Unit-II Audio Systems -12 Marks

Course Outcome Maintain Audio Systems

Introduction A compact disc, or CD, is an optical storage medium with digital data recorded on it. The digital data can be in the form of audio, video, or computer information. When the CD is played, the information is read or detected by a tightly focused light source called a LASER (thus the name optical medium). This article will focus on audio compact discs, which are used to play back recorded music. By the late 1970s, a common set of standards for the optical storage discs had been developed by the joint efforts of Sony and Philips. A consortium of 35 hardware manufacturers agreed to adopt this standard in 1981 and the first compact discs and compact disc players were introduced in the market in 1982.

What is a CD? A compact disc is a thin, circular disc of metal and plastic about 12cm (just over 4½ inches) in diameter. It's actually made of three layers. Most of a CD is made from a tough, brittle plastic called polycarbonate. Sandwiched in the middle there is a thin layer of aluminum. Finally, on top of the aluminum, is a protective layer of lacquer. The first thing you notice about a CD is that it is shiny on one side and dull on the other. The dull side usually has a label on it telling you what's on the CD; the shiny side is the important part. It's shiny so that a laser beam can bounce off the disc and read the information stored on it.

Construction of Compact disc Storage area 15mm 33mm 1.2mm Disc thickness 50mm 116mm 120mm

v Label side Recording side or Audio data On one side of the disc the audio signals are recorded on other side label ,indicating contents of the disc are printed. v Label side Recording side or Audio data

Size and capacity- Diameter- 4.75 inch(120mm) Whole to insert CD- 15mm Actual storage area- 33mm circular Disc thickness- 1.2mm A standard CD can store up to 74 minutes of data i.e. audio information However, most CDs contain only about 50 minutes of music, all of which is recorded on only one side of the CD (the underside).

Material used

Block Diagram of CD Player

Components of CD Mechanism CD pick-up assembly CD Lens: - Collimation Lens - Concave Lens - Objective Lens - Cylindrical Lens 3. Drive motors - Tray, loading motor - Slide, sled, feed motor - Spindle, disc, turntable motor 4. Gear system

Pick –Up assembly

Pick-up Assembly

Lenses used in CD Player 1.Collimation Lens 2. Objective Lens 3. Concave Lens 4. Cylindrical lens

Collimation Lens It is used to produce completely parallel beams of LASER. This lens together with objective lens is used to focus the laser beam to the disc surface.

Objective Lens It is used to focus the LASER beam onto the CD surface and to receive the reflected beam. It sharpens the beam and focus exactly at the center of CD track using focus and tracking coil.

Concave Lens This concave lens is used to concentrate the beam Reflected back from the disc surface, on to the photo-diode array. This lens is mainly used to improve the sensitivity of the photo diode array.

Cylindrical lens It is used to enable the reflected beam from the Cd to assist in creating the necessary signal to make sure that that focus of the laser beam on the playing surface of the disc is maintained.

Drive motors in CD player Types of drive motors used in CD player Tray or loading motor Disc, spindle or turntable motor Slid , sled and feed motor

Tray or loading Motor It is also called carriage motor, moves tray in and out for loading and unloading the disc. Usually plastic tray is driven by a plastic gear box next to the play assembly. The tray of loading motor is controlled by a loading driver IC and a signal from the system control IC processor.

Slide, sled , feed motor It moves the optical pick- up assembly across the disc from the inside to the outside rim of the CD , keeping the objective lens constantly in line with the centre of the optical axis or track. The motor is gear driven to a rotating gear that moves pick-up assembly up and down.

Spindle disc and turntable motor The spindle motor starts to rotate after the disc has been loaded. the disc starts at approximately 500 RPM slows down to approximately 200RPM.

Hi-Fi Amplifier Hi-Fi means High Fidelity. Fidelity means faithfulness. High fidelity reproduction is essentially sound reproduction such that the most critical person can listen without any distortion.

Hi-Fi Amplifier Characteristics Stereo Amplifier becomes Hi-Fi amplifier if the following requirements fulfilled- 1.Signal to noise ratio should be better than 50dB. 2.Frequency response should be flat within +-1dB. 3.The system should posses dynamic range of atleast 8dB. 4.Non linear distortion should be less than 1%. 5.Stereophonic effect should be provided. 6. Environmental conditions should be such as to eliminate the external noise in listening room.

Specifications of Hi-Fi Amplifier Gain: voltage gain= 20log vo/vin(dB) Bandwidth Impedance Distortion i) Frequency distortion ii) Phase Distortion iii) Amplitude distortion iv) Self oscillation distortion

Comparison of stereo and Hi-Fi amplifier Sr.No Stereo Amplifier Hi-Fi Amplifier 1 Less signal to noise ratio. Better signal to noise ratio. 2 Non-linear distortion occurs. Non-linear distortion not more than input/output. 3 Equalizers not used. Equalizers used. 4 Two- way cross-over network with gain control exist. Three- way cross-over network exist.

Block diagram of Hi-Fi amplifier

Block diagram of Hi-Fi amplifier Left channel Matching ckt. Right channel Matching circuit Pre- Amplifier Equalizer Power amplifier L.S. Pre amplifier Equalizer Power Amplifier Matching Network L.S.

Controls available on Hi-Fi Aplifier Balance , Blend and master gain control Loudness control Tone control i)Bass control ii) treble control 4.Quasi stereo switch 5. Mic in 6. Aux in 7.Earphone in

Block diagram of MP3 Player

Block diagram of Flash MP3 Player

A Home theater