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Troublesome PC digital chatter picked up by electric guitars

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Re: Troublesome PC digital chatter picked up by electric guitars

Postby ian2 » Thu May 28, 2020 8:03 am

I wanted to see what happened when the UR242 interface was used with my old PC, but Yamaha/Steinberg don't have a UR242 driver for Windows Vista or Windows XP.

After going into Cubase I paired things down to just this:
[*] PC (Dell Optiplex 3060), connected only to the mains, i.e. with no monitor, no mouse, no keyboard, no network cable
[*] UR242 USB audio interface, powered up from its mains adapter, but without any USB cable connecting it to the PC
[*] USA Strat, with bridge pickup selected, connected by a 10 foot cable to the audio interface
[*] Headphones connected to the audio interface

In the headphones I still got lots of digital chatter emanating from the PC. :frown:

I'd previously found that disabling some power management related BIOS settings improved things a little. From my researches, any of the following could be more fundamental culprits:

[*] The PC motherboard may not be well connected to the PC case, from the perspective of grounding
[*] The power supply in the PC may be of low quality - variations in output voltage may be causing (EMI?) interference
[*] Given that it hasn't been tested, it's possible that there is a poor connection to earth from the mains electricity socket in the room (although note that this factor, if present, didn't affect the combination of my previous PC and E-MU 1616m)

Any thoughts about the above, or other things to try?

Thanks,

Ian
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Re: Troublesome PC digital chatter picked up by electric guitars

Postby Hugh Robjohns » Thu May 28, 2020 11:34 am

From your descriptions, the problem is a one of unwanted shash being radiated from the computer and being picked up by the guitar, and then made audible either by the guitar or the preamps in your interface (or both).

In a perfect world this wouldn't happen, of course, but we've never lived in a perfect world!

The standard way of preventing the radiation and reception of unwanted stuff is Faraday Sheilding -- meaning a conductive enclosure fully surrounding each device's electronics.

Most computers have a Faraday Shield which is the grounded metal case... but because the case is made of separate panels, and because those panels necessarily have holes in them, it can never be a perfect shield. You may be able to improve the effectiveness by ensuring removable panels are grounded more effectively, for example by improving the earth contacts where the panels fit into the case chassis, or by adding grounding wires to screw terminals on the panels, etc.

But this may well affect any warranty on the new computer and is all a bit ad hoc...

So rather than try and improve on the computer shielding to stop noises getting out, a more practical solution is probably to improve the shielding around the guitar's electronics to stop the noises getting in.

Very few guitars seem to come with shielded electronics cavities, even though the high-impedance and unbalanced nature of guitar electronics means that they are particularly prone to picking up unwanted interference. If your guitar has unshielded cavities I'd make sorting that out a priority and this article explains how:

https://www.soundonsound.com/sound-advice/minimising-noise-electric-guitar-rigs

Of course, shielding the electronics is great, but you can't shield the pickups and they are expressly designed to pick up low level changes to the magnetic field around them. Humbucking pickups are designed to reject ambient magnetic fields and focus purely on the effect of the plucked strings, but their construction means they tend to have a different sound character. However, there are humbucking pups that are designed to sound like single coils and Kinman is the manufacturer that seems to receive all the praise in that department.

https://www.soundonsound.com/reviews/kinman-impersonator-54

There are also solutions involving a humbucking coil built into a replacement scratch plate:

https://www.soundonsound.com/reviews/ilitch-electronics-pgncs-t

The other possibility, particularly if the interference is RF, is that it is being demodulated and thus becoming audible, in the preamp itself. A well-designed preamp won't let out-of-band-signals in at all, but a surprising number of designs will and do! And again this problem is made worse by the high-impedance nature of the circuitry. There are ways of dealing with this -- such as adding ferrite beads to the input leads in the preamp, but again this is getting a bit techy and warranty-busting! Experimenting with different preamps is probably an easy route!

And lastly, as with all interference problems, the inverse square law is your friend. More distance between source and receiver means lower levels of interference, and a doubling of distance quarters the level.

HTH
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Re: Troublesome PC digital chatter picked up by electric guitars

Postby Sam Spoons » Thu May 28, 2020 11:37 am

Doubling the distance surely reduces the level of EMI to ¼?
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Re: Troublesome PC digital chatter picked up by electric guitars

Postby ef37a » Thu May 28, 2020 11:52 am

Sam Spoons wrote:Doubling the distance surely reduces the level of EMI to ¼?

Sam "Inverse Square Law" only applies if the radiation pattern is spherical, it rarely is.

There is always, with electric guitars and noise the problem of 'expectation'. They are never going to be as silent say as an SM57! My crude tests some years ago showed a bog S Strat would deliver a noise floor just better than -70dBfs ref a gentle strum of an open E that hit around -18dB but peaking above that.

IMHO if you want a guitar much quieter than that you need to resort to gates.

Dave.
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Re: Troublesome PC digital chatter picked up by electric guitars

Postby Hugh Robjohns » Thu May 28, 2020 12:15 pm

Sam Spoons wrote:Doubling the distance surely reduces the level of EMI to ¼?

