Hunt for RFI offenders

[Don Hills]

2 hours ago, Spacehound said:

...

Computer aren't usually permeable.

Except for the fan inlets and the air exits. And as the fan sucks stuff in it usually has a fine grille over it often made of metal,  and the outlets are usually small slots. ...

 

Computers usually used in domestic situations are exceptionally "permeable". 

The biggest problem is the aforementioned "slots". Slots are good aerials / antennas, acting as dipole radiators. A slot will form anywhere there is not continuous metallic contact between panels. (*)

Side access panels make poor contact along most of their edges, forming slots.

Grilles are usually only bonded to the cabinet at the corners, forming slots along the edges. The metal covers on the back panel covering the card slots usually only make good contact with the case at the ends. And ventilation slots are, well, slots.

 

(*) Old timers may remember when equipment such as computers used to have metal "finger" springs along the edge of the panels, which formed a continuous connection along the joins when the panels were closed. 

[Don Hills]

14 hours ago, Spacehound said:

...

At IBM we didn't give a rats ass about external RFI. We were far more interested in the operators dropping cigarette ash on the  'open' disk packs  as they inserted and removed them.  Later on we found that   didn't matter  much either  

 

What shielding there is on 'home' computers is more about passing 'RF radiating' regulations than anything else.

 

23 years at IBM, myself...

There was the trick of putting an AM radio on top of the CPU, then running a program that ran various delay loops to generate musical tones.

There was no smoking in the computer rooms I frequented as a condition of the Maintenance Agreements, but I agree there was plenty of opportunity for contaminants to get in. I recall an operator putting a pack in a drive, lifting off the cover then sneezing mightily before closing the drive. One of my jobs as an engineer was to clean packs and heads.

 

Home computer shielding is a bit of a joke, especially with the fashion for windows in the cases to show off the tech. Manufacturers have had to resort to tricks like spread spectrum clocking to get the EMI down to allowable levels.

Most of it is at frequencies too high to have any possible effect on audio equipment, though.

[Don Hills]

1 hour ago, sandyk said:

(...) If the Signal Integrity of the data received at the USB input of the DAC is so bloody good, why do so many members need to use (...) widgets (...)  to obtain half decent USB audio (...)

 

In some cases, it's simply because they believe the widgets help. In others, it's because their DACs are poorly designed. Affordable DACs exist where the perturbation of the output by "unclean" input is so low as to be down in the DAC's self noise.

[Don Hills]

11 minutes ago, sandyk said:

..

 This should get up a few Objective noses.

 

Only if you inhale.