Pre-emphasis and De-emphasis in FM Broadcasting | Introduction


In FM broadcasting, sound quality is important. People are always striving to develop new technology to reduce the noise signals in transmission to improve the audio signals and the listening experience. Two of the technologies are Pre-emphasis and De-emphasis. Do you understand them? This share will introduce the definition and the application of Pre-emphasis and De-emphasis to you.


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What is Emphasis?


Actually, Pre-emphasis and De-emphasis can be called emphasis together. But why it is divided into Pre-emphasis and De-emphasis? To answer this question, we need to have a basic understanding of Pre-emphasis and De-emphasis.

The Definition of Pre-emphasis

Pre-emphasis is the concept used in transmitting equipment like FM transmitters. It means that prior to some process, such as the current signals being transmitted over cable, a specific range of input frequency will be boosted or the amplitude will be enlarged. In simple words, the sound volume in a specific range is raised.

The Definition of De-emphasis

On the contrary, De-emphasis is the concept used in receiving equipment like FM radios. It means that before the audio signals are transformed into sound and played, the same range of frequency will apply the transformation opposite to the Pre-emphasis. That's means, the sound volume in the specific range will be decreased.

The Differences of Pre-emphasis and De-emphasis

In conclusion, Pre-emphasis and De-emphasis are similar to each other, but they are applied in different equipment and work reversely. But they work for the same purpose - to improve the audio signals.



How Does Emphasis Work?


Pre-emphasis and De-emphasis work together to improve the audio signals. But how do they achieve this purpose?

The Noise in Audio Signals

The signals in relatively higher frequency have a higher signal resistance capability, but it has a worse noise interference capability because the signals in relatively higher frequency have smaller energy than those in relatively lower frequency. So in radio broadcasting, removing the affection of the noise in the higher frequency is important. And Pre-emphasis and De-emphasis solved the problem by improving the SNR of the signals.



Noise Elimination

Let's see how Pre-emphasis and De-emphasis work together to improve the SNR of the signals.


Determine the Frequency - Pre-emphasis amplifies the high-frequency components through a simple Pre-emphasis circuit. Here's a question, how to determine which range of frequency should be amplified? You will see there is a time interval before the signals are amplified. We call the time interval the time constant. It is calculated via the formula T=RC, where R stands for resistance in the circuit and the C stands for the power supply in the circuit. Typically, 25μs, 50μs, and 75μs these three-time constants are available, and different countries adopt different time constants as standard. For example, in North America and South Korea, 75μs is used, and in Europe, 50μs is used.


Amplify the Frequencies - If the 75μs is used as the time constant, the pre-emphasis circuit will linearly enhance the frequencies higher than 2123 Hz at the rate of 6 dB/octave, and 6 dB means four times. After enhancing the frequencies, the SNR would be improved because the amplified part of frequencies will offset the noise in the signals.


Return the Frequencies - In order to have a normal frequency response, a De-emphasis circuit should be added to the radio receiver. Just as the same as Pre-emphasis circuit, it has a time interval after receiving the radio waves, and it is the same as the one of Pre-emphasis circuit. For example, 75μs is used in the De-emphasis, then it will attenuate the frequencies higher than 2123Hz at the rate of 6dB/octave.




The Applications of Emphasis


In radio broadcasting, Pre-emphasis and De-emphasis are adopted in many applications, like FM broadcasting. Because FM has the feature of high frequency, it is easy to be affected by the noise. Pre-emphasis and De-emphasis can improve the SNR in the signals effectively. In addition to the transmission of the analog signal, the digital transmission adopts Emphasis as well. Similar to analog transmission, digital transmission used Emphasis for correcting the distortion in transmitting signals at high data rates. 


Frequently Asked Questions

1. Q:  What is the Emphasis in FM?

A: It is the process where a signal is somehow changed and returned to normal in the end.


In the process of recording and broadcasting, in order to improve the signal quality, the emphasis is that a signal is somehow altered prior to recording or transmission, and a subsequent process at the other end that returns the signal to its normal state. The most common example in audio recording is noise reduction.

2. Q: Why Pre-emphasis is Used at FM Transmitter?

A: Because it is used to improve the SNR and reduce the affection of the noise.


In the process of detecting a frequency-modulated signal, the receiver would generate a noise spectrum that rises in frequency. Pre-emphasis increases the amplitude of the higher signal frequencies, thereby improving the SNR and reducing the affection of the noise. The best selling FM transmitters from FMUSER are equipped with the latest pre-emphasis technology, if you are interested in it, check it out.

3. Q: What are FM signals?

A: They are the signals that encode the information by changing the instantaneous frequency of the wave.


The FM signals are widely used in computing, telecommunications and signal processing. They are carrying the information in the form of the instantaneous frequency changes of the wave.

4. Q: What is the range of FM signals?

A: 87.5 -108.0 MHz, 76.0 - 95.0 MHz, 65.8 - 74.0 MHz.


87.5 - 108.0 MHz is the most widely used frequency range worldwide. And 76.0 - 95.0 MHz is used in Japan, 65.8 - 74.0 MHz is mainly used in the countries located in western Europe. 




    Speaking of which, we know that Emphasis is such a practical technology in radio broadcasting, it effectively improves the radio signals in transmission. FMUSER is a professional radio broadcasting equipment supplier, you can buy high-quality FM transmitters at satisfying prices. If you need to buy FM radio transmitters with Pre-emphasis, please feel free to contact FMUSER



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