Electromagnetic interference

Chapter Electromagnetic interference

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

  • the recording head when the audio was originally recorded on the tape. This magneticfield induces weak alternating currents in the playback head. These currents are ampli-fied and delivered to a speaker, headset, or other output device.Digital audio tapeDigital audio tape (DAT) is magnetic recording tape on which binary digital data canbe recorded. Digital recording provides better fidelity than analog recording. Thedifference between DAT and conventional analog audio tape is similar to the im-provement afforded by the digital CD over the vinyl audio disk.In digital audio recording, tape noise is eliminated. This is because such noise isanalog in nature, and is therefore ignored by digital sound reproduction equipment.Some electronic noise is generated in the analog amplification stages following D/A con-version, but this is minimal compared with the noise generated in older, fully analog sys-tems. The reduced noise in DAT equipment provides more true-to-life reproductionthan is possible with analog methods.Another advantage of DAT is virtually unlimited copiability. The reason for this isthe same as the reason a computer can repeatedly read and overwrite data on a mag-netic disk. On DAT, the impulses are well defined: high and low states (logic ones andzeros) are represented by distinct magnetized regions on the tape. While analog signalsare “fuzzy” in the sense that they vary continuously, digital signals are “crisp.” Imper-fections in the recording apparatus, the tape itself, and the pickup head affect digitalsignals less than they affect analog signals. Digital signal processing (DSP) can elimi-nate the minute flaws that creep into a digital signal each time it is recorded and playedback. Thus, you can make copies of copies of DAT recordings, and the fidelity will notdeteriorate to any noticeable extent even after dozens of generations.Electromagnetic InterferenceAs hi-fi equipment becomes more sophisticated and complex, the circuits seem to begetting more susceptible to interference from outside sources. If a radio transmitteris operated near a stereo system, the radio signals can be intercepted by the hi-fiwiring and peripherals, and delivered to the amplifier. Unshielded interconnectingcables act as radio receiving antennas. This problem is exacerbated if any of the con-necting cables happen to resonate at the operating frequency of the radio transmit-ter. In the amplifier, the RF currents are rectified, causing changes in the audio gain.Sometimes the signal data can be heard in the speakers or headset. This is known aselectromagnetic interference (EMI).In most cases when EMI takes place in a hi-fi setup, the fault exists in the stereosystem design, not in the radio transmitter. The transmitter system is doing its job: gen-erating and radiating electromagnetic signals. There are several steps that can be takenwhen installing a stereo hi-fi system to minimize the likelihood that EMI will occur.These precautions should be followed:• Connect the stereo amplifier chassis to a good electrical ground.• Use shielded interconnecting cables as much as possible.• Use shielded (coaxial) speaker cables.• Keep all cables as short as possible.Electromagnetic interference601