 | Tech-FAQ Tip: Click Here to Check for PC Errors |
How do Radios work?
We tend to take these little devices, which we find in our cars, offices, or homes, for granted. In other words, we are not at all bothered about how these songs, news and other information reaches our radios and hence to our ears. This article tries to take a sneak peep into the various steps that eventually lead to the voice that we hear from a speaker.
AM and FM radios are the common type of radios we find in our homes. They communicate by means of radio waves - hence the name - which is the lowest frequency of the electromagnetic spectrum. Electromagnetic waves are much similar to sound waves when considering its wave character, but differ in the aspect that it is made up of electric and magnetic vectors, and not mechanical attributes. One more point; radio waves have two aspects we change in order to communicate or send information. They are its amplitude and frequency.
The radio station sends information by embedding it in the radio waves before transmitting it through air by its transmitters. This process is called modulation and it is done by changing any of its basic characteristics, frequency or amplitude. That is, there are two types of modulation techniques, Amplitude Modulation (AM) and Frequency Modulation (FM).
Amplitude Modulation
In Amplitude Modulation, it is the amplitude or strength of the wave that is changed. The earliest AM technique to use by humans was the Morse code, in which the code key will turn the transmitter on and off. The amplitude hits the maximum during the key press and becomes nil when the transmitter is turned off with the key release. Modern AM transmitters vary the signal level proportionally to the sound they are transmitting. That is, positive peaks of the sound produce maximum radio energy and negative peaks of the sound produce minimum energy.
The biggest disadvantage associated with AM is that most man made noises are AM in nature and hence AM receivers have no means to make out that which one is required and which one is not. Also, weak signals are quieter than strong ones, and hence the receiver may need additional circuits to compensate for such signal level differences.
Frequency Modulation
On the other hand, in Frequency Modulation (FM), it is the frequency aspect that is modulated instead of the amplitude. That is, the positive peaks of the modulated signal represent higher frequencies and negative peaks lower frequencies. FM waves are free of the limitations of AM, and as the audio is dependent on the frequency of the signal, no compensation circuits for different signal levels are required.
In fact, there are other types of modulation techniques, which are some variations of the conventional AM and FM techniques. AM variants include Single Sideband (SSB), Double Sideband (DSB), and Vestigial Sideband (VSB) while FM types include Phase Shift Keying (PSK) and Minimum Shift Keying (MSK).
These modulated signals - AM or FM - are detected by the radio receiver, when tuned to that particular frequency, which is then passed through a series of circuits in order to decode and reproduce the original sound. In case of FM, a limiting circuit screens all amplitude variations from the signal while a discriminator circuit converts the frequency variations into the original signal. For AM transmissions, it may require specialized circuits as well to filter the unwanted static.
Many FM stations now transmit subcarriers on the carrier, much like the audio signals, with the exception that they are in a frequency range to high to be heard by human ears. These subcarriers are used to carry weather reports, stock quotes or information about the music being transmitted on the main carrier.
FM stereo itself relies heavily on the subcarrier to bring about the stereophonic effect. The main audio in FM stereo is monophonic, which stops at a frequency of 18 KHz. At 19 KHz is a pilot signal and there is a special stereo subcarrier centered around 38 KHz. The receiver uses this stereo subcarrier in combination with the mono signal to produce left and right channel audio outputs. The 19 KHz pilot signal is used as a reference so that the receiver can properly recover the special stereo subcarrier lying in the 38 KHz band.
Bookmark How do Radios work?
