[SI-LIST] Re: What's different between Pre-emphasis and De-emphasis?
- From: Doug Brooks <doug@xxxxxxxxxx>
- To: jianhaw.tw@xxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Tue, 19 Oct 2004 12:30:23 -0700
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)=
=20
losses (caused by skin effect and by dielectric losses.) These cause a=20
signal distortion because the high frequency signal components are=20
attenuated relative to the lower frequency signal components. One way to=20
handle this is by equalization, or emphasis, increasing the higher=20
frequency components relative to the lower frequency ones, or decreasing=20
the lower frequency harmonics to bring them back into balance with the=20
already attenuated higher frequency ones.
There are two fundamental ways to do this, before the transmission line and=
=20
after the transmission line, and it might be done with active components or=
=20
with passive (filter) components. If it is done before the transmission=20
line it is called pre-equalization, or perhaps pre-emphasis. If it is done=
=20
at the far end of the line it is called post emphasis (or perhaps=20
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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