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From: Eric Jacobsen on 20 Nov 2009 15:18
On 11/20/2009 8:44 AM, Jerry Avins wrote:
> Eric Jacobsen wrote:
>> On 11/19/2009 3:31 PM, Randy Yates wrote:
>
> ...
>
>
>>> And just to clarify, you know I am referring to the situation where you
>>> get THE EXACT SAME 4 values each cycle, right? Wouldn't you agree that's
>>> a pure "signal" without any "noise".
>>
>> The question is how accurately the sampled sequence represents the
>> input signal, or, more specifically to the OP's original question, how
>> well it reveals a representative SNR figure for the ADC. If the ADC
>> only has four output levels, then maybe it's good enough, but in my
>> judgement such a sequence does a very poor job of characterizing an
>> ADC with more levels than that.
>>
>> When one assures that the signal test tone and sample clock are NOT
>> phase locked, then it's far more likely that more of the quantization
>> levels will be exercised (hopefully, eventually, all of them). If the
>> output sequence is analyzed over a window where all or a large
>> fraction of the output codes are included, then one can do a FAR
>> better job of characterizing the ADC than if only four output levels
>> are ever tested.
>>
>> This may mean that a long sequence is needed, and so the output has to
>> be processed over a large enough window to include enough output codes
>> to get the desired information.
>>
>> I didn't interpret the goal as being finding what to do to get the
>> prettiest FFT output, as you're clearly right that phase-locking the
>> test signal to the sample clock will do that. I just don't think
>> that's a good way to exercise the ADC at the high-frequency end for
>> the reasons I mention above.
>>
>> It also requires a better understanding of interpreting FFT outputs
>> than just looking for the highest peak with the lowest non-peak bin
>> energy.
>
> I don't see a disagreement. You address the issue of what the OP needs
> to do to get an accurate measurement. Randy addresses the OP's
> puzzlement at not getting the measurement he expected. Orthogonal!
>
> Jerry

You've taught me another useful thing, Jerry, that I can use the word
"orthogonal" as an exclamatory. ;)

--
Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.abineau.com
From: kongp on 22 Nov 2009 01:38
Must use long time to sampling for correct measurement ?

So, How about the fast response requirement ?

S/N ration is a good parameter for sound?
- I had heard about other: measure in decibel , beam power figure ,
close system for removing frequency 60 Hz, bootstrap modeling(ACG)
etc.

I think of Dragon System: Speech Recognition about testing by child ,
it's not loss that miss spelling may be solution to good quality..


and your formula may not accurate, It is for an ideal !!!

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