For a 1080i hdtv video, would there be much difference in picture quality between an encoding with 15mbit bitrate video and one with 35mbit bitrate video?

It depends.

If the shot involves a lot of motion, then 15Mbps is not adequate for 1080i MPEG-2. If the camera position is in a static position and focused on a news anchor sitting in their chair, then 15Mbps is plenty.

So for a 40min video with lots of action, you don't think 11GB is overkill?

Some sources, such as HBO, use MPEG-2 repeat flags, which reduce the required bit rate for 24p movies by telling home decoders to insert the extra TV fields used for 2-3 pulldown. (Pulldown for 480i and 1080i boosts the TV field rate to 60 Hz, matching the broadcast rate.). Most HD sources actually broadcast the duplicated TV fields used for pulldown, requiring more bandwidth.

The maximum video payload portion of typical ~19-Mbps HD signals--not HD discs--is about 17 Mbps. That's a payload rate used by HDNet, for example, which delivers excellent sports and movies.

But as codec engineer dr1394 points out here (http://archive2.avsforum.com/avs-vb/showthread.php?p=5072024&&#post5072024), without prefiltering (reducing higher frequencies/resolutions), the ideal 'sweetspot' for MPEG-2 is ~23 Mbps. He mentions 30 Mbps peak would be good for MPEG-2 HD discs, and that the newer HD-disc codecs, ideally, might require about half that bit rate.

For lots of HD reception PQ seems increasingly defined as a lack of artifacts, especially when motion occurs and MPEG-2 encoding 'breaks down'. That's an obviously inadequate bit rate. But if HD sources are prefiltered, eliminating higher effective resolutions (http://archive2.avsforum.com/avs-vb/showthread.php?p=5667245&&#post5667245) (finer resolvable details) that might contribute to motion blocking, HD images may seem adequate but still lack all the resolution possible.

On my cable system, for example, any motion artifacts (SD or HD) are very rare, but tests with HDNet's Saturday 6:30 am ET test patterns show that 1920X1080i format resolution is limited to ~1300 lines of effective horizontal resolution maximum. By contrast, some AVS members on smaller systems have reported measuring close to the full 1920X1080 resolution. So, rather than using a criteria, motion artifacts, indicating 'broken' digital encoding, it seems preferable to deliver all the resolutions possible within HD formats. -- John