The best way to test a SCART-SCART cable (?)

[rgr]

I started checking my SCART-SCART cables. The Toshiba VCR (JVC, too) gives a nice blue control image, so all anomalies can be clearly seen.
I didn't limit myself to one-way test transmission -- I sent the TV signal (movie) from the TV to the VCR, and the blue control screen in the other direction (via the same cable).

As a result, it turned out that only on one cable I had a perfect blue picture, on all the others there was a "shadow" (I don't know the correct word) of the TV signal.

The perfect cable is 10mm in diameter, the remaining ones are 6-6.5mm - what a coincidence

[pandy]

External diameter doesn't mean anything nowadays - frequently "thick" are internally very poor quality - open SCART connector and inspect what is inside - search for coaxial cables - if there is no coaxial cable (on component and composite signals) then probably overall cable quality is poor.
VGA cables can be thicker or not but they provide a least 100..150MHz bandwidth.