linuxaudio.org

On 05/28/2010 05:34 PM, fons@kokkinizita.net wrote:

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> On Fri, May 28, 2010 at 05:05:19PM +0200, Robin Gareus wrote:

>

>> Well, interpreting an 'experiment' that one has not conducted

>> him/herself without knowing _all_ parameters of the setup is not

>> something to encourage doing..

>

>> Did you use symmetric (XLR) connections?

>

> Where possible, yes. Note that we are seeing a *modulation*

> effect, the level of the spurious signals is proportional

> to the wanted signal. It's not additive.

a 2nd measurement showing a measurement at - let's say 1215Hz would have

made that clear in the first place. Anyway I take your word for it.
Thanks for /organizing/ this little quiz; it's quite fun!
>> Are the sample-rate settings of both cards identical?

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>

> Shouldn't matter, as the connection is via an analog signal.

>

>> what was the physical distance between the devices

>> (cross-talk)?

>

> It's absolutely not crosstalk.

>

>> Was there any ground-lift equipment (built-in?) in place

>> or maybe you simply forgot to switch off phantom-power?

>

> All those would create additive effects, which is not what

> we see. Phantom power, where available, was off. 

good; those charge-pumps that are used to create 48V from 5 or 12V

often cause ripples in the supply voltage.. which in turn can influence

the oscillator.. resulting in signal modulation.
> Card X

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> has both line and mic inputs, all show the same result.

> Mic inputs are driven via a balanced passive attenuator

> with an output impedance of 50 ohm.

>

>> Anyway, good that you've labeled it 'quiz'; so here's my guess:

>>

>> You're encountering reflections (or even standing-waves) in the cable.

>> (did you uncoil it? or was it a very short one?)

>> Did you repeat the measurement? using different cables for example.

>

> No differnece seen between a cable of 3 m and on of 18 m.

>

> This will answer most other posters, except Chris:

>

>> Why 1015 Hz ?

>

> To clearly separate the signal from any 50 Hz harmonics.

>

>

> One more hint: card X is a very high end one. It should

> be 'perfect', or at least much better than card A.

OT: I would not trust any consumer-grade audio-interface for signal

differences larger than 80dB anyway. I'm amazed that card A shows such a

low noise-floor in the loop-back test.
> Ciao,

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>

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