I have an old (1970s) PCB that generates a (baseband, composite) NTSC video signal. It is a bare board without a containing chassis or case. I am using a shielded cable for the video output line and connecting that to a monitor. On the monitor side, the shield connects to the outer ring of the RCA plug as is standard, and it connects to a composite video connector. (i.e. not a TV/RF signal)
My understanding of shielding on audio/video signal cables has always been that what you're trying to do is protect the signal inside by essentially making the shield of the cable an electrically continuous "part of" both device's chassis on both sides of the cable, so that interference moves around the "outside" of the overall setup and allows the signal to pass unmolested inside:
With my video board, there is no chassis to connect to. If I leave the PCB side of the shield just unconnected, then there is interference and distortion in the video signal. If I connect it to the ground plane on the board, then I get the expected clarity in the signal.
But I'm a bit at a loss as to why that is, or rather, how I should think about it. I (probably!) understand what's going on when both sides of the cable shield connect to the respective chassis which are usually then connected to earth: interference travels around the outer shell of the system, doesn't mess with the circuits inside and is shunted to earth. But what is going on and why does it work to connect to ground plane?