Im no expert but my understanding is that the signal quality is pretty crap (and thats not necessarily TCL's fault as Sky is no  better) due to the level of compression. Also, with cables of the same standard, S-Video will provide a better reproduction of the signal than the composite output the other decoders use. Of course Component video is better again and HDMI allegedly better still but they are all only providing the best transfer they can between decoder and TV - so signal quality to the decoder is still the ultimate determinate.  Having said that I have just made the switch from analogue to digital decoder because the picture quality to my plasma was just about unwatchable on many channels - and despite some comments I have read elsewhere I have to say that the digital picture quality is overall much much better.