding, compression moulding or polymerization process, producing plastic discs (FIG. 11). The signal surface of each disc is then coated with a reflective material (vaporized aluminum) to enable optical read-out, and further protected with a transparent plastic layer which also supports the disc label.