[SI-LIST] Re: what's de-emphasis
- From: LY <long.0.yang@xxxxxxxxx>
- To: Istvan Novak <istvan.novak@xxxxxxx>
- Date: Mon, 2 Nov 2009 11:15:10 +0800
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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