Pre-emphasis is a signal-processing technique used at the transmitter end to compensate for the high-frequency components of the input signal. As signal rates increase, the signal becomes increasingly distorted during transmission. To ensure that the receiving end captures a clean and well-defined signal waveform, it’s essential to restore these high-frequency components. The core idea behind pre-emphasis is to boost the high-frequency parts of the signal right at the beginning of the transmission line, counteracting the excessive attenuation they experience as they travel through the channel. Importantly, pre-emphasis has no impact on noise levels, thereby effectively improving the output signal-to-noise ratio.
Introduction
The theory has proven that the output noise power spectrum of a frequency discriminator increases proportionally to the square of the frequency. However, many real-world message signals—such as speech or music—exhibit power spectra that decrease as frequency rises, with most of their energy concentrated in the lower-frequency range. This can lead to a signal-to-noise ratio at the high-frequency end of the message signal dropping to an unacceptable level. Yet, because the higher-frequency components in these message signals carry relatively little energy—often insufficient to induce even the maximum frequency deviation—the actual signal amplitude responsible for generating the largest frequency deviation typically stems from the lower-frequency parts of the signal. On average, the smaller-amplitude high-frequency components contribute far less to frequency deviation. As a result, FM signals don’t fully utilize the bandwidth allocated to them. It’s important to note, however, that the transmission bandwidth of an FM system is determined by both the highest effective frequency of the message signal (the modulating signal) and the maximum frequency deviation it can accommodate. Meanwhile, the noise spectrum at the receiver input spans the entire FM bandwidth, meaning that the noise power spectrum at the discriminator output becomes disproportionately amplified at higher frequencies.
To counteract this unwanted phenomenon, FM systems commonly employ a technique known as pre-emphasis and de-emphasis. The core idea behind this approach is to leverage the differences between the signal characteristics and noise characteristics, enabling efficient signal processing. Specifically, before noise even enters the system, an appropriate network—called the pre-emphasis network—is used to artificially boost (or "emphasize") the high-frequency components of the modulating signal fed into the transmitter. Then, at the output of the receiver’s frequency discriminator, the opposite process takes place: a de-emphasis network is applied to reduce the emphasis on those same high-frequency components, thereby restoring the original power distribution of the signal. Importantly, while this de-emphasis process also helps attenuate the high-frequency noise components, it leaves the noise itself unaffected. As a result, the overall signal-to-noise ratio at the output is significantly improved.
For example: Signal transmission
The Relationship Among Three Signal Compensation Techniques
During signal transmission, different signals experience varying degrees of attenuation across their frequency components, ultimately leading to signal distortion. To ensure that the received signal maintains a high-quality waveform at the receiving end, it’s necessary to compensate for the degraded signal. Commonly used compensation techniques include pre-emphasis, de-emphasis, and equalization. This section provides an overview of these three signal compensation methods.
Due to the significant attenuation of high-frequency signal components compared to low-frequency ones in the signal path, equalization at the receiver end involves processing the signal to appropriately boost its high-frequency content, based on the attenuation characteristics of the PCB the signal has traversed. This ensures that both low- and high-frequency components are "balanced" at a consistent level, ultimately enhancing the signal's transmission speed and distance between the sender and receiver ports. In contrast, pre-emphasis is applied at the signal’s transmitting end. It proactively amplifies the high-frequency components of the signal in anticipation of the attenuation that will occur as the signal travels through the channel. As a result, by the time the signal reaches the receiver, the natural high-frequency attenuation across the channel is effectively compensated, allowing the receiver to capture a complete and undistorted signal. At the transmitter end, another technique—de-emphasis—can also be employed to reduce the low-frequency components of the signal, thereby counteracting the inherent high-frequency attenuation in the signal path. However, de-emphasis works by attenuating the signal’s energy, which lowers its amplitude and can make it harder for subsequent circuit stages to accurately detect and interpret the signal. For this reason, in practical applications, the pre-emphasis method is more commonly chosen over de-emphasis.
Pre-emphasis
As previously discussed, signal transmission lines exhibit low-pass filtering characteristics, causing significant attenuation of high-frequency components while preserving the lower-frequency ones during transmission. The concept behind pre-emphasis technology is to boost the high-frequency components of the signal at the input end of the transmission line, thereby compensating for the excessive loss of these high-frequency parts as the signal travels. We know that the frequency of a signal is primarily determined by how quickly its amplitude changes; thus, the high-frequency components of a signal typically appear at the rising and falling edges. Pre-emphasis technology essentially enhances the amplitude of these critical rising and falling edges.

