system
Given the following post, respond as if you're commenting on Reddit:
human
When I watch a TV programme from the 70s or 80s on my HDTV, it's blurry almost to the point of unintelligibility. Is that how it looked back in the day? Am I misremembering my youthful TV experiences? Did we not notice the blurriness?

EDIT: Also, videos look terrible, too. Is that for the same reason?
gpt
It's sort of two sided,
First, TV's were smaller with lower resolution (NTSC rather than HD). It's only relatively recently that TV's larger than about 24 inches have become commonplace with the replacement of CRTs with LCD/Plasma TVs. Thus we didn't have to scale for formats to begin with.

Secondly, What makes the problem worse with larger HDTV's is how the scaling works from low resolution media to higher resolution displays. I'd sort of have to stabbing in the dark with the actual implementations that are used so i'll use the simple 'naive' case of linear interpolation. To fit the original to the larger screen the pixels need to be scaled. So as an example, lets say your screen is twice as large as the source format; then 1 pixel in the old format is then a 2x2 pixel square on your screen (because the screen has a higher resolution.) Is turns REALLY blocky looking really quickly. To help with this problem, interpolation is used; which is to say, instead of the one block of 2x2 pixels having the same color, we'll have them share a little bit of their color (linearly) with their neighbors. 

Nice, so now we don't have a super blocky movie. The problem now is that it turns really blurry & has this out of focus feel to it. A solution to this is to use a better sort of interpolation. There are many to choose from.