[Grand Dizzy]

Does anyone have any rough idea how much larger (typically) the compressed filesize is for 1080p HD, compared to 576i SD?

I know HD has 9·375× the number of pixels, but I would imagine there would be far less than 9·375× the data, due to the nature of compression, and the fact that most of the major picture details are already contained in the SD image, with the additional HD information being more "derivative" and less "definitive" (if that makes sense).

Also, what codec(s) do HD camcorders use? Is it all mpeg2?

[senu]

HDV is mpeg2 , but the 1080i variety uses a fairly clever codec.. not the SD variety

HD form ( HDD/Flash card camcorders) uses AVCHD, a completely different codec It is an mpeg4 H.264 derived codec. I use to think it was a Standard until It transpired that Sony Vegas software ( ver7 ) was warmly disposed toward AVCHD from Sony but not Panasonic camcorders
Im not sure about additional HD info being derivative. I think far more is captured and used .

Using Digital stills as an example, a 5Mp image and 8Mp image from Say a DSLR may look similar at 6x4.. They will not at 8 x 10 "
As such HD images are not extrapolated from the little extra recorded over SD they have enough video info to look far more defined at much higher Display resolution
A simple test is to compare the Native and downconverted footage of the same footage from an HDV (tape based) camcorder. in order to make the SD footage, some info has to be discarded

It is possible to upsample SD footage to HD in software. This unlike capturing in HD simply increases file size without any gain in resolution.
One use of such a process is if say one needs to incorporate a clip taken at SD into an HDV edit.

[redsox_mark]

To add to Senu's comments: HDV uses the same bit rate (25 mpbs) as DV, so the file sizes are approx 13 GB per hour.
With AVCHD it depends on the camcorder and quality setting used, but a typical high quality bitrate is about 12 mpbs, so the files will be approx half the size.