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Noise Sources
Modern communications systems are electronic, and electronic circuits always have a certain amount of electrical noise. At room temperature, the molecules in an object that seems completely still actually vibrate at high speeds and in random directions. In electronic components, this produces a small amount of electrical noise. The atmosphere also produces its share of electronic noise, such as from lightning strikes and the effects of solar flares. Radio waves traveling through the air pick up this noise; the farther the waves travel, the more noise they accumulate.
SNR and Intelligibility
Scientists measure noise in communication systems with a figure called the signal-to-noise ratio, or SNR. This is the power of the signal you want to send, divided by the noise you don̵7;t want, and it is usually expressed in decibels. For example, if you have a one-watt signal that contains .01 watts of noise, you divide one watt by .01 and get 100, then take the common logarithm to obtain two, and multiply that by 10 to arrive at an SNR of 20 dB. This is a poor SNR figure; if you had a pocket radio with 20 dB SNR, you would hardly be able to understand speech or music through it. Scientists set a minimum SNR at 25 dB for intelligibility. The higher the SNR, the clearer the signal. A good stereo amplifier, for example, has an SNR of 80 dB or better.
Analog
Analog communications systems use traditional electronic circuitry, such as amplifiers, receivers and transmitters. Though analog systems are simpler than digital ones, noise in them is cumulative. A communications technician is aware of the noise produced at each point in the signal chain; the better the equipment, the higher the SNR. Once noise enters the system, it never leaves, and noise produced in any part of a transmission adds to what is already there. If the overall quality of all components is an SNR of 30 dB or better, the signals are at least intelligible; for high-quality broadcasting, this figure must be 50 dB or better.
Digital
Digital communications systems convert analog signals into digital data, store and transmit them, then convert them back to analog. Digital data has the advantage of being discrete. Digital communications system, like analog ones, are not immune to noise, but designers can use clever tricks to prevent noise from accumulating. For example, digital communications circuits use a strategy called ECC, or error correction code. Extra data piggybacks on top of every byte of a digital signal transmitted. This data contains mathematical information related to the signal; a computer checks the data part against the ECC part and if it detects one or two bad bits it can automatically fix them. Too much digital noise, however, will obliterate data just as surely as it does analog signals. That is why a badly scratched CD will skip: its data has been so damaged that its ECC cannot fix it.