Noise Reduction System: Preemphasis/Deemphasis Filter

In summary, the purpose of the preemphasis filter in a noise reduction system is to boost the amplitude of high-frequency signals before transmission, in order to maintain a consistent SNR across all frequencies. This is especially useful in FM modulation, where the differentiation process can accentuate high-frequency noise. The deemphasis filter at the receiving end then reduces the signal amplitude back to its original size, minimizing any potential loss in fidelity. PDE has been used in various communication systems to improve SNR, but the actual filters used may not be strictly optimum due to cost considerations. Further reading in EE communication texts is recommended for a deeper understanding of PDE and its applications.
  • #1
Swapnil
459
6
I know that purpose of the preemphasis filter in a noise reduction system is to increase the amplitude of the input signal at frequencies where SNR is low prior to transmitting the signal. I also know that a deemphasis filter is located at the receiving end and its purpose is to undo the effects of the preemphasis filter by decreasing the signal amplitude back to its original size.

I get the general idea but I have don't know how all of this fits together. So I was wondering could anyone please tell me some specifics/details on how this type of noise reduction system works and maybe just expand upon what I said above?
 
Engineering news on Phys.org
  • #2
Imagine that a channel has a hard time transmitting some frequencies -- meaning that the channel greatly attenuates those frequencies. When you pass a signal of that frequency through the channel, the receiver only receives a very small amount of energy. The noise in the channel might be relatively constant over all frequencies, though, so the signal's SNR is significantly reduced -- at least, compared to that of another signal of another frequency at which the channel is more gentle.

If you can pre-emphasize those frequency components that the channel does not transmit well, you can maintain the same SNR across all frequencies.

- Warren
 
  • #3
i believe its only used in FM, (my theory is OLD..).

ive always read it as:.

there is more information in the higher frequecies of the audio spectrum, but the modulation techniques used. put more power into the lower frequencies.

so preemphesis is used to boost the higher frequencies or the audio spectrum, and attenuate the lower freqs. making the power spectrum of the signal more linear with the frequency spectrum.
(as i said its been awile).

for amplitude modulated signals i believe the same effect is achived with compresson and expansion. (AM and SSB). reducing the dynamic range i guess in the goal.

and if Preemphises is used for FM modulation, u can assume probably VHF or higher, and its generally not SNR that is the problem. its fidelity.
FM is not a good mode for high noise low signal condition.

but my theory is a bit rusty.
 
  • #4
My take is that you use pre/de-emphasis (PDE) to improve SNR. In general, if you know the spectral characteristics of your "message", the transfer function of your channel, and the noise spectral characteristics, you can come up with an optimum PDE filter pair that maximizes SNR at your receiver.

In general, PDE is used in broadcast FM. The big problem with FM is that your information is in the frequency, which is the time derivative of the instantaneous phase. This differentiation in the demodulation accentuates the higher frequency noise (it gives it a f^2 dependence). If you've ever had to take a derivative of a noisy signal, you know exactly what this means. At the high end of the spectrum, most "messages" (music, speaking, etc.) have very little energy, as explained below. You might be tempted to just filter off the upper end of the spectrum, but the ear will detect this as a loss in fidelity. Try turning down the treble control on your stereo --- many people describe the sound as "muffled" or "muddy". PDE gives the high-frequency message signals a boost so as to be able to be heard above the accentuated high-frequency noise.

I believe the actual PDE filters used in broadcast FM are not strictly optimum, but were chosen such that they can be realized inexpensively (like a simple single-pole RC filter on the receiver for de-emphasis). Nevertheless, the SNR improvements can be considerable --- I'm guessing around 10 dB.

I think PDE is also used in some of the noise reduction techniques used in tape recording (like Dolby, for example) to minimize high-frequency tape "hiss".

To dig deeper, I would suggest some EE communication texts, like Lathi or Haykin. Pierce and Posner is also a real gem, although a bit dated.
 

Related to Noise Reduction System: Preemphasis/Deemphasis Filter

What is a Noise Reduction System?

A Noise Reduction System is a technology used to reduce unwanted noise in audio signals.

What is Preemphasis/Deemphasis Filter?

Preemphasis/Deemphasis Filter is a type of Noise Reduction System that works by boosting and cutting certain frequencies in an audio signal. This process helps to reduce background noise and improve the overall quality of the audio.

How does Preemphasis/Deemphasis Filter work?

Preemphasis/Deemphasis Filter works by boosting the high frequencies of an audio signal before recording or transmission and then cutting them back to normal levels during playback. This process helps to reduce the level of background noise that can be picked up during recording or transmission.

What are the benefits of using Preemphasis/Deemphasis Filter?

Preemphasis/Deemphasis Filter can significantly improve the clarity and quality of audio signals by reducing background noise. This makes it easier to record and transmit audio in noisy environments and results in a better listening experience for the audience.

Is Preemphasis/Deemphasis Filter necessary for all audio recordings?

It depends on the specific situation. Preemphasis/Deemphasis Filter is commonly used for recordings in noisy environments or for radio and television broadcasts. However, it may not be necessary for all types of audio recordings, such as studio recordings with minimal background noise.

Similar threads

Replies
9
Views
2K
Replies
5
Views
2K
Replies
3
Views
1K
  • Electrical Engineering
Replies
3
Views
1K
Replies
5
Views
6K
  • General Engineering
Replies
6
Views
2K
  • Introductory Physics Homework Help
Replies
3
Views
871
  • Engineering and Comp Sci Homework Help
Replies
10
Views
1K
  • Introductory Physics Homework Help
Replies
2
Views
1K
  • Electrical Engineering
Replies
4
Views
955
Back
Top