Simon Thompson wrote: > The GOP length is the number of frames between successive I-Frames. A > long GOP length will, for example, cause a delay on video appearing on > changing channels on a STB or, as editing cuts can only start from an > I-Frame will mean you can't do frame accurate editing.
I disagree with "can't" - the Sony XDCAM EX1 is a serious camera intended for broadcast use that uses long-GOP MPEG2. However, editing is indeed harder since the software needs to be clever about how it handles the content. You potentially have to decode a fair number of frames to show the one you want, and (unless re-rendering) you need to keep up to the previous I-frame before any edits made in your source material throughout the editing process. Final transcoding is awkward too. If you intend to output to another MPEG-2 then you either have to totally re-render the content to have new I-frames (but with associated quality losses) or attempt to piece together the original GOPs for the edit, only generating new sections around edit points. The majority of production houses [1] are still using I-frame based systems - DV/Digibeta/HDCAM - but with the current trends towards MPEG2-based formats - HDV/P2 - it is going to be interesting to see how different types cope with the issues of non-I-frame editing. Some long-timeframe productions will have the opportunity to re-render to a I-frame format at ingest (typically MJPEG), others (current affairs / sports) will have to deal with the formats throughout their workflows as they are. -jeremy [1] Televisual Aug '08 Production Survey (Current usage: 70% Digibeta, 68% DV, 49% HDV, 45% HDCAM) - 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/
