Specialized systems

Chapter Specialized systems

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • and 2.5, 7, and 18 kHz. (These are not standard frequencies; they are given here onlyfor illustrative purposes.) The slide potentiometers are shown as variable resistors fol-lowing the attenuators. Finally, the signals pass through an audio combiner (mixer); thecomposite is sent to the output.The design of a graphic equalizer is not an easy task. The main challenge is to en-sure that the controls do not interact with each other. Judicious choice of filter fre-quencies and responses is also important. If the filters employ active devices such asoperational amplifiers, they must not introduce any distortion into the signal wave-forms. The active devices must not generate significant audio noise. Graphic equalizersare placed at low-level points because they are not built to handle high power. In a mul-tichannel circuit such as a stereo sound system, a separate graphic equalizer can beused for each channel. In stereo equipment, there are usually two equalizers, one forthe left channel and the other for the right channel.Specialized SystemsMobile and portable hi-fi systems operate at low direct current (dc) voltages. Typi-cal audio power levels are much lower than in home hi-fi systems. Speakers aremuch smaller also. In portable systems, headsets are often used in place of speakers.Mobile systemsCar stereos usually have four speakers. The left and right channels each supply apair of speakers. The left stereo channel drives the left front and left rear speakers;the right stereo channel drives the right front and right rear speakers. The balancecontrol adjusts the ratio of sound volume between the left and right channels forboth the front and rear speaker sets. Another control adjusts the ratio of sound vol-ume between the front and rear sets. This gives the impression of “surround sound.”But it is not a true quadraphonic system because there are only two independentsound channels: left (front and rear) and right (front and rear).An automotive hi-fi stereo system usually has an AM/FM receiver and a cassettetape player. More expensive systems incorporate CD players. It’s important to remem-ber that both tapes and CDs are heat-sensitive and should not be stored in a car or truckthat might become hot. On a sunny summer day, the interior of a closed parked car canrise to a level sufficient to damage tapes and CDs.Car stereo systems can relieve some of the tedium of long driving trips. But thesound can present a safety hazard if the volume is excessive. Loud music obscures out-side traffic noises, including sirens and honking horns. A headset, if worn by the driver,presents an even greater danger. In many states, it is illegal to wear a headset while dri-ving a motor vehicle.Portable systemsPortable hi-fi systems operate from sets of dry cells or rechargeable cells and can re-quire anything from 1.5 V (one cell) to 12 V (eight cells).The most well-known portable hi-fi set is the so-called headphone radio. Thereare numerous designs. Some include only an FM radio; some have AM/FM reception ca-pability. Some have a small box with a cord that runs to the headset; others are entirely596 Acoustics, audio, and high fidelity