Smart GOP Splicing, Part 2
Dec 18, 2006 5:57 PM, By Steve Mullen
Smart GOP Splicing Limitations
While it is clearly possible to implement a Smart GOP Splicer, there are limitations to its use:
1. When the splicer determines that a GOP will be created that is shorter than allowed by the MPEG-2 format, it will not execute a smart splice. Encoders have two parameters: maximum length (N) and minimum length (M).
2. When the splicer determines that the quality of a spliced GOP will fall below an acceptable level, it will not execute a smart splice. For example, if the minimum quality level were 50 percent, the sample below would not be smart-spliced.
Below, the worst case, where all but one frame has been trimmed from each GOP. This results in three I-frames in a row. To keep the data rate at an acceptable level, compression must significantly be increased for the two I-frames. This yields a significant decrease in quality.
Whenever GOPs do not meet the criteria to be smart-spliced, they must be decoded and recoded. Therefore, even with a cuts-only timeline, there may be a need for a significant amount of conforming during export.
Likewise, whenever effects have been applied, all rendered frames must be conformed. In the experimental production I created using Liquid, almost every shot was color-corrected, so Liquidâ€™s ability to smart-splice provided no advantage to the final export.
Despite these limitations, a Smart GOP Splicer provides an advantage in two types of productions: news, where effects typically are not applied, and rough drafts of long-form works. Both are production genres in which Avid has a very strong presence.
Bottom line, Smart GOP Splicing is not a panacea for the long export times required for high-definition MPEG-2. Moreover, the situation is far worse for the export of both VC-1 and any type of AVC. AVCHD encoding is reported to be about seven times slower than HDV encoding.
The need for high-speed encoding exists even when the source is not HDV, ProHD, or XDCAM HD. Editing with DVCPRO HD, AVC Intra, HDCAM, HDCAM SR, or even uncompressed: all require encoding to create high-definition DVDs. Until a solution is developed, high-definition export is going to be a painful process.
Additional Trimming Examples
The Table below shows the progressive shortening of an outgoing GOP. In the first section of the Table, the final B-frame has been trimmed away.
The Table below shows the progressive shortening of an incoming GOP. In the first section of the Table, the initial I-frame has been trimmed away.
Note that in all cases, quality remains above 50 percent. Thus, were the quality threshold set at 50 percent, all samples could be smart-spliced.
Continue the discussion on “Crosstalk” the Millimeter Forum.