[SI-LIST] Re: What's different between Pre-emphasis and De-emphasis?
- From: John Wilson <jmwilson@xxxxxxxxxxxx>
- To: lluo3@xxxxxxxxxxxxxx
- Date: Tue, 19 Oct 2004 11:41:55 -0400
Lei is correct. The frequency content is the same for both methods (given the
same relative amplitudes) when referring to pre-emphasis and de-emphasis for
high-speed digital I/O applications (serial links, etc..). It's probably safest
to call it de-emphasis, but pre-emphasis seems to be more common.
john
--
John Wilson
Research Assistant Professor
North Carolina State University, ECE Dept.
441 EGRC, Centennial Campus
jmwilson@xxxxxxxxxxxx
Phone: 919-513-7365
Fax: 919-515-2285
lei luo wrote:
> Istvan:
>
> > 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.
>
> I think above process is still pre-emphasis since the high frequency
> components are emphasized relatively to the low frequency components.
>
> Lei
>
> ----- Original Message -----
> >From: "Istvan NOVAK" <istvan.novak@xxxxxxxxxxxxxxxx>
> To: <jianhaw.tw@xxxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
> Cc: <jianhaw.tw@xxxxxxxxxxxx>
> Sent: Tuesday, October 19, 2004 8:08 AM
> Subject: [SI-LIST] Re: What's different between Pre-emphasis and
> De-emphasis?
>
> > 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
> >
> >
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