well, because the reception is digital , its like you either receive the signal or not ..( calling fellow engineers!!). So even if you get low signal strength , the picture quiality wont be affected AFAIK. The picture quality will be affected by Bit Error Rate (BER) but, as wrong bits make up a wrong picure (read unclear..pixellated etc) .So even if signal strenght increases from 40 to 50 you wont see any visible difference coz there actually isnt any!! In analog transmission , the picture quality is a direct function of the signal strength. This is a major advantage of digital transmission.