Fig. 2. Analog and digital signal

 

We commonly refer to the transmission of voice, music, video and data as information signals as these intended signals contain what we want to communicate. Information signals belong to baseband signals. A baseband signal, or lowpass signal, is a signal at frequencies that are very near zero. This refers to analog or digital data before they are merged with other signals (multiplexed) or modulated. Information signals are at low frequencies, often less than 50 kHz.

There is another type of signal which is called a pulse train. Here the signal is a train of impulses that last a long or a short time. The phase and amplitude of the impulse determines how it is decoded. EKG is a good example of this type of signal occurring in nature.

Two further common characteristics in communications are the frequency spectrum (f) and the bandwidth (B) of a signal. The spectrum of a signal is the range of frequencies that it contains. That is, the spectrum is the combination of all the individual sine waves of different frequencies which make up that signal. The bandwidth refers to the width of this spectrum. For example, if the spectrum of a signal ranges from 2 kHz to 22 kHz, its bandwidth is f_high - f_low = 20 kHz.

 

3. Ознакомьтесь с дополнительными словами и выражениями:

intelligent [ɪn'telɪʤ(ə)nt] – разумный

acquire [ə'kwaɪə] – получать, приобретать

finite ['faɪnaɪt] – ограниченный; конечный

сurrency ['kʌr(ə)n(t)sɪ ]; ['kɜːr(ə)nsɪ] – валюта; средство денежного обращения

vs. (from versus) – против

jerk [ʤɜːk] - двигаться резкими толчками

smoothly ['smuːðlɪ] - гладко; ровно; плавно

derive [dɪ'raɪv] - получать, извлекать

intend [ɪn'tend] – намереваться; предназначать

 

4. Ответьте на вопросы, используя информацию текста:

1. What is a signal in general understanding and in communications? 2. Can you give any examples of signals? 3. What's the difference between a continuous and discrete signal? 4. What is sampling? 5. How can the difference between analog and digital signals be explained? 6. What signals are typically met in nature? 7. What are the three main characteristics of an analog signal? 8. How is a digital signal represented? 9. What are bits? 10. What are baseband signals? 11. What is a pulse train? 12. What is a frequency spectrum? 13. Does bandwidth depend on the frequency spectrum?