How can we reduce this artifact ? By reducing the downsample rate ?? Any other alternative ?
One method might be anti-aliasing after the TV (or playback device, for example) has received the signal, but only on the color channel. Would this help? From what I remember, the reason we downsample is to reduce the amount transmitted over the air or stored, but we might still be able to do some tricks "at the edge" to improve the final image presented to the user.
Another method might be to do some kind of adaptive blocking of the color channel according to places in the image where there is more information, but the overhead involved in this method might be more than it's worth.