[mariush]

The waves you see on the screen could be caused by several things.

A very unlikely reason could be caused by bad power in your area.

Waves on the screen can also happen if the connection between your monitor and the computer is analogue, or in other words if you use a VGA cable to connect your monitor to the PC. If the VGA cable is of low quality (usually thin and cheap, what normally comes with cheap monitors) electric noise from the computer's power supply and the video card and the processor can be picked up through the vga cable and that results in slightly worse image quality on the monitor.
Digital connections (DVI cable, HDMI cable) are digital, which are much less susceptible to noise... the video card sends a digital image and the monitor decodes the image perfectly and noise on the cable has basically no chance of affecting the image.

If this is the reason, if your monitor has a DVI or a HDMI connector, then you would benefit from using a digital cable instead of a vga cable.

If your monitor has only VGA and DVI connectors (typical for older monitors) but your motherboard has only HDMI and VGA connectors, you can buy adapters which convert DVI to HDMI female connector (you plug this adapter into the monitor and then you can plug a HDMI cable into the adapter and into your motherboard's HDMI port)

On older monitors with fluorescent backlight (very thin fluorescent tubes at top and bottom of screen producing light) instead of LED backlight (where monitors use thin strips of blue or white leds producing light), with age it's possible that the power supply of the monitor or the fluorescent tubes themselves degrade a bit, to the point where you'll see those fluctuations of brightness on the screen.
Weaker power supply can cause those waves (could be fixed by replacing degraded capacitors, but it's not something for a total amateur to do). Weak or degraded fluorescent tubes usually manifests as the backlight getting a bit cream , slightly yellowish, or in rarer cases it can be a bit pink-ish light. In other cases, the fluorescent tubes go a bit brown at the ends, so the luminosity on the corners of the monitor is a bit lower.


Oh... (back to if you use vga cable) ... and your computer and monitor should be plugged in an outlet that has functional earth (grounding)... your socket should have 3 or 4 pins, depending on your country, not just the main two. Some of the noise computers create can be eliminated through that earth/ground wire so it's very helpful if both your computer and monitor are properly grounded. Otherwise, the electrical noise could be picked up through the vga cable (as if the cable was an antenna) and affect the quality of the image.

Basically, it's unlikely that just the activity of encoding videos would affect the quality of your image you see on the screen.