CD player and CD are the most successful masterpieces of contemporary laser application technology. CD players and CDs are simply called "CD players" or "CD records". CD is the abbreviation of English compact disc——Digital audio, which means small digital audio record. Compared with the traditional record player, the CD player has many incomparable advantages: it can provide excellent high fidelity and high purity sound quality; Stereo separation between left and right channels is 85 dB, frequency response is between 5 ~ 20,000 cycles (Hz), harmonic distortion is 0.004%, there is no jitter problem, the life of the record is very long, and it is hardly worn. The dynamic range is over 90 dB, which is close to that of a large symphony orchestra. It can faithfully and clearly reproduce the most delicate sound recorded on the record: the strong bass makes people feel that their legs are swinging, the sound of the violin is full of fragrance, the climax is repeated, and even the texture of the singer's lips when he is silent ... all these make fans "burn" and intoxicated.

When briefly describing the working principle of CD player, we might as well make a comparison with the traditional CD player. The record surface of ordinary record player is engraved with a continuous track "groove", in which various analog audio signals are recorded. When the stylus of the pickup directly contacts the groove of the audio track, the pickup picks up the analog signal of the record from the audio track with the continuous change of the amplitude and depth of the groove. The record is driven by the phonograph (clockwork or motor) and rotates clockwise with equal angular velocity. The stylus moves along the groove from the outer circle to the inner circle of the record, constantly reading the signal on the record. After these signals are processed and amplified by the circuit, the sound of the record is emitted by the speaker. Ordinary records are generally made of plastic, because the stylus is in direct contact with the record, so the stylus is prone to play distortion due to friction and wear.

The working principle of CD player is completely different from that of ordinary CD player. The signal recorded on the record is a digital signal, which is a continuous "pit" track, namely "0" and "1" digital symbols. The depth of these "pits" is generally 0. 1 micron (1 micron =11000 mm), the track pitch is 1.6 micron, and there are 625 tracks per millimeter. The number of tracks on an optical disc is about 20,000, which is total.

The pit on the CD is concave. The scanning laser beam on the optical pickup comes from the lower part of the record, so the "pits" protrude from the laser beam. When the laser beam emitted by the laser pickup scans and focuses on the aluminum-plated "pits" on the record, diffuse reflection occurs, and the signal detected by the optical pickup is "0"; When the laser beam hits a place without pits, the light is reflected back to the optical path and picked up, and the signal is "1". With the rotation of the record, the long and short pits constantly sweep the laser beam, and the density and intensity of the reflected light will change accordingly, forming a continuous signal stream. After photoelectric conversion, current-voltage conversion, amplification and shaping, the digital sound signal recorded on the record is obtained.

Digital sound signal contains modulation, synchronization, error correction and other information, and it must be decoded, digital filtered and D/A (digital/analog) converted to get analog sound signal.

CD is actually a kind of CD, which is the carrier or "medium" of information storage. The substrate of an optical disc is made of glass or plastic. The key of manufacturing is to form a recording film on the substrate and carve a recording groove. Most of the entire disk surface is a data track for storing information or data. A spiral groove with a pitch of 1.65 micron (the diameter of hair is about 70 microns) and a width of 1 micron is carved in this area. The groove consists of countless pits. Each groove is divided into 32 sectors, which is convenient for storing all kinds of information. And how is this groove as thin as hair 1/70 made? Of course, this is inseparable from the magical light of the laser. The specific method is as follows: firstly, a thin protective adhesive layer is coated on the substrate, and the laser beam is focused into fine light with a diameter below 1 micron to expose the adhesive layer. In order to ensure that the pitch of spiral grooves is equal everywhere, it is necessary to equip the laser with an automatic focusing system and an automatic tracking system, because the exposure time base rotates at a constant speed. After exposure, it is developed and dried, and then a thin conductive layer and a nickel film are coated on the substrate. At this time, grooves have been formed on the nickel film. Nickel is separated from the glass substrate and copied to the substrate with recording film again to obtain a complete optical disc with pre-grooves.

The substrate of an optical disc is not just a piece of glass or plastic. It must be precisely polished, with light transmittance of over 90%, good rigidity, high-speed rotation resistance, good affinity for recording film and low thermal conductivity. At the same time, the requirements for recording film materials are also very high. It is expected to have good reproducibility and high sensitivity, and the storage life after recording information should be more than 10 years. In the process of optical disc processing, the requirements for the environment are also very strict, so that the dust that is difficult for human eyes to distinguish will also cause errors or even distortions. In addition, strict testing and sealing are essential.