Thank you for your answer but all of this is related to a problem with display.. i had no problem before my upgrade to Catalina... so after talking with your customer support, still not solved the problem.. finally solved it myself :
When you hold Option and click on the scale resolution, more resolution options appears! I guess that in previous version all options were displayed and now in catalina, only fill resolution full refresh rate appear! If you hold Option at the same time you click the scaled, all option appears : high resolution, low resolution, many refresh rates.