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Author Topic: Snake  (Read 8056 times)

Arnold B. Krueger

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Re: Snake
« Reply #30 on: June 27, 2010, 11:52:52 pm »

Darin Brunet wrote on Mon, 28 June 2010 04:21

Hello Everyone,

I am new to this forum and have read many good replies and learned much from many of you. Mr. Buckalew, you stated that foil shield w/ drain is poor cable for SCIN. Is there an acceptable length maximum for this type of cable to minimize this problem?  I have used Belden 5500FE cable for several installs and have not noticed much (if any) audio degradation. Perhaps I am staying within the limits?

Is there a better recommended cable of similar OD that will fit my Amphenol IDC XLR connectors?



SCIN (Shield Current Induced Noise) is as its name suggests a consequence of there being signfiicant current flowing through a cable's shield, which is not supposed to ever happen.

The chassis at both ends of a piece of cable are supposed to be at pretty much the same voltage, so only negligable amounts of current should flow.

Using a cable's shield to strong arm two interconnected chassis so they have the same voltage w/r/t ground is probably not the best thing to do.

If you haven't been having problems with SCIN, it may be due to your system being well-designed and well-installed. I hope you can learn to live with this situation! Wink
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Darin Brunet

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Re: Snake
« Reply #31 on: June 28, 2010, 12:15:37 am »

Mr. Krueger,

Thank you for the prompt reply. I do try my best to think things out in the planning stage rather than re-wire a premises Smile Let me ask you, does the SCIN issue also pertain to speaker level signals or does the voltage on the speaker cable swamp out that issue? I have seen 12AWG shielded speaker cable with a drain wire.

~D
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Andy Peters

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Re: Snake
« Reply #32 on: June 28, 2010, 12:21:15 am »

Darin Brunet wrote on Sun, 27 June 2010 21:15

Mr. Krueger,

Thank you for the prompt reply. I do try my best to think things out in the planning stage rather than re-wire a premises Smile Let me ask you, does the SCIN issue also pertain to speaker level signals or does the voltage on the speaker cable swamp out that issue? I have seen 12AWG shielded speaker cable with a drain wire.

~D


There is no reason to use a shielded speaker cable.

-a
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Darin Brunet

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Re: Snake
« Reply #33 on: June 28, 2010, 12:29:44 am »

Andy,
Would the shield, if terminated at the amp (a Faraday Shield?) help the signal integrity between several runs of speaker cable in a conduit? Or is it simply not necessary at all?
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Lee Buckalew

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Re: Snake
« Reply #34 on: June 28, 2010, 12:43:48 am »

Quote:

The confusion starts right here. When one says that "there are no reasons to pick one construction type, etc. over another" the statement has a context that is usually well understood.

Here is the usual context for that statement:

(1) It is presumed that the cable is generally appropriate for the application. For example, two components with balanced inputs and outputs are being connected by a cable composed of two twisted conductors housed within a shield that is itself insulated. The differences that are being considered are things like choice of solid versus stranded wire, vinyl versus polyethylene versus teflon insulation, thickness of insulation, tightness of twisting, etc.

(2) The signal being transmitted is an analog audio signal whose voltage and current were used to select the general kind of cable. IOW the differences in construction are not something crazy like using speaker cable for mics or vice-versa. We are most certainly not talking about digital audio.

I think that these two common-sense conditions would pretty disqualify much that has been said in this thread lately to attack the idea that there are no reasons to pick one construction type, etc. over another.

I don't see why the bandwidth of this conference should be wasted extensively discussing the adverse consequences of obviously bad practice like using single center conductor coax as if it were mic cable in a balanced circuit, or speaker cable to transmit digital audio.

We should all know better and that is that!


Arnold,
There are often questions asked about using wrong cable types for the wrong signal.  That is why I addressed the "cable is cable" comment and pointed out reasons for certain cable construction.  Precisely so that this forum would not be misunderstood to advocate that "cable is cable" so it doesn't matter.

I obviously misunderstood the issue of digital transmission and will study up on this.

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Lee Buckalew
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Lee Buckalew

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Re: Snake
« Reply #35 on: June 28, 2010, 01:04:05 am »

Andy,
Everything that I have said about analogue circuits agree's 100% with what you stated.  The statement about my misinterpreting Whitlock is inaccurate unless he did not mean what he wrote.  

As far as digital audio transmission distances I still know of no synchronous digital audio transmission scheme over coper that is rated for more than a 1000' maximum, most are under 600'.
I will study digital transmission systems so that I have a more clear understanding of the properties involved and I don't get athletes tongue again.

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Lee Buckalew
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Lee Buckalew

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Re: Snake
« Reply #36 on: June 28, 2010, 01:10:49 am »

SCIN really only becomes an issue under certain conditions.
As Arnold stated, one case would be if there is current flowing between devices over the shield this could lead to a problem with SCIN.  

Here is a list from the AES paper that I referenced in a previous post and a link to the paper.

http://www.audiosystemsgroup.com/AES-SCIN-ASGWeb.pdf

SOURCES OF SHIELD CURRENT
Some common causes of shield current in audio ca- bles are:
1. Potential differences between points where shields of opposing ends of an audio cable are connected. The potential differences typically result from IR losses (that is, voltage drops) due to leakage currents of motors, lighting equipment, and electronics equipment.
2.   Wiring faults in equipment unrelated to the au- dio system.
3.   Magnetic induction.
4.   Ground currents from building equipment such as variable-frequency-drive motors.
5.   Radio signals, most commonly broadcast sig- nals, but also noise from lighting circuits and other incidental or unintentional radiators.



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Lee Buckalew
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Re: Snake
« Reply #36 on: June 28, 2010, 01:10:49 am »


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