LECTURE 11

AUDIO SYSTEMS

·  In the telephone transmitter, a carbon microphone can change sound waves into audio-frequency voltages or audio-frequency currents.

·  These voltages or currents are called analog or audio (AF) signals.

·  AF signals can be amplified through the use of an audio-amplifier.

·  AF signals can be stored on a disk / record / magnetic tape.

·  AF signals can be digitized (converted from analog signals to digital pulses) as in a digital audio system.

·  The monaural audio-amplifier system uses only one channel or one path for input, amplification and output.

·  The stereo audio-amplifier system has two amplifiers or two channels for the sound to be amplified to reproduce more lifelike sounds or almost 3-dimensional (stereophonic) sound.

·  Through the stereo system, it is possible to amplify the sound from each microphone separately.

·  Audio systems were further improved in an effort to obtain as natural a reproduction of sound as possible.

·  A process involving 4-channel stereo systems was developed.

·  The 4-channel systems were called quadraphonic, quadrasonic, multichannel, or surround-sound.

·  All these systems may not produce true four-channel sound, but they were attempts to reproduce sound almost as human ears would hear it if it were played “live”.

·  This is called a high-fidelity sound.

·  Several elements are common to the monaural, stereo and four-channel amplifier systems, which are microphones, phonograph cartridges, connecting audio cables, audio amplifiers, audio receivers, CD players, magnetic tape players, loudspeakers and headphones.

·  Input devices can be microphones and phonograph cartridges.

·  Output devices can be tape recorders, CD players and radios.

·  Any of these input and output devices can be monaural, stereo or four-channel.

·  A microphone changes the energy of sound waves into electric energy.

·  The mechanical energy of the moving air striking the microphone’s diaphragm is changed into electric voltages and currents.

·  These represent the audio frequencies and volumes of the original sound wave.

·  Two of the more common types of microphone are dynamic microphone and crystal microphone.

·  The dynamic microphone has a diaphragm connected to a lightweight form with a small coil of wire wrapped around it.

·  The high-fidelity dynamic microphone can respond to the frequencies ranging from about 40 Hz to 20 kHz.

·  Crystal microphone heart consists of Rochelle salt crystals or of a ceramic element.

·  When the crystal is vibrated mechanically, an alternating voltage is developed between its two opposite faces, known as the piezoelectric effect.

·  There are two common kinds of cartridges used in pickup arms for audio systems, which are crystal cartridge and magnetic cartridge.

·  The crystal cartridge has a Rochelle salt crystal or a ceramic element, to which is fastened a needed or stylus.

·  As the needle moves in the grooves of a record, it swings from side to side.

·  This mechanical motion is applied to the crystal, and a voltage is produced (known as the piezoelectric effect).

·  The magnetic cartridge performs the same function as the crystal cartridge and also work by electromagnetic induction.

·  Two important characteristics are often used to compare magnetic cartridges, which are frequency response and compliance.

·  Frequency response is the range of audio frequencies to which a magnetic cartridge will respond without distortion.

·  In high-fidelity cartridges, the frequency response range may be from 20 Hz to 20 kHz.

·  Compliance is a measure of the ability of the needle to move easily within the cartridge assembly.

·  The compliance is often indicated in centimeters-per-dyne (cm/dyn) units.

·  An example would be 25 * 10-6 cm at an applied force of 1 dyn.

·  A high cartridge compliance is desirable since it increases the frequency response of the cartridge and reduces needle and record wear.

·  The main function of an audio amplifier is to amplify or increase or enhance the volume of the input signal.

·  The power output of an amplifier is rated in a number of ways, e.g. music, peak and Root Mean Square (RMS).

·  RMS power rating is the most meaningful rating for comparing different amplifiers.

·  RMS is obtained by applying a sine wave at different frequencies to the input and by measuring the power output across a resistive load.

·  The power values at the different frequencies are squared and totaled, and the mean or average is found.

·  Then the square root is taken to give the RMS value.

·  The loudspeaker changes electric energy into sound.

·  A single loudspeaker can produce a good frequency response.

·  High-fidelity systems often use three loudspeakers, called the woofer, the midrange speaker and the tweeter.

·  The low frequencies (20 Hz to 1 kHz) are handled by the woofer, the middle frequencies (800 Hz to 10 kHz) are handled by the midrange loudspeaker, and the high frequencies (3.5 kHz to 20 kHz) are handled by the tweeter.

·  The volume is the output sound level of a circuit.

·  The volume control of an amplifier circuit is most often a potentiometer (a variable resistor type).

·  Tone refers to the balance of low frequencies or bass and high frequencies or treble in the sound output.

·  A tone control lets user adjust the frequency response of an audio amplifier so as to affect this balance.

·  A basic tone-control circuit consists of a capacitor and a potentiometer connected in series across the output of an amplifier stage.

·  Magnetic tape has a special coating of iron oxide.

·  Sounds are induced on this coating electromagnetically.

·  For stereo, four-track tapes are used where two-tracks are used at the same time, one for each channel.

·  The compact disc (CD) player is a digital audio system that uses an invisible laser beam to read data on the disc.

·  Three new processes involved to make the CD player technology possible:

1.  The creation of digital data that can represent the complex waveforms of music (sound).

2.  The development of a laser beam with an optical (light) system that is tiny and sensitive enough to transmit and record digital data representing musical waveforms.

3.  The development of a mechanical system that can accurately control and play back the data approaching the original sound.

·  Analog-to-Digital Converter (ADC) is a device used to convert audio signals (analog form) into a digital form where a binary digital code is used.

·  A binary code, which has only two states, on or off (1 or 0) can be used to indicate different levels of amplitude of an audio signal.

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