[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


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

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

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