What is Signal?
In our daily lives, we often need to communicate with other people. Especially during these pandemic times, we want to communicate safely with minimum physical contact. Of course, with the advance of information technology, we can achieve that. We can do many things online — meetings, conferences, family gatherings — with such ease. All we need to have is a computing device — PC, laptops, smartphones — and an internet connection. But sometimes, we have a few problems with the internet connection; the ‘signal’ weakens, which drops the connection speed and quality, and consequently, disturbs our online activities. We are already familiar with the word ‘signal,’ but we may be having difficulty in defining that word. What is ‘signal,’ really?
A signal is defined as a physical phenomenon that carries information.
In modern times, information is often transmitted electrically, which means the physical phenomenon involved is electrical voltage and current. Here, information is any physical features that are meaningful — voice, text, images, etc. As most information is not electrical, we may ask, why should the information be carried electrically?
Electricity is chosen as an information carrier because it is very flexible.
With some ingenious engineering (which is studied in electrical engineering), we can control and process the signal easily. Electronic devices can be made small, as small as smartphones. Electrical signals can be amplified with a simple circuit. As normally we can’t see or hear electricity, the voltage and current can’t disturb us while traveling along the wires (except of course, if we directly touch the conductors of the wires — we may get electrocuted). And electrical signals can even travel wirelessly through air, by the means of radio waves. Such flexibility leads engineers to favor electricity as an information carrier.
Let’s compare to its mechanical counterpart — vibrating strings for example, which we can use to send voice over a short distance with the help of a pair of cans. Why don’t we use that? It is because communicating via such a device is intractable. Mechanical devices are often large and bulky, and maintaining a network of those strings would be unrealistic. Amplification is hard — if possible. It is also hard to store the signal if we need a device with a memory. To be capable of carrying waves, those strings need to be tense at all times. These requirements limit the ability of mechanical signals to carry information (though in the case of doomsday, we may need to use this if electricity is not available).
Now we shall see how the signal can take form. There are two types of signal, analog and digital. An analog signal is a signal that has continuous values. A digital signal is a signal that has discrete values that represent certain numbers. These types of signals have their uses and advantages, but almost — if not all — physical phenomena we can sense are analog, meaning the most natural form of signal is analog. Digital signals are produced by the process of digitization. Some small amount of information is lost in this process, but even with the presence of disturbance in transmission, we can recover the received signal that is exactly same with the one transmitted on the sender. This is impossible with analog signals, as both the signal and disturbance are continuously-valued, thus impossible to exactly guess the original signal without disturbance. Nowadays, digital signal transmission is more favored than analog transmission.
A simple case of music streaming via the internet
Let’s see a simple case of signal transmission in real life — the internet. Suppose we want to listen to music by streaming. First of all, the music — the sound of music — is recorded in a studio. Microphones and instrument amplifiers convert sound waves to analog electrical signals. The signal is then digitized with a computer, resulting in a digital signal. The digital signal containing the music is then stored as a music file in a storage device. The music file is acquired by the streaming service provider from the recording label, then stored in their storage that can be accessed via the internet.
When we request to play the music in the streaming app, we are sending a request to the streaming server to send the music file to our device. The server then proceeds to send the file to our device via the internet. The music file is sent as separate packets, which travels hundreds — even thousands of kilometers. It may even be sent underground or underwater — via international networks of optical fiber. Then the packets reach their way to our device, to be grouped again. The digital signals formed from the packets are sent to a device that converts digital to analog signals. Finally, the analog signal reaches our speaker, which converts them to sound. And that is how the music we listen reaches us via the internet.
Communication is of much importance in our daily lives. A simple activity of communication we do via the internet has a complex mechanism behind it. It is the clever engineering of signals that keeps us connected. We should be grateful if we are privileged with communication facilities, because we may reflect, what can we do without being connected to everybody else?
