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Crosstalk issue

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Crosstalk issue

Postby Beans57 » Mon Nov 11, 2019 2:29 am

Hi there

I have a fairly modest home recording setup: e-pianos, synth, few mics and semi-hollow feeding Spirit mixer and RME UC interface. To date, I've had this 'very' limited and use the mixer to permit doing things that don't require a computer 'ON'. I record to both DAW (Samplitude) and an older AW1600.

Anyway, wiring this a bit more for a bit broader thing, I came across something odd. I have the following inputs in one section of the board:

Rhodes:J48:mixer (48V)
Guitar:J48: mixer (48V)
Synth:mixer (line in)

And from each of these, I have 'Direct Outs' going into the RME inputs. Balanced cables.

Now, the last day or so spent recording the Rhodes with no guitar input onto the mixer at all, the synth hooked up but powered down. In the RME mix software, I'm getting bleed through of the Rhodes sounds at a very low volume into both the guitar and synth hardware inputs. I recorded one of these tracks instead of the Rhodes input in Samplitude, gained up and plain as day the sound.

If I disconnect the 'Direct Out' of either the guitar or synth to the RME, bleed through gone. So something at/near mixer.

Fiddling about with 'all' kinds of things . . . moving cables about closer/further etc, I have found that if I go back to the unbalanced 'Direct Outs' from the mixer to the RME, less bleed through.

And that is where I am today. What is the sense of 'less' of this crosstalk with these unbalanced cables? And backing up, this crosstalk; is there any other reasons for it?

And that is where I am today. Perplexed as hell.

Of note: I only have 'one' type of balanced cable for those "Direct Outs" and all of those from a single purchase from a single supplier (ironically, to try and cut down on some noise I was hearing at low levels when connecting the Rhodes. I didn't have all of these inputs at the time of that earlier work so I had no opportunity to see this back then.

As well, there was no change moving the inputs further away from each other.

Open to any and all suggestions, opinions and questions of this.

Thanks in advance.
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Re: Crosstalk issue

Postby ef37a » Mon Nov 11, 2019 9:47 am

Let me see if I understand this Beans. You are getting crosstalk from one of the RME line inputs to the other?

Assuming so, can you feed a signal into the UC from a "floating" source? A phone might do but won't have much level, a CD from a laptop might be better. I want to determine you see what the crosstalk is like without the possible complication of earth paths from the mixer.

Also, what is the nature of the xtlk? Is it low level but full bandwidth or more an HF "twittering"? Be useful to see the level of the xtlk as a dBfs figure?

Dave.
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Re: Crosstalk issue

Postby Wonks » Mon Nov 11, 2019 9:51 am

What's the actual model of the Spirit mixer?
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Re: Crosstalk issue

Postby Hugh Robjohns » Mon Nov 11, 2019 10:35 am

Beans57 wrote:And that is where I am today. Perplexed as hell.

I suspect the crosstalk is probably occurring via the ground side of the direct wiring. Perhaps the grounding of the RME is not as solid as it should be...

If the mixer is purely for local monitoring and the RME for recording, I'd used the balanced DI box outputs to feed the RME and the direct feeds to the local mixer.... and see if that fixes the problem.

H
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Re: Crosstalk issue

Postby Beans57 » Mon Nov 11, 2019 6:30 pm

H folks . . . all kinds of useful suggestions and questions . . .

The mixer is the Soundcraft Spirit M8.

Yes to the crosstalk from one M8 input to other M8 inputs.

I will see about the bandwidth and character of this crosstalk tonight. It 'is' low level, but as this is the first time I expanded my wiring of this setup, I never had notice it earlier. I may at some points have had two inputs together into the M8, but it would have been those unbalanced cables and I don't recall any evidence of crosstalk (i.e. audible or visual) ever.

I will try the floating source; I'll use the outputs from my AW1600 and feed direct into the RME.

I'll also see about revamping my hookups but I'd kind of like to have the physical setup of connections as I have now, but if no resolution then yeah . . . do whatever to get rid of this. But this might permit the ID of a potential ground issue that is behind this . . .

--------------------------------------------------------------

That attenuation of the crosstalk simply by switching back to the unbalanced 'Direct Outs' from mixer to RME has me puzzled . . . . .

Thanks folks. Back in a couple days.
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Re: Crosstalk issue

Postby Hugh Robjohns » Mon Nov 11, 2019 7:04 pm

If you're into DIY cable making, I suspect you could cure the problem by using pseudo-balanced wiring* between the link out of the DI boxes to the mixer's balanced line inputs.

