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

[Pieter Palmers]

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).

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.

> 
> 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.

Pieter