Additionally, pre-emphasis can be implemented using either time-domain or frequency-domain techniques.
The pre-emphasis circuit is implemented using time-domain techniques by adjusting the amplitude of the bit signal to be transmitted accordingly. Specifically, when the current bit signal differs from the previously transmitted bit, its amplitude is increased by an appropriate multiple. However, if the current bit matches the previous one, no adjustment is made at all. This approach effectively achieves the pre-emphasis function.

Frequency-domain techniques implement pre-emphasis circuits by incorporating a high-pass filter. This filter boosts the energy of the high-frequency components in the signal being transmitted, effectively compensating in advance for the attenuation these components experience as they travel through the transmission line. However, this approach to implementing pre-emphasis circuits has several drawbacks: First, when designing a pre-emphasis circuit, engineers typically need to know the attenuation characteristics of the specific transmission line and identify the frequency range over which the circuit should operate. Yet in practical applications, circuit designers often lack precise information about the line’s length, type, or other critical parameters, making it difficult to accurately define the circuit’s operational frequency range and the amount of pre-emphasis required. Second, while the pre-emphasis circuit enhances the high-frequency components of the signal, it also amplifies high-frequency noise at the same time, leading to a deterioration in Near-End Crosstalk (NEXT) performance at the transmitter port. Finally, since pre-emphasis increases the energy of high-frequency signal components—and given that signal power consumption is proportional to the square of the signal voltage—this approach inevitably results in a significant rise in system power dissipation. For these reasons, time-domain techniques are increasingly preferred for implementing pre-emphasis processing.

De-emphasize
The process of restoring an already-emphasized transmit signal back to its original waveform. De-emphasis circuits are also applied at the transmitting end, performing the exact opposite function of pre-emphasis circuits: while pre-emphasis boosts the low-frequency components of the transmitted signal, de-emphasis reduces them. This ensures that after the modified signal passes through the transmission line and experiences high-frequency attenuation, the balance between low- and high-frequency components is maintained. By attenuating the signal's energy, de-emphasis reduces the amplitude of the signal sent over the transmission line, thereby minimizing signal crosstalk. However, this approach also makes the signal more susceptible to noise interference. As a result, de-emphasis circuits are used less frequently. The underlying principle of de-emphasis is somewhat similar to that of pre-emphasis—though the implementation methods differ slightly. Pre-emphasis enhances the amplitudes at the signal’s rising and falling edges while leaving other parts unchanged, whereas de-emphasis keeps the edge amplitudes constant while weakening the signal elsewhere.
 

Balance
The pre-emphasis and de-emphasis techniques introduced earlier effectively compensate for signal loss during transmission, enhancing signal quality. However, these methods also have certain limitations—specifically, when crosstalk is present on the line, pre-emphasis and de-emphasis can amplify the high-frequency crosstalk components, thereby intensifying its detrimental effects. To address these shortcomings of pre-emphasis and de-emphasis, equalization techniques were subsequently developed. Unlike pre-emphasis and de-emphasis, which operate at the transmitter end, equalization is applied at the receiver, where its characteristics closely resemble those of a high-pass filter.

An equalizer is essentially a high-pass filter—specifically, it functions as an equalizer. Equalizers are typically implemented using filters to compensate for distorted pulses. As a result, the demodulation output samples received by the decision device are either corrected by the equalizer or free from intersymbol interference. Adaptive equalizers, on the other hand, continuously adjust their gain based on real-time digital signals transmitted, following a specific algorithm. This allows them to dynamically adapt to random changes in the channel, ensuring the equalizer always operates at its optimal state—and consequently delivering superior distortion-compensation performance.