[SI-LIST] Re: what's de-emphasis
- From: Adiu <adiu_panli@xxxxxxxxx>
- To: LY <long.0.yang@xxxxxxxxx>, istvan.novak@xxxxxxx
- Date: Mon, 2 Nov 2009 05:23:14 -0800 (PST)
Thanks for all replies.
My application of this pre/de-emphasis is SerDes signals.
It is more clear to me now that pre/de-emphasis functions similarly (the pulse
waveform shape looks same), except that de-emphasis produces smaller amplitude.
However, going to the implementation details, I still have some concerns.
To explain my concerns more clearly, I am attaching a one-page word document.
In the document,
First, an arbitrary 1v nominal waveform is shown.
Second, the pre-emphasis implementation block digaram captured from a Altera
document is shown.
Third, the pre-emphasis waveform obtained based on the Altera implementation
diagram is shown. my concern: this waveform seems to me more like the
de-emphasis waveforms based on the dicussion.
Fourth, I try to plot the pre-emphasis waveform based on the discussion. my
concern: is this correct?
Could any one provide me the block implementation diagrams for both
pre-emphasis and de-emphasis?
Thanks again for everyone's time.
Best regards,
Adiu
--- On Sun, 11/1/09, Istvan Novak <istvan.novak@xxxxxxx> wrote:
From: Istvan Novak <istvan.novak@xxxxxxx>
Subject: Re: [SI-LIST] Re: what's de-emphasis
To: "LY" <long.0.yang@xxxxxxxxx>
Cc: "Bill Wurst" <billw@xxxxxxxxxxx>, "Adiu" <adiu_panli@xxxxxxxxx>, "prasad"
<hariprasad.palli@xxxxxxxxx>, Joseph.Schachner@xxxxxxxxxx, si-list@xxxxxxxxxxxxx
Received: Sunday, November 1, 2009, 10:25 PM
If you search the SI-list archives you should find past discussions on
this same topic with more detailed explanations.
LY wrote:
> Istvan:
> Could you make it clear about the pre/de-emphasis in
> telecommunications and SerDes signaling?
>
> Long Yang
> Joan Crawford - "I, Joan Crawford, I believe in the dollar.
> Everything I earn, I spend." -
> http://www.brainyquote.com/quotes/authors/j/joan_crawford.html
>
>
> On Mon, Nov 2, 2009 at 08:13, Istvan Novak <istvan.novak@xxxxxxx> wrote:
>
>> Bill and All,
>>
>> Rather than saying that this definition is wrong, we should say that
>> this does not apply to digital SerDes signaling.
>> If one reads the entire text on wikipedia, it is clear that it refers to
>> telecommunications applications, where the high-
>> frequency components are in fact suppressed. The unfortunate fact is
>> that deemphasis means different things in
>> telecommunications and in SerDes signaling.
>>
>> Regards,
>>
>> Istvan Novak
>> SUN Microsystems
>>
>>
>> Bill Wurst wrote:
>>
>>> Adiu,
>>>
>>> As Joe S. indicated, the definition for de-emphasis is clearly wrong. A
>>> more correct way of stating de-emphasis would be:
>>> De-emphasis: Improving the signal to noise ratio by
>>> decreasing the magnitude of lower frequency signals
>>> with respect to higher frequency signals.
>>> Even with this corrected definition, you will note that what both
>>> pre-emphasis and de-emphasis accomplish is similar. Thus, given the
>>> same input waveshape, the waveforms output from either process will have
>>> the same shape.
>>>
>>> Either pre-emphasis or de-emphasis is usually accomplished at the
>>> transmitter, while equalization is done at the receiver. Although all
>>> three work in different ways, they all accomplish the same purpose,
>>> i.e., to flatten the frequency response of the overall system by
>>> compensating for distortions introduced by the transmission medium.
>>>
>>> Best regards,
>>> -Bill
>>>
>>> /************************************
>>> / William C. Wurst, PE /
>>> / billw@xxxxxxxxxxx /
>>> / Advanced Electronic Concepts, LLC /
>>> / www.aec-lab.com /
>>> ************************************
>>> =========================================================
>>> prasad wrote:
>>>
>>>
>>>> i believe, its a matter of reference with which we are defining. since the
>>>> technique is much evident when we see frequent transitions, the amplitude
>>>> of
>>>> those bits is higher than the bits having the lesser transitons. extending
>>>> this to a combination of lets say, 0111, the transtion from 0 to 1 will
>>>> have
>>>> higher amplitude than the further 1s. so we say that 0to 1 is pre
>>>> emphasized
>>>> or 1to 1 is deemphasized w r t 0 to 1.
>>>>
>>>>
>>>>
>>>> 2009/10/31 <Joseph.Schachner@xxxxxxxxxx>
>>>>
>>>>
>>>>
>>>>> 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.
>>>>>
>>>>>
>>>>> Adiu wrote:
>>>>>
>>>>>
>>>>>> Hello, Everyone
>>>>>>
>>>>>> What's pre-emphasis and de-emphasis?
>>>>>> I can get the following definitions 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"
>>>>>>
>>>>>> There is not problem to understand the above definitions. However, I
>>>>>> have difficulities in the following concerns:
>>>>>> 1. is pre-emphasis always at the transmitter?
>>>>>> 2. is de-emphasis always at the receiver?
>>>>>> 3. what's difference between de-emphasis and equalization?
>>>>>> 4. I have not problems to draw the pre-emphasis pulse waveforms with
>>>>>> respect to a normal waveform. However, I am not quite sure of the
>>>>>> de-emphasis pulse waveforms. The attached figure shows a simple example.
>>>>>> Could someone confirms for me if the waveforms are draw correctly?
>>>>>>
>>>>>> any comments are appreciated.
>>>>>>
>>>>>> best regards,
>>>>>> adiu
>>>>>>
>>>>>>
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