That approach would avoid a common direct ground connection shared between sources, since the grounded side of the unbalanced sources would connect to the cold inputs of the mixer... and without a shared ground, I suspect there would no direct path for crosstalk.

*Wire TS plugs (for the DI box link output) to TRS plugs (for the mixer balanced input), with tip to tip, and TS sleeve to TRS ring. No connection to the TRS sleeve.

H
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Re: Crosstalk issue

Postby Beans57 » Mon Nov 11, 2019 9:10 pm

Spent a bit of time today on this as its confounding me . . .

The crosstalk varies, but generally full bandwidth and at times approaching -20 dB if I select one of the 'cross' channels when this is occurring.

Bottom line on the situation re: cabling is that the type of cable for the input that I intend on using doesn matter (bal or unbal) but if 'any' other channel - with an input to that channel connected or not, uses a balanced cable from mixer 'Direct Out' to RME inputs, crosstalk occurs. I actually found one other balanced cable from any manufacturer and tried it and crosstalk resulted as well (was worrying about the single source of these bal cables). I placed unbalanced back in all across for the 'Direct Outs' and no crosstalk.

Want to try and use bal and unbalanced direct outs sometime in the next day not to connect to RME inputs but to my AW1600 recorder and see if crosstalk present.

I walked away today with a small list of things to try courtesy of you folks, and with frustration with this issue. Get some other things done around here, and I'm thinking pull cabling completely and start with minimal cabling to see if I could reproduce the issue.

Thanks again
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Re: Crosstalk issue

Postby Wonks » Mon Nov 11, 2019 9:11 pm

I had an M12, and I think there may be a design flaw somewhere in the circuit boards. With a loud mic input signal (normally capacitor drum overheads on my M12), I had the peak light on the overhead channel and several other of the channels coming on, even with the input gain on the overhead channels turned to minimum. When turned up a bit, the sound was fine with no distortion, but about half of the peak lights on other channels pulsed at the same time.

At first I thought it was the multicore I was using, with the main out line level signal being picked up on the mic level inputs. But then I used the multicore and all the same mics on another mixer and there wasn't an issue.

I can't be certain, but I'd say there's a strong likelihood that there's an inherent issue with this series of mixer that allows signal linking of some sort between the channels and affecting the signal strength indicators, and which may be a partial cause of some of the problem you're having.
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Re: Crosstalk issue

Postby Beans57 » Fri Nov 15, 2019 4:51 pm

Well, ferreted out the answer to this . . . . . .

Long and short, those 'Direct Out' jacks are unbalanced. That is why they work well with unbalanced lines, and have this issue with balanced lines.

The Spirit M8 manual that I have has an error in it. Early in the manual (pg 14), it presents those jacks as 3-pole, balanced outs. I referenced this when getting those balanced lines for that reason. Trying to figure this out more of late, I reviewed that entire document and later in the manual (pg. 35), it presents the 'Direct Out's in a group of connections that are 'unbalanced'.

Looking at the schematic for the unit, they are presented as 'unbalanced'.

I can sleep better.

Oh yeah . . . the noise that I thought to kill switching to balanced had nothing to do with the unbalanced (short) cabling to the RME. That was a ground issue elsewhere in my hookup.

Here is another question though that I don't have enough knowhow for an answer:

I could revise those balanced cables to have ring and sleeve connected so as to be able to use them with this unbalanced output. But is there any benefit at all in doing so when I could simply use (as I am) the unbalanced lines? I'm my understanding of things, those cables would be equivalent in function with that revision to the balanced cable.

thanks again for the input on this
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Re: Crosstalk issue

Postby ef37a » Fri Nov 15, 2019 5:12 pm

Does the specification give the output impedance of those unbalanced outs?
If so and it is an 'easy' value such as 100 or 120 Ohms you could make up some pseudo balanced cables.

Probably not worth is because of the presnt short run but that might change?

Dave.
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Re: Crosstalk issue

Postby Wonks » Fri Nov 15, 2019 5:26 pm

75 ohms for the direct outs.
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Re: Crosstalk issue

Postby Beans57 » Fri Nov 15, 2019 5:29 pm

Not having the manual on hand now, believe it was 75 ohm.

Those short cable runs (4 ft) will be that way forever. Have that mixer on upper surface of a tiltable rack top with RME rack mount beneath.
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