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MillimeterEncoding Best Practices, Part 2
Jun 23, 2008 12:00 PM, By Jan Ozer
Table 1. SD video quality by codec
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SD Tests
I produced all SD test files at 640x480 at 30fps, with a data rate of 468kbps for video and 32kbps for audio. I produced using two-pass variable-bit-rate encoding (VBR) at the highest supported profile for H.264. I used encoding VC-1 parameters provided by Microsoft’s encoding wiz Ben Waggoner. With Flix Pro, I set all quality settings for VP6 at the highest supported levels.
To assess quality, I looked at three factors. To test still-image quality, I captured frames from each compressed file and compared them side by side. To test motion quality, I played the files back in realtime and looked for artifacts such as mosquitoes, banding, jitter, and other artifacts that appear only during realtime playback. Finally, to test smoothness, I loaded each compressed file into Inlet Technology's Semaphore tool to detect whether the encoder had dropped any frames to meet the target data rate.
Figure 1. All codecs look the same in this easy-to-compress, low-motion shot.
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In each test, I ranked the codecs. Lower scores are better, as summarized in Table 1. As you can see, H.264 was first, followed by VP6-E, and with VC-1 last.
Let’s start with still quality because that’s easiest to illustrate. As you would expect, all codecs performed well with easy-to-compress, low-motion footage, and they pretty much look the same in Figure 1.
Figure 2. In this high-motion sequence, VC-1 is starting to lose detail—and more importantly, it's dropping frames.
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As motion and detail in the video increases, compression becomes more challenging, and you start to see some differences between the technologies. For example, in Figure 2, VC-1 starts to look faded and shows less detail in the shirt and in the woman’s face. That said, at least with these SD clips, the quality differences were quite modest, and probably wouldn’t be noticed absent side-by-side comparisons.
Note that other encoders produced VC-1 files without dropped frames, and that it may have been possible to produce files with Expression Encoder 2 that didn’t drop frames—although this may have changed other results, such as still-image quality or motion quality. However, I used the encoding parameters supplied by Microsoft, and those are the results I’m reporting.
Figure 3. While the New VP6 codec didn’t noticeably improve still-image quality, motion artifacts were much less visible.
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Interestingly, I didn’t see much difference between the old VP6 and new VP6 in still image tests, and neither codec dropped frames. However, the new version of the codec was noticeably quieter than the existing VP6 during realtime playback.
To illustrate this, I doctored up Figure 3, reducing brightness in the midtones to illustrate that the background wall as encoded by the updated VP6 codec was smoother than the wall as encoded by the VP6-E codec that I officially tested, which showed some banding, a hint of blockiness, and some mottled colors. During realtime playback, this was the source of much of the distracting noise that lowered VP6-E’s score.
