Signal Filtering
Filters may remove (band-reject) or allow (band-pass) portions of analog (possibly audio signals) that contain a range of high and low frequencies that are not necessary to transmit. In some cases, additional signals (at different frequencies) may be combined with audio signals prior to transmission. These signals may be multiple channels or may be signals that are used for control purposes. If control signals are added to an analog signal that is transmitted, they are usually removed from the audio signal in the receiver by filtering.
Figure below shows typical audio signal processing for a communications transmitter. In this example, the audio signal is processed through a filter to remove very high and very low frequency parts (audio band-pass filter). These unwanted frequency parts are possibly noise and other out of audio frequency signals that could distort the desired signal. The high frequencies can be seen as rapid changes in the audio signal. After an audio signal is processed by the audio band-pass filter, the sharp edges of the audio signal (high frequency components) are removed.
Signal Amplification
Signal amplification is a process of sensing an input (usually low level) signal and converting the signal into a larger version of itself. An amplifier device provides this conversion process. Amplifiers increase both the desired signal and unwanted noise signals. Noise signals are any random disturbance or unwanted signal in a communication system that tends to obscure the clarity of a signal in relation to its intended use.
Figure below shows how a signal may be amplified. This diagram shows that the input signal is increased in value by an amplifier that can vary its gain (amount of amplification).
Signal Shaping
Audio signals may be processed by shaping circuits to add or remove emphasis of frequency (tone) or intensity (volume). When the signal processing involves differences of amplification of specific frequency components of an input signal, it is called pre-emphasis and de-emphasis. Signal processing that involves relative changes in the amount of amplification dependent on the level of input signal, it is called companding and expanding.
Some analog transmission systems use pre-emphasis circuits to amplify the high frequency components of the audio input signal which allow the modulation system to be more effective. Certain modulation systems do not respond well to low amplitudes of high frequency input signals. By boosting the high frequency component of the input audio signal, the modulator better translates the input signal into a modulated carrier signal. When pre-emphasis is used for transmission, a matched de-emphasis system is used in the receiver to convert the boosted high frequency component back into its original low signal level.
The intensity of an audio signal can vary dramatically because some people talk loudly and others talk softly. A system that reduces the amount of amplification (gain) of an audio signal for larger input signals (e.g., louder talker) is called companding. The use of companding allows the level of audio signal that enters the modulator to have a smaller overall range (higher minimum and lower maximum). High signals and low signals input to a modulator may have a different conversion level (ratio of modulation compared to input signal level). This can create distortion so companding allows the modulator to convert the information signal (audio signal) with less distortion. Of course, the process of companding must be reversed at the receiving end, called expanding, to recreate the original audio signal.
Figure below shows the basic signal companding and expanding process. This diagram shows that the amount of amplifier gain is reduced as the level of input signal is increased. This keeps the input level to the modulator to a relatively small dynamic range. At the receiving end of the system, an expanding system is used to provide additional amplification to the upper end of the output signal. This recreates the shape of the original input audio signal.
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