I have problems when I use the mame cores with 15khz video output. It’s impossible to get a proper picture, all is distorted and badly scaled. I tested with “alternate render” method in the options but that don’t help I have no problems with other cores. I never seen that on modern displays, does it occurs only with low res modes ?
I use FBA core when It’s possible since the picture is good with it but I have plenty of other problems with some games but that’s another story 
Thanks to help me
yeah, our mame-git core has the issue you described. You should be able to get a proper picture with any other mame core, though. that is, any of the ones with a year after the name.
Thanks, it works now ! But I have to use the precise resolution as It doesn’t seems to read the custom viewport parameters For example 224p viewport Y in a 240p resolution will stretch the picture to 240p. It won’t display a letterbox. In fact it will work if I change the Aspect Ratio Index from custom to another value then back to custom. I don’t have this problem with the other cores.