graininess in old videos converted to new formats

[djflakf1]

Hello, greetings to everyone. I've registered on this forum to seek the expertise of the community regarding a question I've had for some time.

There are certain videos and programs from the 70s, 80s, 90s, and 2000s that, when originally broadcast during those eras, didn't exhibit visual flaws because they were optimized for the technology of the time. However, when these videos are transferred to higher resolutions like 720p, 1080p, or 4k, they develop a noticeable grain, similar to the interference you'd encounter when tuning to a TV channel with no signal. I apologize if I'm struggling to find the right word; I'm referring to a form of visual noise in the video. I've illustrated this issue with the images below.

I'm curious to understand the cause of this noise or grain in the video and why it's not visible when viewed in its original format.

In general, I want to know more information about this; there are old movies that don't have this grain. I've observed this in VHS or Betamax videos converted to new formats. I'm a newcomer with this question, and I hope it doesn't bother you.

[Selur]

Since this does not look like grain on the screenshot but more like compression artifacts, I see multiple scenarios:
a. the higher resolution version was created by re-encoding a low resolution version. The lower resolution had noise or artifacts, which got enhanced by whatever upscaling was done.
b. the lower resolution source got smoothed a lot during or before the upscaling, then grain was added, but too much grain for the bitrate and target format chosen. So after the encoding one got lots of artifacts instead of grain.
c. the lower resolution version was upscaled, and the output had lots of light gradients which would have caused banding when going for 8bit compression (and not increasing the data rate a lot). So to avoid banding grain was added which due to low bit rate compression turned to artifacts.
Note that these are just wild speculations based on a single screenshot which does not really tell much about the nature of the noise/grain/artifacts,...

there are old movies that don't have this grain

Yes, rescanning of a clean film content and properly filtering and encoding is possible, but often creating a HD version by using a SD source is just cheap and still sells.

Cu Selur

didn't exhibit visual flaws because they were optimized for the technology of the time

more like some stuff simply was hidden by the technology of the time