2-pass encoding

on Saturday, February 23rd 2013 I 0 Comment

With the increasing rates of bandwidth and computing power, the video quality is constantly evolving. Aware of this, we are improving the quality of daily videos encoded by our system.

2-pass encoding

The two pass encoding gives better results than one pass.
The first pass considers the
Rate-Distortion methods for video using a first set of quantization parameters (QP). Estimated rates helps to select quantization parameters for the second pass to minimize fluctuations in quality between frames.
Let's see how it works in detail:
For instance, you have a
2 minutes video. A part of it, is very dynamic, the other part is more static. So that the video can be properly read in HQ, it takes at least 750kbps for the biterate global level.

A biterate of 300kbps is enough to play the static part of the video, and 750kbps is correct for the dynamic part.

When using 1-pass encoding, the process selects 750kbps for both parts, and thus ignores their differences, which leads to a noticeable fluctuation between frames.
The 2-pass encoding will not make such mistakes. The use of statistics from the first pass allows the encoder to estimate the value of each frame in bits. This results in a better allocation of bits between the two parties.
The first
dynamic part  will get more bits while the second part will have fewer bits.

We must note that there is only one flaw with this technique: The 2-pass encoding is almost 30% slower than 1-pass encoding. So if you are in a hurry, do not use this encoding mode.

For those looking for the best possible encoding, just check the "Enable 2-pass" option, which is on your video settings page Video settings> WebTV

This feature is currently available only for the product WebTV.

Saveria Maroselli
As support manager, Saveria guides you daily while you're using your favorite CMS, but not only,... Know more about this author

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