With me its the opposite. Analog Cable looks like crap using the built in tuner of my Sony HDTV, and looks much better on the AverTV. I have both cables running through a signal amplifier.
The AverTV uses some digiital comb filtering to clean up SD signals, this unfortunately isnt defeatable or adjustable.
While this can be pleasing to the eye to most, some folks just don't like the look of the processed video. That is probably the difference in what you are seeing. Its really a matter of taste. For whatever reason to me the Aver provides a much sharper SD pic than my tv can. I never watch SD using my tv's built in tuner but prefer the Aver instead. Again its a matter of preference.
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