Ground in scart cables - Page 1
Forum section: electronics / beginners

author: fabiodl
author_rank: Frequent Contributor
timestamp: March 12, 2015, 05:53:27 am
content: The scart connector has lots of grounds, for instance one for each RGB channel.for instance, pin 13 is red's ground, pin 9 is green's ground and pin 5 is blue's ground.Let us assume to have a scart to scart cable, where each color is brought by a different coax.After connecting each coax shield to the respective ground, is it better to connect the grounds together inside the cable's connector or is it better to leave each ground independent, and let the circuitry inside the TV set/VCR connect them?What are the reasons?

author: rudika79
author_rank: Regular Contributor
timestamp: March 12, 2015, 06:32:59 am
content: Hi,One potential problem could be ground loop.

author: German_EE
author_rank: Super Contributor
timestamp: March 12, 2015, 08:05:26 am
content: Please leave each ground independent. Inside the TV set (or VCR or set top box) the separate RGB grounds are preserved as long as possible and end up at the SCART switching IC. The same happens with the audio path.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 12, 2015, 08:55:36 am
content: Generally, they're all brought together to a common ground anyway, but...There are often ferrite beads used on the RGB pairs. Sometimes it's only one for all three. In which case, common grounds won't matter (in principle, at least).Often, they are done individually, though, and shorting the grounds together elsewhere will partially foil the intent of those ferrite beads.So, better to leave them separate. Also, keep the shield tight in, as close as possible and all that. I don't know what the backshell looks like, but I'm guessing it's not crimped coax joints, so... anywhere you have to separate shield from core, and wire them to pins, stuff like that.It really only matters with noisy environments and long runs, but if you want best quality or operation, it's always better to do it well.Tim

author: fabiodl
author_rank: Frequent Contributor
timestamp: March 12, 2015, 03:00:40 pm
content: Thank you to all. My conclusion was also that if the pins are there, they are there for the reason of being separated,otherwise they would have put just one.What is the main reason for this?Is the reason ground loops, i.e. current from the audio not loading the connector to ground (coax sleeve) of colors?Is it that each pair is treated differentially, so that if a noise signal couples with a coax it will couple with both the inner conductor and the outer sleeve?Is it for other reasons?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 12, 2015, 08:47:21 pm
content: Quote from: fabiodl on March 12, 2015, 03:00:40 pmThis is the main one, which guards against induction of high frequency interference. Hence the ferrite beads that are frequently used inside equipment. Acts as a sort of transformer, keeping that difference of voltage where it should be.The other possibilities, like crosstalk, can be seen as corollaries of this; interference isn't a one-way street, but cables can (and do) radiate and receive interference, back and forth, and this can happen within a single cable.Another reason for multiple ground pins: extra current capacity -- for power cables, of course.Tim