[LAU] Aliasing noise when going D->A->A->D

[Ken Restivo]

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On Wed, May 16, 2007 at 11:52:35AM +0200, Pieter Palmers wrote:
> Ken Restivo wrote:
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> >
> >I've noticed a nasty aliasing noise when connecting an analog output
> >from a digital source, to the analog input of another digital device.
> >
> >
> >For example, going out of a cheap soundcard on a PC, into a nice (or
> >cheap) A-D converter, an ugly high-pitched aliasing noise appears
> >even with no program data. It's not audible to my ears when listening
> >just to the analog output of the hda-intel; one must plug its output
> >into an A-D in order to hear it, and then it's rather loud.
> >
> >I've noticed this with my expensive Edirol firewire interface and
> >with my cheap Audiophile USB interface. The common factor is when the
> >sound being digitized comes from an ugly hda-intel sound chip. All
> >three are running 48khz
> >
> >Would this happen even with expensive D-A's?  Is this a common
> >problem when connecting D-A back to A-D?
> >
> >I'm asking because I'm going to have to plug the analog outs from one
> >of these interfaces (the firewire or the USB, not the hda-intel) into
> >someone's PresonusFirebox/ProTools rig on Friday, and I want to know
> >if I'm going to have to deal with this aliasing noise.
> I don't think this is aliasing noise since the HDA will have some sort 
> of reconstruction filter, and the Edirol/MA-Audio interface will have a 
> decent anti-aliasing filter.
> 
> It will probably have to do with the power supply noise rejection of the 
> codec attached to the HDA chip. Or the headphone amp that is definitely 
> present in it. The load presented by a headphone (can be as low as 
> 32ohm) is different from the load presented by a device input (multiple 
> kiloohms). This can cause a change in PSSR behavior. It's related to 
> ground-loop type of behavior. (if you can make the sound change by e.g. 
> moving the mouse or changing the stuff displayed on the screen, most 
> likely you're seeing bad PSSR. The sound comes from the dc-dc converters 
> from the LCD panel).

Yep, that's probably it then. The sound squiggles and chirps with mouse movement or window redraws.

> 
> Most likely you won't be experiencing this when connecting the edirol to 
> the presonus device. The only way to find out is to try it. Be sure to 
> use balanced cables.
> 

Cool, thanks.

> >
> >He runs at 96Khz, IIRC. I dunno if that will make the noise better,
> >or worse. It'd be nice to go S/PDIF out, but I don't know if that'll
> >work either, if the sample rates don't match.
> It only works if the samplerates are the same and the two cards are 
> sync'ed. Analog will be easier.

Again, thanks.

- -ken
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