What Is Amplitude Modulation (AM)?
The first amplitude modulated signal was transmitted in 1901 by a Canadian engineer by the name Reginald Fessenden. He used a continuous spark transmission and placed a carbon microphone in the antenna lead. This transmission was very crude; signals were audible over a distance of a few hundred meters. The quality of the audio was not good.
Amplitude modulation commonly referred to as AM, is a type of modulation where the amplitude of the carrier wave is varied in some proportion with respect to the modulating data or the characteristics of the signal such as a vocal or musical sound composed of audio-frequency waves. As for the mechanism, when amplitude modulation is used there is a variation in the amplitude of the carrier. Here, the voltage or the power level of the information signal changes the amplitude of the carrier. In AM, the carrier does not vary in amplitude. However, the modulating data is in the form of signal components consisting of frequencies either higher or lower than that of the carrier. The signal components are known as sidebands and the sideband power is responsible for the variations in the overall amplitude of the signal.
Amplitude modulation is used in electronic communication, most commonly for transmitting a piece of information with a radio carrier wave. Although one of the earliest used forms of modulation, it is still used today, mainly for long, medium and short wave broadcasting. Currently, this method is used in many areas of communication such as in portable two-way radios, citizens band radios, VHF aircraft radio and in modems for computers. It is also used to mention the mediumwave AM radio broadcasting.
There are three main types of amplitude modulation. They include:
- Double sideband-suppressed carrier modulation (DSB-SC).
- Single Sideband Modulation (SSB).
- Vestigial Sideband Modulation (VSB).
Facts About Amplitude Modulation (AM)
- In AM, the amplitude of the carrier wave is modified in order to send the data or information.
- AM method of audio transmission was successfully carried out in the mid-1870s.
- In AM, a radio wave known as the “carrier” or “carrier wave” is modulated in amplitude by the signal that is to be transmitted.
- The frequency and phase remain the same.
- AM radio ranges from 535 to 1705 KHz or Up to 1200 bits per second.
- AM has poorer sound quality compared with FM, but is cheaper and can be transmitted over long distances. It has a lower bandwidth so it can have more stations available in any frequency range.
- Its modulation index varies from 0 to 1.
- Bandwidth requirement is twice the highest modulating frequency. In AM radio broadcasting, the modulating signal has bandwidth of 15kHz, then the bandwidth of an amplitude-modulated signal is 30kHz.
- AM is more susceptible to noise because noise affects amplitude, which is where information is “stored” in an AM signal.
- In AM, if two or more signals are received at the same frequency, both are demodulated which causes interference.
- It requires low bandwidth in the range of 10 kHz.
- There is wastage of power is more as a major part of the power carried by the carrier wave does not contain the information.
- It operates in the medium frequency (MF) and high frequency (HF).
- The number of sidebands is constant and equal to 2.
- Zero crossings in modulating signals are equidistant.
- AM transmitters and receivers are less complex than FM and PM but synchronization is needed in the case of SSBSC carriers.
- The received signal is of low quality.
- It has a large transmission range. It can transmit over long distances.
- It is a less costly method.
- It has a relatively simple circuit design.
Also Read: Difference Between Delta Modulation And Differential Pulse Code Modulation
What Is Frequency Modulation (FM)
Frequency modulation is a technique or a process of encoding information on a particular signal, either analogue or digital. In this process, the frequency rather than the amplitude of the carrier wave is made to vary in proportion to the varying amplitude of the modulating signal. Generally, a modulating signal is information or message that has to be transmitted after being converted into an electronic signal.
Frequency modulation just like in amplitude modulation also has a similar approach where a carrier signal is modulated by the input signal. However, in the case of Frequency Modulation (FM), the amplitude of the modulated signal remains constant.
Frequency modulation is mostly used in radio broadcasting. It offers a great advantage in radio transmission as it has a larger signal-to-noise ratio. Meaning, it results in low radio frequency interference. This is the main reason that many radio stations use FM to broadcast music over the radio. Frequency Modulation is also used in radar, telemetry, seismic prospecting and in EEG, different radio systems, music synthesis as well as in video-transmission instruments.
Facts About Frequency Modulation (FM)
- In FM, the frequency of the carrier wave is modified in order to send the data or information.
- FM was developed in 1930s by Edwin Armstrong in the United States.
- In FM, a radio wave known as the “carrier” or “carrier wave” is modulated in frequency by the signal that is to be transmitted.
- The amplitude and phase remain the same.
- FM radio ranges in a higher spectrum from 88 to 108 MHz. or 1200 to 2400 bits per second.
- FM is less prone to interference than AM. However, FM signals are impacted by physical barriers. FM has better sound quality due to higher bandwidth.
- Its modulation index is always greater than one.
- Bandwidth requirement is twice the sum of the modulating signal frequency and the frequency deviation. If the frequency deviation is 75kHz and the modulating signal frequency is 15kHz, then the bandwidth required is 180kHz.
- FM is less susceptible to noise because information in an FM signal is transmitted through varying the frequency, and not the amplitude.
- In FM, if two or more signals are received at the same frequency, the receiver captures the stronger signal and eliminates the weaker one.
- It requires high bandwidth in the range of 200 kHz.
