[TriLUG] Television & RF interference -- yes computer related -- I think

[Joseph Mack NA3T]

On Thu, 7 Mar 2013, James Jones wrote:

> After finally getting around to transferring my notes to a 
> block drawing ( see attached pdf ),

these are stripped by the mailing list. But I'm not going to 
be able to pinpoint it from the diagram anyhow.

> Your thoughts on this are appreciated. As a side note, 
> their cabling is more of a rat's nest and it looks like I 
> get to help them organize it.

Other people have had good suggestions, eg making sure the 
power cabling is earthed and wireless mikes (which you've 
eliminated). The lesson from them is not so much what to 
look for, but how to think about it.

At the outset you have to face

o you may not solve it, one of the problems being that you 
can't make the problem occur on demand. There is nothing 
harder to fix than an intermittant. You will have to accept 
bad recordings in the short term and maybe long term. To 
know when it happens, you'll need the recording device 
running 24*7, which presumably will require a human on hand.

o it may take a long time. Before you start, decide how much 
time are you prepared to put into it. When that time comes, 
will you really quit, or will you just keep going?

If you do fix it, the explanation will almost certainly be 
obvious after the fact, but it's unlikely that you'll be 
able to find it by rational means. In that case you'll have 
to take the monkey-typewriter-sonnet approach.

Finding the problem here is no different, thinking wise, 
than fixing a software bug that only appears intermittantly.

You'll have to find out what all the cables are doing and 
simplify it as much as possible (are all the cables 
shielded? - I don't know what you have - are the twisted 
pairs individually shielded? it the bundle shielded? is the 
shield earthed at each end? - this itself will be a 
nightmare.). Make a plan for what to do when the next RFI 
event happens and be prepared to start disconnecting cables. 
Have some idea of how you'll interpret what is going on if 
the RFI remains/stops.

You need to get to the simplest setup that reproduces the 
problem and have a list of what stops it. Can you take out 
the video switcher and record directly from the camera? (You 
may need to borrow gear from a different manufacturer, but 
with similar specs, to substitute to see if anything 
different happens.) You may wind up in the unhappy situation 
that eliminating RFI means you can't record.

Joe

-- 
Joseph Mack NA3T EME(B,D), FM05lw North Carolina
jmack (at) wm7d (dot) net - azimuthal equidistant map
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