[SI-LIST] Re: what's de-emphasis
- From: Joseph.Schachner@xxxxxxxxxx
- To: Adiu <adiu_panli@xxxxxxxxx>
- Date: Fri, 30 Oct 2009 17:11:19 -0400
Of the two definitions you got from Google:
Pre-emphasis: " Improving the signal to noise ratio by increasing the
magnitude of higher frequency signals with respect to lower frequency
signals"
De-emphasis: " Improving the signal to noise ratio by decreasing the
magnitude of higher frequency signals with respect to lower frequency
signals"
The Pre-emphasis definition is acceptable. The De-emphasis definition is
clearly wrong, in my opinion. De-emphasis goal is the same, to increase
magnitude of higher frequency signals with respect to low frequency
signals. It just is a slightly different way to get that effect.
I think pre-emphasis and de-emphasis are much easier to understand in the
time domain, rather than the frequency domain.
In the time domain, as text with with one dash per UI (transitions don't
appear) :
Each diagram is 4 UI low, 5 UI high, 4 UI low
_
_____ ... ____ ... _
____
____ ____ ... ____ ___ ... ____ ___
_ _
The desired pulse With pre-emphasis With deemphasis
The first UI All but the first UI
after a transition after a transition
is higher amplitude are lower amplitude
The reason to do this is because a normal channel attenuates high
frequencies more,
so the (relatively) sharp rising edge of a transmitted pulse will become
slower
at the far end, and the first UI would not reach the desired height by the
middle
of the UI. That is, it would cause the eye opening to be reduced.
By having the TX pump out a larger transition, after the loss due to the
channel,
at the far end, the signal looks better - more like what we desired.
Standards
like PCI-Express don't actually call for pre-emphasis, rather they call for
de-emphasis. The effect is the same - the transitions are sent larger than
subsequent bits; but it is achieved by using the nominal level for the
first bit
after a transition and a smaller level for all subsequent bits. This makes
a nice
but smaller eye.
--- Joe S.
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