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Smart GOP Splicing, Part 2

Dec 18, 2006 5:57 PM, By Steve Mullen


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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.

Click to enlarge image

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.

Click to enlarge image

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 Liquid’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.

Click to enlarge image


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.

Click to enlarge image

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.


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