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Can someone explain 'crosstalk' please?

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Can someone explain 'crosstalk' please?

Postby mjfe2 » Sun Jun 18, 2017 4:42 pm

I understand the basic concept but I'm wondering how it manifests itself practically? Is it simply another potential source of noise in a multichannel system?
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Re: Can someone explain 'crosstalk' please?

Postby jagraphics » Sun Jun 18, 2017 4:59 pm

My wife can explain "crosstalk" but I doubt her explanation will help :-)
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Re: Can someone explain 'crosstalk' please?

Postby The Elf » Sun Jun 18, 2017 6:07 pm

It's essentially where one signal bleeds into another.

A simple form of it might be a multi-track tape recorder, where signals recorded on adjacent tape tracks would be heard when either one was soloed. In this latter example, this was why it was imperative to keep such as drums and vocals at opposite sides of the tape width!

You can also get cross-talk along cables, in mixer electronics and in all other bits of analogue gear. Generally it's not a desirable thing, but within reason, and give well-designed equipment, it's usually not a huge problem.
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Re: Can someone explain 'crosstalk' please?

Postby CS70 » Sun Jun 18, 2017 10:44 pm

mjfe2 wrote:I understand the basic concept but I'm wondering how it manifests itself practically? Is it simply another potential source of noise in a multichannel system?

If you have a stereo mix, when the two channels have crosstalk you basically gets some bleed (either full spectrum or only some band) of one to the others, so the stereo effect will suffer (in the worst possible case, with full spectrum full bleed you would have with the two channel carrying identical signals, i.e. a dual mono).

If you have many channels, you would get basically bleed between pairs, concretely raising the fader for one channel would also increase some frequency for the audio coming to one or more other channels.

You have crosstalk also in "virtual channels", like the bands of radio of Wifi spectrum - in these cases it manifests itself as overlapping signals or lower bandwidth due to packet loss.
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Re: Can someone explain 'crosstalk' please?

Postby Hugh Robjohns » Mon Jun 19, 2017 9:11 am

mjfe2 wrote:I understand the basic concept but I'm wondering how it manifests itself practically? Is it simply another potential source of noise in a multichannel system?

Crosstalk is the leakage of sound from one channel into another.

In some situations it can be a serious problem -- like when linear time-code breaks through to the adjacent track on a multitrack recorder.

In others it can be a kind of benign character -- like the tonewheel leakage in a console Hammond that gives a background 'singing' sound of all notes at once.

Or it can actually be beneficial -- such as the mechanical crosstalk in a gramophone pickup that narrows the stereo width slightly at HF to give a more accurate and stable stereo image.

Crosstalk is an analogue phenomenon -- if it exists in a digital system it is because someone has written code specifically to introduce it -- but it occurs everywhere.

Recording spill where a mic picks up unwanted noises from distant sources is really a form of crosstalk. Electronic circuits can suffer crosstalk through capacitive coupling between wiring or PCB tracks, or by contaminated ground references. Tape machines can suffer crosstalk through misaligned tape heads, tape guides, or too strong signal levels. and so on and so on...

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Re: Can someone explain 'crosstalk' please?

Postby ef37a » Mon Jun 19, 2017 1:11 pm

"Crosstalk is an analogue phenomenon -- if it exists in a digital system it is because someone has written code specifically to introduce it -- but it occurs everywhere."

Oh! When I was at the network hardware factory (whom the wife now always calls 'The B***tds! But that is another story and only by pm) There was MUCH talk of Crosstalk in CATX cables? Where cables were bundled there was 'alien crosstalk' and that was controlled by the rate of and staggering the rate of individual pair twists.

There were some formidable equations bandied about for 'return sum power' or some such.

I just made test beds and assembly fixtures!

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Re: Can someone explain 'crosstalk' please?

Postby jagraphics » Mon Jun 19, 2017 1:52 pm

Hugh Robjohns wrote:
mjfe2 wrote:Crosstalk is an analogue phenomenon -- if it exists in a digital system it is because someone has written code specifically to introduce it

sort of... In digital systems the problem is lesser and different but still there. In all electrical systems there are electro-magnetic waves that can affect conductors in their vicinity. So cross talk, leakage, bleeding and interference can occur in many unexpected places even in digital systems. This is why you need properly screened cables or cores and well designed PCB's. I have seen badly designed SBC PCB's that had "noise" and had to be redesigned or additional de-coupleng capacitors added.
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Re: Can someone explain 'crosstalk' please?

Postby CS70 » Mon Jun 19, 2017 1:57 pm

Crosstalk is analogue, but can have effects on digital formats because the bits are generated by A/D conversion from a signal transported over an analogue medium, so if there's signal degradation in the analogue, the A/D conversion will generate wrong bits (or, more commonly, the data packet checksum will fail and the entire packet will be discarded).

Then of course there's audio code in "console" plugins that actually simulates that on purpose (that's what Hugh was talking about) - virtual crosstalk on virtual channels transporting virtual audio!: D
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Re: Can someone explain 'crosstalk' please?

Postby Hugh Robjohns » Mon Jun 19, 2017 2:40 pm

Let me clarify my post: crosstalk is everywhere! Anywhere that there are two dissimilar signals within the same system they will interact with each other in some way and to some extent, somehow. And that applies equally to whether the system is analogue or digital.

However, crosstalk manifests directly as an integral part of the wanted signal in analogue systems, whereas it is inherently excluded from the wanted signal in a digital system, and only becomes significant if it starts to corrupt the digital signal beyond the capabilities of the system's error correction facility.

In the case of ethernet cables, there is capacitive and inductive crosstalk between the cable pairs -- which is minimised by using twisted pairs with different lay lengths. But, provided it is kept below a certain level, it has no effect on the data content and is not, therefore, 'audible'.

In analogue systems, like mixing consoles, tape machines, etc, it often is directly audible.

I think we're all singing from the same hymn sheet, but I could have made my point clearer, so thanks for highlighting that.

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