Amplitude Modulation and Frequency Modulation are two types of radio transmission on which our communication once depended upon. In order to broadcast information in the form of voice, music, or data that exists at low frequencies, between the range of 300 and 3 KHz, a carrier signal is used. The carrier signal exists at a much higher frequency compared to the frequency of the information being transmitted. The carrier frequency can be better explained as the station where listeners tune in their stereo and the baseband signal as the voice of a broadcaster.
Even though AM and FM both accomplish the transmission of information in the form of electromagnetic waves, they differ in their modulation method, bandwidth specifications, and propagation properties. AM is the process in which the amplitude of the carrier wave is varied in proportion to the transmitting information, referred to as the baseband signal, while the carrier frequency remains constant. On the other hand, in FM the frequency of the carrier signal varies with the amplitude of the baseband signal.
Since AM is concerned with any changes in the carrier’s amplitude, noise in the form of static, electrical or atmospheric interference has a great effect in its transmission. In FM, the information is contained in the frequency of the modulated signal and that is why it is said to be immune to noise and its transmission is not affected. Bandwidth in telecommunication refers to the range of frequencies, measured in hertz, that is required in order to maintain a communication without interference, loss, or corruption of information.
In AM the required bandwidth is twice the highest frequency in the signal being transmitted. FM in contrast requires a bandwidth twice the sum of the baseband signal frequency and the frequency deviation. An advantage that AM has over FM is that AM requires less bandwidth. AM ranges from 535-1705 KHz separated by intervals of 10 KHz. On the other hand, FM exists in the range of 88-108 MHz separated by intervals of 200 KHz. Since FM stations require greater bandwidth in between its carrier frequencies, in order to avoid overlapping stations, they are more expensive.
Signal propagation in AM and FM depends on their frequency spectrum and define how signals behave when obstructed. Since the transmission through AM happens at low frequencies, 535-1705 KHz, its signal consists of wavelengths of approximately 300m, having the ability to diffract around significant obstacles. FM high frequency signals, 88-108 MHz, have wavelengths of around 3m giving them a great disadvantage when obstructed by structures with areas of tens or even hundreds of meters. When no obstacles are present AM and FM propagate equally, but in poor conditions as previously described, AM have better propagation.
In conclusion, even though FM was developed in order to fill in for the disadvantages that AM represented, such as noise vulnerability, AM is still used in today’s communication. AM is more cost efficient than FM because it requires less bandwidth, development of AM transmitter and receivers are cheaper and more accessible, and if the purpose of communication is speech broadcasting AM serves its purpose. If sound quality is the concern in communication, FM is the best way to go. FM in its nature is immune to any type of external interference, but it is more expensive and complex to develop.