I have an AV cable that connects Atari 1040STE with a AV to VGA converter. For some reason the vertical edges are not sharp, rather jagged or pixelated. I red that old atari would have an issue with modern display, as refresh rate of 15Hz is not supported. I was under the impression the converter would do the trick, but on the other hand, if I recall correctly refresh rate applies to highest resolution output to vga directly. Can you suggest where to look for a clue on how to fix it? Provided is a photo of the issue. Also included the components I used, maybe I went the wrong path. Perhaps any electronics inside is to blame and needs checking? Any feedback welcome as this is my first step in retrocomputing.
You are seeing dot crawl which is common when sending composite video into a low quality comb filter such as in that Video to VGA Converter box. To get rid of it, you will need a better comb filter (good ones are hard to find) or avoid composite entirely by converting from the STE's analog 15 kHz RGBHV to either the 31 kHz VGA that your monitor supports, or HDMI, DVI, or even S-Video depending on what the monitor accepts.
Some VGA monitors support 15 kHz RGB signals directly. For those monitors, all you need is a "passive" Atari ST to VGA cable. Unfortunately, it seems your monitor does not support 15 kHz RGB, so for you and others with the same problem, here are some other solutions:
If the monitor takes HDMI or DVI, the Open Source Scan Converter is a good choice that adds almost no lag. If price is a concern, look for a SCART to HDMI converter on eBay. It looks like this:
In either case, you will also need an Atari ST to SCART cable like this one.
Here is a video comparing this and other converters on the Atari ST.