[SI-LIST] Re: What's different between Pre-emphasis and De-emphasis?
- From: "Clark Foley" <clarkf@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Tue, 19 Oct 2004 12:15:53 -0800
Based upon the signal shape, there is no difference between "pre" and "de."
This perversion of once meaningful terminology (i.e. emphasis) is a crude
attempt to describe the method by which the "emphasis" at the front of a
line is created.
Allow me to EMPHASIZE that emphasis, pre or post, describes a ratio of high
frequency to low frequency content. It does not describe absolute
magnitude.
Once a long, long time ago, there was "post emphasis" and "pre emphasis."
This terminology clearly depicts the location at which the compensation is
placed. "Post" means at the end of the line or load. "Pre" means at the
front of the line or source. "De" is irrelevant and adds confusion rather
than clarity.
If you look at a picture that has overshoot to compensate for the dispersive
effects of the line, you cannot tell if the peak was increased or the logic
level decreased. All that you can say is that the high frequency components
are higher relative to the low frequency components.
What is important is to ask what is the emphasis ratio and its adjustment
range, what is the transition time and what is the peak or peak-to-peak
voltage of the signal. This is information that a designer can use to
assess EMI, signal-to-noise ratio and receiver sensitivity. Neither "pre"
nor "de" tell you enough information.
Regards,
Clark Foley
Maxim Integrated Products
(P.S. I realize that I used the word "overshoot" in my message. This is the
excursion of the signal beyond the logic level to which it will settle,
whether it be logic one or zero!)
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On
Behalf Of Istvan NOVAK
Sent: Tuesday, October 19, 2004 4:08 AM
To: jianhaw.tw@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Cc: jianhaw.tw@xxxxxxxxxxxx
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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