x264 HD Benchmark FAQs
by graysky
updated by Adrian Wong & Dashken

x264 HD Benchmark FAQs

Q: The benchmark doesn't run on my machine, HELP!

The only reason the benchmark won't run is because you neglected to install AviSynth; re-read the initial setup section.

Q : What is AviSynth and why did I have to install it?

AviSynth is a frameserver. It allows the MPEG-2 format video to be fed to x264 as if it were a standard avi file. x264 cannot read MPEG-2 videos without it.

Q : Why is there no sound in the video?

Transcoding audio streams and also muxing them with the resulting video is beyond the scope of this benchmark. The whole point is to compare pure video encode times.

Q: What does 1080p mean?

Well, technically, it's 1080p24. The 1080 means there are 1080 vertical lines of resolution. The p means the video frames are progressive (i.e. not interlaced), and the 24 means there are ~24 frames per second of video (there are actually 23.976 frames per second).

Since all high definition videos use the "letterbox" 16:9 aspect ratio, you take 1080 x (16/9) = 1920 which gives you the horizontal resolution. The video is therefore 1920x1080 aka 1080p.

Roughly to scale, here are the differences in resolution between the various common standard formats for video:

If you think of them like your digital camera (i.e. in mega pixels or MPixels):

SD (Standard definition) = 720x480 = 0.35 MPixels
HD (High definition) = 1280x720 = 0.92 MPixels
Full HD = 1920x1080 = 2.07 MPixels

Q: What's the difference between progressive and interlaced frames?

If you've ever made a cartoon flip-book in a notepad, you've made a progressive frame movie in a sense. Progressive frames are just pictures set to display in a series. If you stop at any given frame, you'll see 100 % of the image as if it was a photograph. These photos are displayed in sequential order 1, 2, 3, etc.

Interlaced frames on the other hand are more complicated. Over simplified, an interlaced image is like half of a progressive image (the odd numbered lines) followed by the other half of a progressive image (the even numbered lines) displayed at a higher framerate and in a different order (termed 3:2 pulldown). For more on these concepts, see the following Wikipedia articles: here and here.

Q: What's 3:2 Pulldown?

This is getting more difficult to explain without pictures, so I'll point you to the following links: here and here.

Q: Why does it encode two passes per file?

You can encode video in a single pass, but two passes will typically give a higher quality result. The first pass scans through the entire clip analyzing it so that the 2nd pass can use more bits on particular scenes and less on other scenes thus giving a higher quality (and more efficient) result.

In a single pass encode, the encoder doesn't have any idea what's coming up next in the video and is forced to guess based on what it's currently seeing. By the way, other video formats take advantage of multi-pass encodes such as Xvid, DivX, MPEG-2 (this is the format DVD movies use), etc.

Q: Why does the first pass encode faster than the 2nd pass?

The first pass is just a scanning pass. This analysis can occur much faster than the actual encode (the 2nd pass) can. No doubt, you see this reflected in your benchmark results.

Q : Why doesn't the first pass use 100 % of my multi-core processor?

As mentioned above, the 1st pass simply isn't as CPU-intensive as the 2nd pass. It's actually relies more on memory bandwidth than CPU performance. Therefore, it's not uncommon for less than 100 % usage on the first pass.

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