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'Digital' noise...

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'Digital' noise...

Postby starman9 » Wed Oct 16, 2013 5:20 pm

I've always experienced what I can only describe as a low level 'digital noise' with my present system (year old Carillon PC, RME Multiface 2 connected to PCI). As I've hardly used it since purchase, I have never got round to doing anything about it. But now, finally, I am getting going with it all and need to properly resolve this issue.

It worsens if I play a soft instrument... but there's actually a certain low level noise at all times. It sounds just like a similar issue I had years ago with a laptop, which I eventually cured by removing its earth (yes I know I shouldn't have...). The noise is actually very little with my present setup, but still I'd rather not have it!

I connect a number of hardware keyboards + the RME outs to an old Studiomaster P7 mixer. I notice there are NO offending noises if I listen, instead, to the headphone out of the RME, even at blisteringly loud levels.

Any thoughts/ideas that could help?
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Re: 'Digital' noise...

Postby oggyb » Wed Oct 16, 2013 5:49 pm

Is it a frequency-modulated squelchy whine? Does it change when the PC is "working" or if you move the mouse?
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Re: 'Digital' noise...

Postby Mike Stranks » Wed Oct 16, 2013 9:49 pm

I refer the Hon Gentleman to THIS thread...
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Re: 'Digital' noise...

Postby starman9 » Thu Oct 17, 2013 12:59 am

oggyb, It's not normally a whine as such, more a low level 'burbling'(!) with added 'zipper' noise when the mouse is moved or PC is 'working'.

Thanks Mike. Just had a very brief skim through some of that thread. The bit of advice that caught my eye was to use a DI box for the outputs. But I still get this noise (through my mixer) when my audio outs are completely disconnected, which presumably means a DI box would not solve the issue. I will read the thread properly when I've had some sleep, though. Perhaps there are some other goodies in there that are relevant.
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Re: 'Digital' noise...

Postby Mike Stranks » Thu Oct 17, 2013 8:11 am

Seems like you're describing classic ground-loop issues - ties-in exactly with what I was experiencing before I took action.

... and it''s not a DI box that's needed as a quick-fix the problem of ground-loops - it's an isolation transformer. I've found THESE to be versatile and of good quality. (Others say that the Maplin ones with attached phono/RCA connectors - which are much cheaper - are of sufficient quality)

Just a quick further word about the difference between a DI and an Isolator as the terms are often used imprecisely... A DI may] include some form of transformer which will give the isolation needed to break a ground-loop, but the primary purpose of a DI is to take a line/guitar-level unbalanced signal and convert it to balanced mic-level. An isolator doesn't alter levels or any other aspect of the signal in any way. It simply provides electrical isolation of the 'out' from the 'in', thus breaking the loop.
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Re: 'Digital' noise...

Postby ef37a » Thu Oct 17, 2013 9:00 am

Hi Mike,
Maplin seem to have cottoned onto how handy that isolator is (# VW43W) as it is now 15 quid! This..
http://www.amazon.co.uk/hq-CAR-NF01-Skytronic-Ground-Isolator/dp/B000NVWB9O

Seems to be the same thing? I checked one out with a Leader osc/mV meter and scope some time ago and they look pretty good but this was at 1 volt levels and so they are probably not really "stooodio" grade transformers. But, for 4 quid they are surely worth a punt as a test device? If that cures the noise then look at some really good traffs from OEP, Sowter et.al.

But!The use or transformers of whatever quality in such a good signal train goes against! I would use the Amazon ones to prove this is an earth loop then look at removing screens from cables, maybe with 1nf RF linking cap?

And you are quite right OP. Removing mains earths is VERY BAD PRACTICE!!! DO NOT DO IT!!!

Dave.
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Re: 'Digital' noise...

Postby James Perrett » Thu Oct 17, 2013 10:30 am

Personally I would just disconnect the screen at one end of the balanced connection between the interface and the mixer.

Edit: just saw that Dave has already suggested the same thing above.
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Re: 'Digital' noise...

Postby starman9 » Fri Oct 18, 2013 4:02 am

Thanks a million guys.

But the problem remains even when the outputs of my RME interface are not connected to my mixer (or anything!). It occurs as long as the PC is switched on, in fact. So does that fact mean that the isolator will not do the trick in my case? Presumably when the RME is not connected to my mixer, it is then already isolated, right?
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Re: 'Digital' noise...

Postby James Perrett » Fri Oct 18, 2013 9:02 am

Are you saying that the mixer makes these noises with nothing connected to it?

It might help if you could draw a diagram of how things are connected.
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Re: 'Digital' noise...

Postby ef37a » Fri Oct 18, 2013 10:00 am

James Perrett wrote:Are you saying that the mixer makes these noises with nothing connected to it?

It might help if you could draw a diagram of how things are connected.

Indeed James and to me this,.." I notice there are NO offending noises if I listen, instead, to the headphone out of the RME, even at blisteringly loud levels."
Does not compute!

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Re: 'Digital' noise...

Postby starman9 » Fri Oct 18, 2013 2:48 pm

Ok, I think I'd better clarify!

I have a number of keyboards, which are connected to my Studiomaster P7 mixer (an old analog mixer), along with a L+R from my RME interface.

I work in the box but I monitor everything via the mixer (and if I record the audio from any of my hardware keyboards, I use the mixer group outs, which are connected to the inputs of the RME).

I tried listening to the headphone output of the RME just in an attempt to get further in identifying the source of the problem but I don't ever usually use the RME headphone out for monitoring.

Similarly, just as an experiment, I totally disconnected the outputs of the RME but could still hear those funny little noises coming through the mixer.

Hope that's clear. Any ideas welcome and thanks again for the help so far.
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Re: 'Digital' noise...

Postby James Perrett » Fri Oct 18, 2013 3:12 pm

Try disconnecting the group outs from the mixer to the RME. Does the noise go away?
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Re: 'Digital' noise...

Postby starman9 » Fri Oct 18, 2013 10:51 pm

James Perrett wrote:Try disconnecting the group outs from the mixer to the RME. Does the noise go away?

Well actually, the 'digital' noise does go away when I do this... BUT it is replaced by a constant buzz (not terribly loud, but still audible).
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Re: 'Digital' noise...

Postby James Perrett » Mon Oct 21, 2013 4:32 pm

Sounds like you need to connect everything to the RME using balanced connections with the shield disconnected at one end. If you have decent cables with connectors with removable covers it will be easy to do but if you use cables with moulded ends you'll probably need to buy/make some adaptors.
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Re: 'Digital' noise...

Postby ef37a » Mon Oct 21, 2013 5:35 pm

James Perrett wrote:Sounds like you need to connect everything to the RME using balanced connections with the shield disconnected at one end. If you have decent cables with connectors with removable covers it will be easy to do but if you use cables with moulded ends you'll probably need to buy/make some adaptors.

I would make up adaptors anyway, saves making good, full length cables non-standard. If possible incorporate the RF "keeper cap".
Another advantage of adaptors is that you can just make up a pair and try them in various lines. Two might be all you need!

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Re: 'Digital' noise...

Postby Martin Walker » Wed Oct 23, 2013 12:38 am

Hi starman9!

I suspect this Q & A that I wrote for SOS January 2010 may help:

www.soundonsound.com/sos/jan10/articles/qa0110_4.htm

Here's a suitable pic to illustrate the wiring of a pseudo-balanced cable:

Image

Hope this helps!


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Re: 'Digital' noise...

Postby Martin Walker » Wed Oct 23, 2013 1:38 am

Oh, and here's a photo of a pseudo-balanced cable showing how to fit the resistor inside the jack sleeve:

Image

Hope it helps!


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