JVC LT-23D50BK Service Manual page 54

Note 1:
At low signal level, the changings on the Noise Reduction
function are not effective when there is menu on the
screen. If the menu is removed, the effect of this function
is seen. At normal broadcasting, Noise Reduction
changings are effective at once.
Note 2:
At moving pictures, while Noise Reduction is at maximum
level, some picture distortions may occur. If Noise Reduc-
tion is set to medium or minimum, the picture will improve.