TEXT 1. Modulation
1. Выучите следующие слова и выражения:
detect [dɪ'tekt] - замечать, обнаруживать
carrier signal - сигнал-переносчик, сигнал несущей частоты
current ['kʌr(ə)nt ]; ['kɜːr(ə)nt] – электрический ток
alternating current (AC) - переменный ток
direct current (DC) - постоянный ток
delay [dɪ'leɪ] - задерживать, замедлять
pulse code modulation (PCM) - импульсно-кодовая модуляция, ИКМ
pulse amplitude modulation (PAM) - амплитудно-импульсная модуляция, АИМ
pulse duration modulation (PDM) - широтно-импульсная модуляция, ШИМ
pulse position modulation (PPM) - фазово-импульсная модуляция, ФИМ
sideband - боковая полоса (частот)
single sideband (SSB) - с одной боковой полосой, однополосный (о системе передачи)
two-way radio - одновременная двусторонняя радиосвязь
Phase Shift Keying (PSK) - фазовая манипуляция
Quadrature Amplitude Modulation (QAM) - квадратурная амплитудная модуляция (используемая в высокоскоростных модемах техника модуляции, сочетающая фазовую и амплитудную модуляцию)
2. Прочитайте и устно переведите текст:
One way to improve the transmission of a signal is modulation. Modulation is the process of facilitating the transfer of information over a medium. For example, sound transmission in air has limited range for the amount of power your lungs can generate. To extend the range your voice can reach, we need to transmit it through a medium other than air, such as a phone line or radio. The term modulation refers to how information (voice in this case) is encoded so that it can be successfully sent through a medium (wire or radio waves), and can later be detected by a receiver.
There are two main reasons to blend data into a carrier signal. The first reason is to reduce the wavelength for efficient transmission and reception (the optimum antenna size is Ѕ or ј of a wavelength). A typical audio frequency of 3000 Hz will have a wavelength of 100 km and would need an effective antenna length of 25 km! By comparison, a typical carrier for FM is 100 MHz, with a wavelength of 3 m, and could use an antenna only 80 cm long. The other reason is to allow simultaneous use of the same channel, called multiplexing. If there were a group of people transmitting signals between 20 Hz and 20 kHz the probability of one signal interfering with another would be very high. With modulation, each unique signal can be assigned a different carrier frequency (like radio stations) and still share the same channel.
The process of modulation means taking either an analog or a digital signal and turning it into an analog signal. The difference between a digital modulation and analog modulation is the nature of the signal that is modulating the carrier. The carrier is always analog - for most of radio and telecommunications today, it is alternating current (AC) in a given range of frequencies.
Common modulation methods include: 1) amplitude modulation (AM), in which the voltage applied to the carrier is varied over time; 2) frequency modulation (FM), in which the frequency of the carrier waveform is varied in small but meaningful amounts; 3) phase modulation (PM), in which the natural flow of the alternating current waveform is delayed temporarily. These are sometimes known as analog, or continuous wave modulation methods, to distinguish them from pulse code modulation (PCM), which is used to encode both digital and analog information in a binary way. The varieties of PCM include pulse amplitude modulation (PAM), pulse duration modulation (PDM), and pulse position modulation (PPM).
Radio and television broadcast stations typically use AM or FM. Most two-way radios use FM, although some employ a mode known as single sideband (SSB).
More complex forms of modulation are Phase Shift Keying (PSK) and Quadrature Amplitude Modulation (QAM). Optical signals are modulated by applying an electromagnetic current to vary the intensity of a laser beam.
An electromagnetic carrier can be of any frequency depending on the medium and the communications needs. Most media dictate what type of carrier (its frequency, amplitude) can propagate through it and the type of distortions it will suffer while travelling through it.
Anything that is wireless is analog – always. Wired signals can be digital or analog. Communications inside a computer are examples of pure digital communications, digital data over digital medium. LAN communications are digital data over analog medium. The AM and FM radios are examples of analog data over analog medium.
In general, when we talk about a digital system, we are usually talking about digital information over an analog medium. However, there are exceptions. PCM, for example, is a form of modulation where there is no carrier, so that makes a pure digital system.
3. Ознакомьтесь с дополнительными словами и выражениями:
facilitate [fə'sɪlɪteɪt] - облегчать; устранять трудности
lung [lʌŋ] – (n) легкое
temporary ['temp(ə)r(ə)rɪ] - временный
exception [ɪk'sepʃ(ə)n ], [ek-] - исключение
LAN (local area network) - локальная сеть
4. Ответьте на вопросы, используя информацию текста:
1. Why is it necessary to use modulation? 2. What does the term modulation refer to? 3. What are two main reasons to blend data into a carrier signal? 4. Is it possible not to use the analog carrier? 5. How is the process that allows simultaneous use of the same channel called? 6. What kind of signals is modulated by applying an electromagnetic current to vary the intensity of a laser beam?
5. Найдите в тексте слова и выражения, эквивалентные данным ниже:
Ограниченная дальность; среда, отличная от воздуха; наложить информацию; природа сигнала; данный диапазон; в зависимости от среды; подвергаться искажениям.
6. Переведите на русский язык следующие словосочетания:
Sound transmission, a carrier signal, typical audio frequency, simultaneous use, the probability of one signal interfering, a different carrier frequency, continuous wave modulation methods, wired signals.
7. Вставьте пропущенные предлоги:
1. Modulation improves the transmission … a signal. 2. If we want to extend the range our voice can reach, we transmit it … a medium other than air. 3. The first reason to blend data … a carrier signal is to reduce the wavelength for efficient transmission and reception. 4. AM, FM and PM are referred … as analog, or continuous wave modulation methods to distinguish them … PCM. 5. Examples of analog data … analog medium are the AM and FM radios.
8. Вставьте пропущенные слова:
modulation, digital,amplitude, carrier, modulate
1. To … is to impress the characteristics (intelligence) of one waveform onto a second waveform by varying the amplitude, frequency, phase, or other characteristics of the second waveform. 2. ... is the means whereby a signal of some type is entered into and carried by an electronic signal carrier. 3. … modulation describes a broadcast situation in which the level of voltage that is carried over the medium vary noticeably over time. 4. A signal in a communications channel modulated to carry analog or digital signal information is called a …. 5. Communications inside a computer are examples of pure ... communications.