Pre-emphasis and De-emphasis

In signal processing, there are two important techniques called pre-emphasis and de-emphasis. They help to improve the quality of any communication especially audio signals on the transmitter and receiver sides. In this article, we deal with the concept of pre-emphasis and de-emphasis, their advantages and their application in various fields of communication systems.

Table of Content

What is Pre-emphasis and De-emphasis?
Pre-Emphasis
De-emphasis
Comparison Between Pre-emphasis and De-emphasis

What is Pre-emphasis and De-emphasis?

The presence of noise is also an issue in FM and we know that noise usually has higher amplitude and higher frequency. When the amplitude of a high-frequency noise is higher than the current component in the modulation signal, it causes high-frequency interference. To deal with this issue, most FM circuits use a technique called pre-emphasis during transmission and de-emphasis during receiving. Pre-emphasis and de-emphasis circuits are commonly used in FM transmitters and receivers to improve the output signal-to-noise ratio. Pre-emphasis and De-emphasis are used to improve the fidelity of FM transmission of the audio signal.

Pre-emphasis and de-emphasis circuits are integral components used in frequency modulation (FM) systems.

Pre-emphasis is used at the transmitter
De-emphasis is used at the receiver

Pre-Emphasis

The higher the frequency modulation in FM, the greater the effect of noise. For higher modulation frequency (fm), this effect can be reduced by increasing the value of the measurement factor (mf). This can be done by increasing the difference Δf. δf can be increased by increasing the amplitude of the modulation signal at a higher modulation frequency.

The process of optimizing the frequency components of a signal to improve the signal-to-noise ratio. Therefore, the higher the amplitude of the modulation signal, the more we can improve its ability to resist the noise of the modulation. This results in an improvement in signal-to-noise ratio, or SNR. Artificially enhanced frequency change is called pre-emphasis. Preliminary analysis is performed on the transmitter before frequency adjustment is made.

Pre-emphasis Circuit

In the pre-emphasis circuit:

R1, R2, and C form a high-pass filter.
The audio signal is applied to the input.
The output is taken across capacitor C.
Below is a High Pass Filter with transfer function
50µsec pre-emphasis corresponds to frequency response curve

The formula to calculate the time constant (τ) of the pre-emphasis circuit is:

𝛕= R1 ✖ C1

Where

R1is the resistance of resistor R1.
C1 is the capacitance of capacitor C1.

Pre-emphasis Characteristics

Advantages of Pre-emphasis

It help us to amplify high frequency signal components so that they have a higher magnitude than the noise components. It improves Signal to Noise ratio or SNR.
It has an upper cutoff point where signal enhancement begins to phase.
This is a simple high pass filter with amplification
The pre-emphasis has time constant of 50µs.

Disadvantages of Pre-emphasis

If there are too many constraints, overmodulation will occur.
Pre-emphasis amplifies high-frequency components, including noise, which can degrade the signal-to-noise ratio.
Pre-emphasis is most effective in systems with high signal-to-noise ratios and wide bandwidths, making it less suitable for some applications.
Implementing pre-emphasis requires additional circuitry and introduces complexity, potentially increasing the cost of the system.

Application of Pre-emphasis

Enhances signal clarity by boosting higher frequencies before modulation.
Increases speech intelligibility by boosting higher frequencies in voice signals.
Improves audio fidelity by emphasizing higher frequencies during recording.
Improves perceived audio quality by allocating more bits to higher frequencies during encoding.
Enhances detection of weak radio signals from celestial objects by boosting certain frequency components.

De-emphasis

The process applied at the end of receiving assistance or compensation for the first time for artificial products that are frequently replaced is called highlighting. It is the process of high frequency component of message signal to get back the original transmitted message signal. The artificially boosted high frequencies signals are bought to their original amplitude using the de-emphasis circuit. After demodulation, emphasis is made at the receiver.

The De-emphasis does exactly reverse of Pre-emphasis counterpart, it is used at the receiver part.

De-emphasis Circuit

In the de-emphasis circuit:

R and C form a low-pass filter.
The pre-emphasized signal is applied to the input.
The output is taken across resistor R.

The formula to calculate the time constant (τ) of the deemphasis circuit is:

𝛕= R ✖ C

Where:

R is the resistance of resistor R.
C is the capacitance of capacitor C.
It is a simple Low Pass Filter with time constant of about 75µsec.
Emphasis reduction of 75 µs corresponds to a frequency response curve with a 3dB drop in frequency with an RC time constant of 75 µs.

f = 1/2πRC = 1/2π×75×10^-6=2,122Hz

De-emphasis Characteristics

Advantages of De-emphasis

It helps adjust the pre-amplified signal to its normal amplitude level.
It uses Low pass filter.
The de-emphasis has a cutoff frequency of 2122Hz.
De-emphasis has cutoff frequency of 75µs.

Disadvantages of De-emphasis

There must be risk of overmodulation.
De-emphasis reduces the amplification of high-frequency components, which may lead to a loss of detail in the signal, particularly in the presence of noise.
Like pre-emphasis, de-emphasis must be matched between transmitting and receiving systems, which can be challenging in heterogeneous environments and may require additional calibration.
If not properly implemented, de-emphasis can introduce distortion to the signal, affecting its fidelity and quality.

Application of De-emphasis

Deemphasis attenuates boosted higher frequencies in received FM signals, restoring original frequency balance.
Normalizes frequency response of received signals, improving speech clarity.
Deemphasis ensures faithful reproduction of tonal balance in pre emphasized recordings.
Compensates for pre emphasis during encoding, ensuring accurate signal reproduction.
Restores frequency balance of received astronomical signals for analysis.

Comparison Between Pre-emphasis and De-emphasis

Parameter	Pre-emphasis	De-emphasis
Circuit Used	High pass filter	Low pass filter
Time constant	T=RC= 50µs	T=RC= 75µs
Definition	Boosting of higher frequencies	Removal of higher frequencies
Used at	FM transmitter	FM receiver
Implementation	High-pass filter in transmitter circuitry	Low-pass filter in receiver circuitry
Purpose	Compensates for expected attenuation during transmission	Restores original frequency balance of received signal
Advantages	Improves signal-to-noise ratio, enhances fidelity	Improves signal-to-noise ratio, enhances fidelity
Applications	FM broadcasting, audio recording, digital communication	FM demodulation, audio playback, communication receivers

Characteristics of Pre-emphasis and De-emphasis

We will see the characteristics of pre-emphasis and de-emphasis given below :

Characteristics of Pre-emphasis

Boosts higher-frequency components of a signal before transmission.
Compensates for the expected attenuation of higher frequencies during transmission.
Implemented using a high-pass filter in the transmitter circuitry.
Improves signal-to-noise ratio (SNR) by amplifying higher frequencies.
Enhances the overall fidelity and intelligibility of the transmitted signal.
Commonly used in FM broadcasting, audio recording, and digital communication.

Characteristics of De-emphasis

Attenuates higher-frequency components of a received signal.
Reverses the pre-emphasis applied during transmission to restore the original frequency balance.
Implemented using a low-pass filter in the receiver circuitry.
Restores the original frequency characteristics of the transmitted signal.
Helps improve the SNR and overall fidelity of the received signal.
Essential in FM demodulation, audio playback, and communication receivers.

Conclusion

The pre-emphasis and de-emphasis helps to improve the quality of audio signals on the transmitter and receiver sides. The pre-emphasis boosts higher frequencies at the transmitter whereas de-emphasis reduces the frequencies at the receiver. They are used together to improve signal to noise ratio increasing audio signal fidelity over communication channels.