[LAD] A little quiz about audio measurements...

[Olivier Guilyardi]

On 05/28/2010 08:07 PM, Ralf Mardorf wrote:
> 
> 
> Ralf Mardorf wrote:
>> Olivier Guilyardi wrote:
>>> On 05/28/2010 07:36 PM, Ralf Mardorf wrote:
>>>  
>>>> Folderol wrote:
>>>>   
>>>>> On Fri, 28 May 2010 19:20:54 +0200
>>>>> Ralf Mardorf <ralf.mardorf at alice-dsl.net> wrote:
>>>>>
>>>>>  
>>>>>     
>>>>>> Veronica Merryfield wrote:
>>>>>>          
>>>>>>> You can't trust a loop back test.
>>>>>>> Any instability or dither on the reference clock of card A (fifo
>>>>>>> clocking say) is not going to show in a loop back test.
>>>>>>> Vrnc
>>>>>>>                 
>>>>>> Is Veronica Merryfield the winner?
>>>>>>             
>>>>> I'm highly suspicious of the USB link, but can't quite put my finger
>>>>> on why.
>>>>>         
>>>> Card A is the USB card. For USB there could be several issues, but I
>>>> don't have knowledge about buffering etc., but I guess it's card A and
>>>> that there's  a "instability" = jitter. I don't know what dither for
>>>> CLK
>>>> is. I guess the winner is Veronica Merryfield.
>>>>     
>>>
>>> I mentioned the clock problem first ;-) However, I thought it the
>>> other way
>>> around: I said that clocks being asynchronous that would generate
>>> artefacts, but
>>> Veronica seems to say that these are hidden when using a single clock.
>>>
>>> That's pretty much the same thing to me :p
>>>
>>> -- 
>>>   Olivier
>>>   
>>
>> Did you also say for what card? A or X? If so, is Oliver the winner?

I said that the problem should be the same with the cards inverted, see below.

>>
>> Btw.:
>>
>> Ralf Mardorf wrote:
>>> Gabriel Beddingfield wrote:
>>>> On Fri, May 28, 2010 at 12:39 PM, Gabriel M. Beddingfield
>>>> <gabrbedd at gmail.com> wrote:
>>>>  
>>>>> The 100 Hz (being 2x 50Hz, the power freq. in Italy)
>>>>> suggests that it is probably related to some manner of
>>>>> power supply.  However, I have no theory why we're
>>>>> getting 2x 50Hz (and I think I need one :-)).
>>>>>     
>>>>
>>>> Doh!  When the AC wave is rectified, it results in a signal that is 2x
>>>> the freq. because the negative part gets inverted.  That's why we see
>>>> 100 Hz instead of 50 Hz.
>>>>
>>>> -gabriel
>>>>   
>>>
>>> On card A or X?
>>> Why AM and not additive signals? 
>>
>> Is the jitter caused because of residual ripple?
>>
>> Summarized:
>>
>> Residual ripple for the DC could cause clock jitter and this for card A.
> 
> And more:
> 
> Clock jitter would cause AM (<-- not my knowledge, somebody else wrote
> it) instead of analog hum, that would cause an additive signal.
> 
> @ Oliver: Didn't you talk about syncing both cards? That's irrelevant.

Here's what I said:

"Well, it looks to me like this is caused by the fact that the audio cards
clocks are not in sync. When you loopback card A with itself, both input and
output are sampled with the same clock. But when you plug the output of card A
into card X, you're dealing with two clocks which are not synchronized, and that
result into these "artefacts". If I'm right then you should observe some
/similar/ issues when repeating the experience with the cards inverted."

Unfortunately, today my mails aren't posted on the lists, I don't know why. Some
days it works, and the day after it doesn't. So look at the second post from
Fons, there's my answer.

What I said is about syncing yeah. Maybe that Veronica is being more precise
mentioning clock instability or dither. But I'm very low in regard to theory
anyway, I'm the intuitive guy ;-)

--
  Olivier