Okay, you have a nice shiny new high definition TV receiver. You work through its setup menus and discover you can set its high definition output resolution to 576p, 720p or 1080i. Which should you use?
Preferably, none of them. Some HDTV receivers also offer a ‘through’ option (eg. the Toshiba HDD-J35 and the Topfield TF7000HT). This sends the picture on at whatever resolution its was transmitted at. So when you switch to an SD channel, the picture comes through at 576i. On SBS HD or ABC HD it will be 576p, on WIN HD or SCTEN HD it will be 1080i.
Almost certainly the vertical resolution of your display (in widescreen mode) will be one of the following: 480, 576, 720, 768, 788, 1024 or 1080. That, I think, covers all the vertical resolutions of all the displays I’ve seen (that 788 is for LCoS projectors with a 1,400 pixel wide resolution). In terms of common resolutions, there are 480, 576, 720, 768 and 1024. As you can see, only two of those match the output options available from your HD STB.
One way or another, much of the HD programming is going to have to be scaled from the signal’s native resolution to the display resolution. The question is, where? The STB can do it. Or your display can do it (except for CRT TVs, which often demand a 576i or, at most, 576p signal). Which one in your setup that does the best job is for you to work out by experimenting. But what you do not want is for both of them to be doing it.
Plasma and LCD displays are most commonly 480, 768 and 1024 pixels tall. None of these matches the output resolution available from an HD STB.
Scaling can be done to a very high standard, but inevitably any manipulation of the picture induces some degradation. You typically won’t notice this if you’re using high quality gear. But what you don’t want is for scaling to be performed twice. And that’s what will happen if you’re using, say, 1080i output from your STB to watch 576i or 576p material on a 768 or 1024 pixel tall plasma.
The exception is if you have a 720p projector — a very common resolution. In that case, once again, experiment with setting the HD STB to 720p output, and to ‘through’ output. See which gives the best results. In my experience, the scalers in projectors tend to be a bit better than those in HD STBs. But one caution: find out if there is a ‘film mode’ or ‘cinema’ option in the projector’s menus. These sometimes force ‘weave‘ deinterlacing, which means that video sourced interlaced material will appear with marked combing, and look horrible. If that’s the case, switch that mode off. It seems to make no noticable difference with film-sourced PAL material anyway (I think these are primarily for NTSC signals).