Yes, quite right -- typing faster than I'm thinking again!

Double the distance means the intensity is then 1/(2 squared) which is 1/4 of course!

I've corrected my post. Thanks Sam. Quick catch! :thumbup: :oops:
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Re: Troublesome PC digital chatter picked up by electric guitars

Postby Hugh Robjohns » Thu May 28, 2020 12:18 pm

ef37a wrote:"Inverse Square Law" only applies if the radiation pattern is spherical, it rarely is.

Yes, also very true ...

Okay, forget 'inverse square law'... Let's just say that moving further away from the source of noise will help a lot... :lol:

And even a small increase in distance can often make a worthwhile improvement.
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Re: Troublesome PC digital chatter picked up by electric guitars

Postby ian2 » Wed Jun 03, 2020 11:32 pm

Result, I think :bouncy: - on Sunday I opened up the case and measured the resistance between the bare metal of the case and various screws that I figured ought to be connected to ground. Generally the resistance was indicated to be about 0.4 Ohms but in one or two cases it was rather more - in one case something around 100 Ohms; that was one of the screws that held a hinging piece of the case onto the main part of the case. I tightened those screws, in addition to checking that the other screws were tight.

In the midst of all this I removed a couple of screws - concerned with the hinging part of the case - that I probably needn't have removed. Then, in trying to get the case back together again I bent two corners of one of the panels so badly that it was never going to go back on without some remedial action. I had to completely remove the hinged part of the case, in order to be able to beat the bent panel corners back into a vaguely flat shape with a hammer(!). To do this I had to disconnect a number of cables linking the motherboard and the hinged panel.

After reassembling everything I was chuffed to find that, with the guitar at my normal distance from the PC, the digital chatter from the PC was no longer apparent in the audio signal from my Strat's bridge pickup. There was a little bit of what I took to be 50Hz hum, but quite likely in the same ballpark as with the combination of my previous PC and previous audio interface.

I've read that a PC motherboard naturally creates a myriad of tiny ground loops. So - what made the difference on Sunday? Was it improving the integrity of the Faraday cage by tightening those screws, or was it reducing or eliminating one of those tiny ground loops by the re-seating of one of the cables I temporarily detached from the motherboard?

The main thing is that I feel I can now record overdriven electric guitar sounds with an acceptable signal-to-noise ratio. :bouncy:

Thanks to everyone for your input. And I may yet Investigate whether my Strat's control cavities are shielded.
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Re: Troublesome PC digital chatter picked up by electric guitars

Postby zenguitar » Wed Jun 03, 2020 11:41 pm

Result !!

Excellent news :thumbup:

Andy :beamup:
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Re: Troublesome PC digital chatter picked up by electric guitars

Postby ef37a » Thu Jun 04, 2020 6:35 am

Yes, well done Ian. You have found that every piece of metal screening needs to be bonded to earth or it acts as an aerial!

Another area to be wary of is the solder pads under the M3 screws securing the MOBO to the casing via posts. The screw can read low R to case but not make intimate contact with the pad both sides and this leaves part of the ground plane 'floating' (yes, the plane will find ground somewhere but that length of track will have inductance and 'sit it up' at RF.)

Now that you have a nice quiet setup it is worth making a 'silent' reference recording. With the guitar connected and parked and the controls set as for playing (make a note) run a recording for say 30 seconds. You can of course listen critically to that but even better is to put it through a 'Real Time Analyser' such as the free RightMark Analyser (as a 16 bit 44.1kHz .wav) and save the piccy.
As time goes on you see and you add to your kit, a low noise floor can creep away from you and a monthly or so silent check can catch things before they catch you having done something a bit critical.

Dave.
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Re: Troublesome PC digital chatter picked up by electric guitars

Postby Martin Walker » Thu Jun 04, 2020 10:10 pm

ef37a wrote:Now that you have a nice quiet setup it is worth making a 'silent' reference recording. With the guitar connected and parked and the controls set as for playing (make a note) run a recording for say 30 seconds. You can of course listen critically to that but even better is to put it through a 'Real Time Analyser' such as the free RightMark Analyser (as a 16 bit 44.1kHz .wav) and save the piccy.
As time goes on you see and you add to your kit, a low noise floor can creep away from you and a monthly or so silent check can catch things before they catch you having done something a bit critical.

What a great idea Dave!

I've done this in the past while investigating review interface problems, but never thought to make a 'silent recording' as a matter of course when everything's working well, for subsequent reference purposes.

Clever!


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Re: Troublesome PC digital chatter picked up by electric guitars

Postby ef37a » Fri Jun 05, 2020 10:17 am

Well thank you Martin!

Born of slight necessity I fear!
When son was doing his music he would re configure and mess with things in an attempt to get 'The Sound' he was after for his guitar or in some cases because something SEEMED not to work and he would just re-plug in an attempt to fix it.

I would come home from work and attempt to sort things out but very often there would be a hum we did not have a couple of days ago. resolved therefore to make a log of the system as best it could get so I would know instantly if things had changed!

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