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Editing long-GOP video

Nov 1, 2008 12:00 PM, By Steve Mullen

Perform MPEG export using smart GOP splicing.

Figure 2. Removing an I-frame (in red) would create an illegal GOP (in blue)

The upper half of Table 2 has the final B-frame of the outgoing GOP and the initial I-frame of the incoming GOP — as represented in Table 1 — trimmed away. The lower half of Table 2 displays the two new five-frame GOPs created by the re-encode process.

Notice the re-encoding process has handled the difficult situation created when the I-frame of a GOP is trimmed away. As shown in Figure 2 on page 36, simply removing an I-frame would create an illegal GOP (shown in blue).

Table 2 and Table 3 have bit rate values (shown in red) that indicate the data rate relative to a nominal 18Mb data rate. In Table 2, this value is 104 percent and indicates the increased data rate created by trimming away two frames. Values above 100 percent indicate an overload.

Figure 3 shows data rate increase above 100 percent as a function of removing two, four, six, eight and 10 frames from two six-frame GOPs.

Figure 3. Data rate as a function of removing two, four, six, eight and 10 frames from two six-frame GOPs
Select image to enlarge.

To prevent data rate overload, a smart GOP splicing system, after decoding all frames in a pair of GOPs, encodes them using higher compression. Therefore, the amount of data in each frame is reduced. For example, Table 2 shows I-frame data has been reduced from 1.44MB to 1.39MB.

Table 3 shows the progressive shortening of outgoing and incoming GOPs. In the second example, the final two B-frames of the outgoing GOP plus the initial I- and B-frame of the incoming GOP have been trimmed away.

Table 2 and Table 3 have a value (below the data rate value) that indicates the quality of the two GOPs after re-encoding. As you can see, increased compression causes a relatively significant loss of quality as the pair of GOPs is trimmed shorter. Thankfully, the lowest quality transitions occur at the shortest durations.

The Sarnoff patent addresses this issue by this statement, “With respect to rate control (which ultimately determines overall picture quality of the recoded portion 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.”

Table 3. Simulated data rate increase above 100 percent as a function of removing two, four, six, eight and 10 frames from two six-frame GOPs
Select image to enlarge.

Given that a six-frame GOP carries 3.6MB, the average amount of data for each frame is 0.60MB. Knowing this value, we can estimate the quality of the frames in the four GOPs centered on the transition. The values in blue cells in Table 3 present average post re-encoding data within the two transition GOPs as well as the preceding and following GOPs. Figure 4 shows these values, in megabytes from Table 3.

Although quality does vary depending on GOP length, average data size (shown in purple) is 0.622MB per frame. When compared to the nominal value of 0.60MB, the result indicates a smart GOP splicer keeps overall quality reasonably consistent for the four GOPs centered on the transition.

Smart GOP splicing restrictions

There are restrictions to using a smart GOP splicer when exporting to MPEG-2 — or any interframe format.First, even though Media Composer and Xpress Pro HD support smart GOP splicing, both NLEs typically render effects to DN×HD. Therefore, timeline segments where effects have been applied are no longer interframe video and will need to be encoded.

Second, some NLEs support a less powerful version of a smart GOP splicer. During export, these NLEs scan a timeline looking for those edited clips — with no applied effects — that begin with an I-frame. These clips are marked so they are not decoded and encoded during export. All other clips must be re-encoded. Obviously, these NLEs do not offer the potential of significantly reduced export times.

This type of smart GOP splicing is used by Final Cut Pro when already exported sequences — which inherently begin with an I-frame — are loaded without trimming into a master sequence. This sequence can then be exported without re-encoding.