Re: Tip needed for Coax termination

[Keith Doxey <ukha@xxxxxxxxxxx>]

Quoting W T Taylor <bill-taylor@xxxxxxx>:

> KD> Make sure that all 3 leads are EXACTLY the same length
>
> That's a bit OTT.
>

It probably is a bit OTT. Should have omitted the word "EXACTLY"


> If we assume the speed of propagation in a cable is
> 2/3 of the speed of light, 200,000,000 m/s, and we're using MPEG2
> pictures; one picture element lasts for one 720th of 52 microseconds,
> i.e. 72nS, then you need about 14M of cable to delay the signal by 1
> pixel.
>
> Assuming that less than 1/4 pixel of colour timing error will
> be unnoticeable, then the cables only need to be matched to 3.5M,
> which may well be more than the total cable length anyway.
>

I hadnt worked anything out mathmatically but based it on experience.

A projector at work seemed badly out of convergence on the RED. Putting the
Crosshatch test pattern up the pattern was perfectly aligned. Looking at
the
connecting cables from the Extron box on the floor to the BNC sockets on
the
wall revealed a bundle of 4 x Coax and a separate single coax

One of the Coax cables in the bundle had gone faulty ( the RED) and had
been
replaced with a separate BNC lead that was longer than the bundled cable
harness. The extra length was enough to displace the red image giving the
poor
convergence. I swapped leads over so that the longer lead was used for
CompositeSync and the three good leads of equal length in the harness were
used
for RGB.

This gave a well converged image that was then offest slightly to one side
of
the screen. This was corrected by the H_Sync adjustment on the Extron Box.

Agreed that in the above case, the problem was probably worse due to the
higher
resolution of the PC display but the point I was trying to make is that all
the
cables should be the same length. If you run multiple cables from Point to
Point and they are run individually rather than pulled as a bunch then it
is
very easy to get differences in length.

Multicore coax is much neater and can also be used to carry other signals
as
well eg 5 core coax can be used for

RGBHV
RGBS + Composite
Component + S-Video

Its also very difficult to get substantially different lengths on the cores
:-)

Keith

www.diyha.co.uk
www.kat5.tv


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