This is probably something that could be posted to EEVBlog about how to easily test 75ohm coax cables for various characteristics that we'd want to know for composite or S-Video cables and signals (what waveforms to use, and whether a termination load should be used during the measurement, etc).
I think it is well established that S-Video and composite are supposed to use 75ohm cables and connectors, but you're kind of just hoping that manufacturer built those cables to spec as I doubt many of us have actually measured a cable's impedance.
There's a relatively simple way to measure cable impedance described here:
https://www.jackenhack.com/cable-imp...ow-to-measure/
Getting to see the signal on an oscilloscope or a spectrum analyzer would also show you if the cable is picking up outside frequency noise (due to poor cable shielding). For picking up stray noise, I think you could just attach the cable to the oscilloscope just on the one end and leave the other end unconnected and just cover the other end with aluminum foil so that ambient noise doesn't enter the cable there and see how close to zero volts the waveform stays. A wide or fuzzy line would suggest the cable is picking up noise and shielding of that cable may be poor.
The outside frequency noise won't be constant though since it'll depend on what kind of electrical noise is present in the environment you are using the cable in. Fluorescent lights and brushed motor vacuum cleaners used to be big ones for a variety of RF interference back in the day. Nowadays you might be more likely to get RF interference from switching power supplies or other electronics nearby.
I imagine it may be possible to just feed a square wave through multiple composite/s-video cables and then display the output simultaneously of each using different channels on an oscilloscope. The ones with the least noisy/fuzzy/wide waveforms should be the lower noise (better signal to noise ratio) cables. Square waves can also demonstrate "ringing" (signal overshoots and then undershoots) if cables and connectors are not impedance matched so that may be directly related to how much ringing you see in an actual composite video signal around sharp contrasted lines, particularly black vertical lines on a white background.
Almost sounds like another way to verify impedance of a cable would be to use a variable resistor as a load on the output side when feeding it a square wave from the other side and then turn the variable resistor until the ringing stops for the square wave (meaning impedance matching has occurred), then measure the resistance of the variable resistor that it ended up at:
https://electronics.stackexchange.co...smission-lines
That's all theoretical, so take that all with a biiiiig grain of salt since I am not an electrical engineer haha.
