> For 720p25 you might need more than 3.5Mbps for more > demanding scenes. (Except increasing the bitrate or using a > better encoder will make iPlayer look better than the > broadcast...) > > You do get an awful lot better results when you > are not compressing in real time, of course, because you > can use all the MPEG4 forward references, the ones you > don't get when you real time encode. >
Real-time encoding with Bi-predictive frames (B-frames) in H.264 doesn't work like that. There's a frame delay in order for B-frame encoding to take place. Most encoders worth their while also have a lookahead for deciding frame-types and bit rate allocation. (Sometimes this is called "2-pass" realtime, which is a bit of a misnomer for marketing reasons. Some marketing people for manufacturers seem to spread this myth that more passes is always better). Using x264 with a recent CPU, if you ran it at realtime even at 720...@3mbit you'd most likely do better than the £50k+ broadcast encoder at 1080i merely because we're generations ahead of most (if not all) of the H.264 hardware and software out there. Naturally, with 2-pass you can allocate bits more efficiently but the benefits aren't as significant as they once were. - Sent via the backstage.bbc.co.uk discussion group. To unsubscribe, please visit http://backstage.bbc.co.uk/archives/2005/01/mailing_list.html. Unofficial list archive: http://www.mail-archive.com/backstage@lists.bbc.co.uk/
