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Diagram of beat frequency. In acoustics, a beat is an interference pattern between two sounds of slightly different frequencies, perceived as a periodic variation in volume whose rate is the difference of the two frequencies. With tuning instruments that can produce sustained tones, beats can be readily recognized.
The modulation wavelength is double that of the envelope itself because each half-wavelength of the modulating cosine wave governs both positive and negative values of the modulated sine wave. Likewise the beat frequency is that of the envelope, twice that of the modulating wave, or 2Δ f .
The difference frequency, f audio = |f IF − f BFO | = 1000 Hz, is also known as the beat frequency. The other, the sum frequency, (F if + F bfo) = 89000 or 91000 Hz, is unneeded. It can be removed by a lowpass filter, such as the radio's speaker, which cannot vibrate at such a high frequency.
Frequency modulation and phase modulation are the two complementary principal methods of angle modulation; phase modulation is often used as an intermediate step to achieve frequency modulation. These methods contrast with amplitude modulation , in which the amplitude of the carrier wave varies, while the frequency and phase remain constant.
When the two signals are mixed, a "beat" frequency equal to the difference between the two frequencies is created. Adjusting the local oscillator frequency correctly puts the beat frequency in the audio range, where it can be heard as a tone in the receiver's earphones whenever the transmitter signal is present. Thus the Morse code "dots" and ...
The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called heterodyning, resulting in a signal at the difference or beat frequency. Intermediate frequencies are used in superheterodyne radio receivers , in which an incoming signal is shifted to an IF for amplification before final ...
Amplitude modulation produces an output signal the bandwidth of which is twice the maximum frequency of the original baseband signal. Single-sideband modulation avoids this bandwidth increase, and the power wasted on a carrier, at the cost of increased device complexity and more difficult tuning at the receiver.
Perception of second-order AM has been interpreted as resulting from nonlinear mechanisms in the auditory pathway that produce an audible distortion component at the envelope beat frequency in the internal modulation spectrum of the sounds. [58] [59] [60]