[SI-LIST] Re: What's different between Pre-emphasis and De-emphasis?

  • From: Ray Anderson <reanderson@xxxxxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Tue, 19 Oct 2004 13:11:44 -0700

I think most of the respondents to this thread are saying about the same 
thing, except in different words. Let me throw in my 2 cents worth on 
the topic:

* Pre-emphasis: Increases the high frequency content of a signal 
relative to the low frequency content..
* De-emphasis:  Decreases the low frequency content of a signal relative 
to the high frequency content.

The end result of either of the above is to generate a signal with 
increased high frequency spectral content relative to the low frequency 
content with the intent of compensating for frequency dependent losses 
in a channel. Ideally the rising response of the signal and the falling 
response of the channel will cancel to provide a flat response. If done 
correctly the signal at the far end of the channel will have the same 
ratio of  HF and LF content as the driving signal would have had without 
pre- or de-emphasis.

Pre-emphasis or de-emphasis (as defined above) are applied at the driver.


* Equalization: Can be either passive or active, lumped or distributed.

Passive equalizers may be realized as RLC filters that have a highpass 
characteristic. The intent is to introduce a complementary frequency 
dependent response  into the channel to counteract the natural falling 
frequency dependent response of the channel. Some cable manufacturers 
have implemented a distributed version into the cable by utilizing 
specially designed  conductors built with a ferromagnetic annular tube 
integrated into the conductors.

Active equalizers are typically adaptive and are constructed using DSP 
techniques to realize an appropriate FIR filter that compensate for the 
channels frequency dependent response.

Failure to compensate for a typical channel's frequency dependent 
response can result in ISI and other types of DDJ that can contribute to 
eye-pattern closure at the receiver.


If H(w) is the channels lowpass response and 1/(H(w) is the signal's (or 
equalizer's) highpass response then the composite response is H(w) * 
1/H(w) = 1 (or a flat response). Obviously this is a simplistic 
explanation, but it illustrates the concept.

Also note, that various forms of spectral compensation (pre-emphasis or 
de-emphasis and equalization) may be applied in combination.
Hopefully this doesn't confuse the issue.


Ray Anderson


Doug Brooks wrote:

>Sorry all, I can't help but try to simplify the answer that I believe he=20
>might be looking for here.
>
>In a lossy transmission line situation, there are high-frequency (harmonic)
>losses (caused by skin effect and by dielectric losses.) These cause a
>signal distortion because the high frequency signal components are
>attenuated relative to the lower frequency signal components.  One way to
>handle this is by equalization, or emphasis, increasing the higher
>frequency components relative to the lower frequency ones, or decreasing
>the lower frequency harmonics to bring them back into balance with the
>already attenuated higher frequency ones.
>
>There are two fundamental ways to do this, before the transmission line and
>after the transmission line, and it might be done with active components or
>with passive (filter) components. If it is done before the transmission
>line it is called pre-equalization, or perhaps pre-emphasis. If it is done
>at the far end of the line it is called post emphasis (or perhaps
>de-emphasis). Some people may use these terms to mean:
>Pre-emphasis --- amplifying the higher frequency components to compensate=20
>for the higher frequency losses
>De-emphasis --- attenuating the lower frequency components to bring them=20
>back into balance with the higher frequency losses.
>
>I'm not sure I like the terms, but this might be what the context is.
>
>Doug Brooks
>http://www.ultracad.com
>
>
>
>
>At 11:33 PM 10/18/2004, you wrote:
>  
>
>>Hi All,
>>Just want to ask one simple question which already
>>confused me a long time.
>>What's different between Pre-emphasis and De-emphasis?
>>Do they have different technology?
>>Or can anyone indicate which are good documents?
>>
>>Thanks a lot.
>>
>>JH Oct.19,2004
>>
>>    
>>
.
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