First of all, a Tivo Roamio being used for
OTA tv is
incapable of showing a signal level higher than 72 maximum. If you watch the signal screen as it calibrates to the channel being received, it'll go down then UP, and then settle to it's final value.
It's true they can show a higher signal level when used with a cable tv connection, just NOT for OTA tv for whatever reason. This has been hashed out over at the Tivo Community forums.
So, be very happy with a signal level of 72, as that's the best! 40 is the lowest before you'll have serious issues. You need to concern yourself with signal QUALITY though. So your pixelation problem is another issue, and I'll leave that to being answered by Rabbit and others. They'll either get you going, or you'll KNOW you'll have to settle for what you have.
You should also run an "FMFOOL" report, and post a screen print of that here.
http://fmfool.com/