• Blogs
• Forums
• Tradeshows
• Partners
• White Papers
• Events

Smart GOP Splicing, Part 2

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

In the last edition of HDV@Work, I examined the promise of an encoding feature called Smart Splicing, which is a feature of Avid Liquid that can increase the efficiency of MPEG-2 encoding. This should hold special interest for HDV producers, who in the next year will be called upon to deliver high-definition DVDs.

To learn more about Smart Splicing, I created a simple simulation. The Table below shows a yellow cell representing the last B-frame before the outgoing (pale green frames) GOP, and the rightmost yellow cell represents the first I-frame after the incoming (pale blue frames) GOP. The pale green and blue cells represent two un-trimmed, 6-frame, Closed GOPs.

Notation in the cells indicates that each I-frame is 1.44Mb; each P-frame is 1/2 of an I-frame (0.72Mb); and each B-frame is 1/4 of an I-frame (0.36Mb). The total data in one GOP is 3.6Mb; therefore, the bit rate for five GOPs (one-second) is 18Mbps.

According to the Sarnoff patent mentioned in the last edition, “With respect to rate control (which ultimately determines picture quality of the transition clip…)…due to masking in the human visual system, a small degradation in video quality at a scene change is often imperceptible to a viewer.” To explore this concept, I included a green “quality” indicator in my simulation.

The Table below shows the progressive shortening of outgoing and incoming GOPs. The red percent indicates the amount of bit rate overload caused by the trim — which in turn becomes the amount by which data for each frame are reduced by increasing compression. In the first section of the Table, the final B-frame of the outgoing GOP and the initial I-frame of the incoming GOP have been trimmed away.

Values in blue are the reduced data for each frame. (To save space, not all permutations are presented.) The Chart below shows the increased data rate caused by trimming.

Given the 18Mbps bit rate, each GOP carries 3.6Mb; therefore, the average amount of data for each frame is 0.60Mb. The blue cells’ values are average data for all frames within the two transition GOPs, plus data from the preceding (ending with the yellow B-frame) GOP and the following GOP (beginning with the yellow I-frame).

The Chart below shows these values based upon GOP lengths of 1 to 6 (untrimmed GOP). The average data per frame (shown in pink) is 0.622Mb, which yields a data rate of 18.64Mbps. Therefore, despite the significant increase in source data caused by trimming, a Smart Splicer keeps data output quite consistent.

The Table’s green percent value indicates post-trim image quality. Quality decreases significantly when both GOPs are trimmed. (See the end of the article for alternate trimming examples.) If, however, a GOP’s structure is optimized to better match its shorter length, quality can be increased. Below the orange P indicates the alteration.

Below you will see: an IBB structure changed to IBP and an IB structure changed to IP. (The former alteration enables the B-frame to truly be bi-directional.)

The data values in the previous Chart reflect GOP optimization. In order to make GOP alterations rapidly — and other splicing decisions — Liquid supports an “index” file for each HDV file. The index provides relevant data for each frame and GOP.

The “smarter” the splicer, the higher the splice quality. The Chart below shows Quality based upon GOP length. Chart 3.jpg

Remember that all Charts and Tables are based upon my simulation and do not represent the actual performance of any real-world splicer.

next page

© 2008 Penton Media, Inc.