Amplitude modulation (AM) is a form of modulation in which the amplitude of a carrier is altered to convey information. There are variations of AM in optical, electronic, and electromagnetic (i.e., wireless) transmission systems. The information conveyed is analog, including such things as voice, music, and video. A form of analog modulation can also be used to convey digital information, but it is usually called amplitude shift keying (ASK) and will not be discussed here.
AM in More Detail
An AM transmission is one in which a carrier at a particular frequency is established, and then modulated by adjusting the peak amplitude of the sine wave of the carrier to correspond to a given baseband signal. In AM, the baseband signal is an analog signal. In ASK, the baseband signal is a digital signal.
This modulation technique has the effect of creating a signal that has 2/3 of it's power at the carrier frequency, but creates two sidebands in the frequencies immediately above and below the carrier. These two sidebands are equal in bandwidth to the modulating signal, and are mirror images of one another (i.e., they convey identical information). This means that only 1/6 of the power of an analog signal is being used to carry meaningful information, making it a modulation technique with low power efficiency.
Over the years, many variations of AM have been created to try to deal with this inefficiency and preserve the frequency space. They include double-sideband suppressed-carrier (DSBSC), single-sideband modulation (SSM), and vestigial sideband (VSB) modulation.
Examples of AM Systems
AM is found in radio transmission systems, although most people who still listen to the radio prefer to listen to FM radio. The AM radio bands received by most radios are consider medium wave and occupy the frequencies from 520 to 1,610 kHz. There are also long wave (153-279 kHz) and short wave (2.3 - 26.1 [[MHz]) radio systems. In North America, long wave is reserved for aeronautics navigation aids. Short wave is used by a variety of audio services, and by ham radio operators.
Medium wave signals are the ones we are using when we listen to the AM radio, and they are less popular because of some of the characteristics of AM. AM is highly susceptible to disruption by other electromagnetic energy. The quality of the reception is affected by distance, reflection, and interference. The channels allocated in the medium wave frequencies are not adequate for high-fidelity stereo music. It is for this reason that most AM stations feature talk radio and most music is now found on FM radio stations.
Another analog transmission system is broadcast television. Because the cable television system has its roots in broadcast television, the analog channels on the cable TV network also use a form of AM called VSB modulation. In this system the transmitter (i.e., the camera) scans the pixels of an image and assigns each an amplitude value reflecting its luminescence. A baseband signal is created that reflects the luminescence value of each pixel scanned, in sequence. This baseband signal is then modulated onto a carrier in a particular channel, and the lower sideband is partially filtered. The video information is carried on the upper sideband. Analog video transmission is rapidly giving way to digital video transmission.
- Amplitude shift keying (ASK)
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