[SI-LIST] Re: What's different between Pre-emphasis and De-emphasis?
- From: "Istvan NOVAK" <istvan.novak@xxxxxxxxxxxxxxxx>
- To: <jianhaw.tw@xxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Tue, 19 Oct 2004 08:08:26 -0400
JH,
While preemphasis and deemphasis were primarily used in
FM broadcasting and communications during the several
past decades, these terms got somewhat new meanings
in high-speed digital applications. Preemphasis means
that you boost high-frequency components, deemphasis
means the opposite: you suppress high-frequency components.
In FM communications, the channel itself can be assumed to
be flat in frequency response. Preemphasis will cancel
with deemphasis, and this trick is used to suppress the
high-frequency boost of noise that occurs in the
demodulation process.
In digital high-speed applications, the purpose of
preemphasis-deemphasis is to compensate for the
frequency dependent transfer function of traces
and/or cables, and usually they ARE NOT used in pairs,
because all what we want is to compensate for the
loss of high-frequency components.
Preemphasis in high-speed digital applications means that
you boost the high-frequency components, usually by
amplifying somewhat the leading edge after a longer
period of no transitions. The problem with this approach
is that leading edges will require more voltage swing at
the driver, and the silicon may not like it.
The other possibility to achieve the same affect is
instead of boosting the leading edge, to suppress everything
else. This is called deemphasis: you transmit the
leading edge with the nominal amplitude, but make
subsequent levels smaller.
Again, as opposed to FM communications, you
want to do either preemphasis or deemphasis, not
both.
Note that these are the simplest forms of channel
compensation, more sophistaced forms also exist.
Best regards,
Istvan Novak
SUN Microsystems
----- Original Message -----
From: "jianhaw.tw" <jianhaw.tw@xxxxxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Cc: <jianhaw.tw@xxxxxxxxxxxx>
Sent: Tuesday, October 19, 2004 2:33 AM
Subject: [SI-LIST] What's different between Pre-emphasis and De-emphasis?
> 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
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
