Your converter box must convert the signal from the high-res digital signal it receives from the Roku to a low-res analog signal, then encode it for the 480i NTSC (or if your converter and TV support it a 576i European PAL) signal required by your TV. Some converter boxes do a better job of this than others.
The TV must then decode the signal so it can be displayed on the screen. All these conversions add some element of degradation to the image quality.
If your set has component connections (red, green, blue video plus red and white audio) a converter to component instead of composite video (yellow, red, and white) will give an improvement in video quality.
Thanks for the info.
