bit depth

the resolution and bit depth are two different things: let say pal is 720x576 in rgb R, G and B channel must all be the same size (720x576) beacuse if youwould make on half resoultion it would look really off that is why in the old times YUV (colorspce was invented partly because reduced bandwith and partly because backward compatibilty with black and white standard Y) in this case Y consist of black and white image and two other channels make up the croma information so the croma can be 1/2 or even 1/4 of the orignal resolution (PAL DV 4:2:0 or mpeg2 compression), the bit depth resembles the different levels that can be stored in every channels (one channel is 2d array of pixels) one pixel (8 bit 256, 10 bit 1024)

hope this clears it up a bit - if not make it evene more complicated
-Kaspar