- There is no wastage of power as all transmitted power is carried by the information signal.
- It operates in the upper VHF and UHF range where noise effects are less.
- The number of sidebands having significant amplitude depends upon the modulation index.
- Zero crossings in modulating signals are not equidistant.
- FM transmitters are more complex than AM because the variations of modulating signal to be converted and detected from the corresponding variation in frequencies.
- The received signal is of high quality.
- It has a smaller range of transmission. It cannot transmit over long distances.
- It is a more costly method when compared to AM.
- It has a complex circuit design.
Also Read: Difference Between Modulation And Demodulation
Difference Between AM And FM In Tabular Form
| BASIS OF COMPARISON | AM | FM |
| Description | In AM, the amplitude of the carrier wave is modified in order to send the data or information. | In FM, the frequency of the carrier wave is modified in order to send the data or information. |
| Year of Development/ Transmission | AM method of audio transmission was successfully carried out in the mid-1870s. | FM was developed in 1930s by Edwin Armstrong in the United States. |
| Radio Wave | In AM, a radio wave known as the “carrier” or “carrier wave” is modulated in amplitude by the signal that is to be transmitted. | In FM, a radio wave known as the “carrier” or “carrier wave” is modulated in frequency by the signal that is to be transmitted. |
| Frequency & Phase | The frequency and phase remain the same. | The amplitude and phase remain the same. |
| Range | AM radio ranges from 535 to 1705 KHz or Up to 1200 bits per second. | FM radio ranges in a higher spectrum from 88 to 108 MHz. or 1200 to 2400 bits per second. |
| Sound Quality | AM has poorer sound quality. | FM is less prone to interference than AM. |
| Modulation Index | Its modulation index varies from 0 to 1. | Its modulation index is always greater than one. |
| Bandwidth Requirement | Bandwidth requirement is twice the highest modulating frequency. | Bandwidth requirement is twice the sum of the modulating signal frequency and the frequency deviation. |
| Noise Interference | AM is more susceptible to noise because noise affects amplitude, which is where information is “stored” in an AM signal. | FM is less susceptible to noise because information in an FM signal is transmitted through varying the frequency, and not the amplitude. |
| Two Or More Signals | In AM, if two or more signals are received at the same frequency, both are demodulated which causes interference. | In FM, if two or more signals are received at the same frequency, the receiver captures the stronger signal and eliminates the weaker one. |
| Bandwidth | It requires low bandwidth in the range of 10 kHz. | It requires high bandwidth in the range of 200 kHz. |
| Wastage of Power | There is wastage of power is more as a major part of the power carried by the carrier wave does not contain the information. | There is no wastage of power as all transmitted power is carried by the information signal. |
| Operation | It operates in the medium frequency (MF) and high frequency (HF). | It operates in the upper VHF and UHF range where noise effects are less. |
| Number Of Sidebands | The number of sidebands is constant and equal to 2. | The number of sidebands having significant amplitude depends upon the modulation index. |
| Zero Crossings | Zero crossings in modulating signals are equidistant. | Zero crossings in modulating signals are not equidistant. |
| Complexity | AM transmitters and receivers are less complex than FM and PM but synchronization is needed in the case of SSBSC carriers. | FM transmitters are more complex than AM because the variations of modulating signal to be converted and detected from the corresponding variation in frequencies. |
| Signal Quality | The received signal is of low quality. | The received signal is of high quality. |
| Transmission | It has a large transmission range. It can transmit over long distances. | It has a smaller range of transmission. It cannot transmit over long distances. |
| Cost | It is a less costly method. | It is a more costly method when compared to AM. |
| Circuitry Design | It has a relatively simple circuit design. | It has a complex circuit design. |
Also Read: Difference Between PCM, DM, ADM, And DPCM
Advantages And Disadvantages of Amplitude Modulation
Advantages
- It is simple to implement
- Demodulation of AM signals can be done using simple circuits consisting of diodes.
- AM transmitters are less complex.
- AM receivers are very cheap as no specialized components are required.
- AM waves can travel a long distance.
- AM waves have low bandwidth.
Disadvantages
- It is not efficient in as far as power use is concerned.
- It requires a very high bandwidth that is equivalent to that of the highest audio frequency.
- Amplitude modulation is prone to high levels of noise.
- It is not efficient in terms of its use of bandwidth. It requires a bandwidth equal to twice that of the highest audio frequency.
- Reproduction is not high fidelity.
- The applications of amplitude modulation limits to VHF, radio and applicable one to one communication only.
- There is decline in signal quality on the receiving end.
Advantages And Disadvantages of Frequency Modulation
Advantages
- It gives noiseless reception because amplitude limiters are used to remove the amplitude variations caused by noise.
- Operating range is quite large.
- The efficiency of transmission is very high.
- Easy to apply modulation at a low power stage of the transmitter.
- It is possible to use efficient RF amplifiers with frequency modulated signals.
Disadvantages
- Requires more complicated demodulator
- FM transmission and reception equipment is expensive as the circuitry is complex.
- It has infinite number of sidebands
- The channel bandwidth is much higher.
- The Area of reception for FM is much smaller because reception is limited to line of sight.