Two pass encoding, also known as multi-pass encoding, is a video encoding strategy used to retain the best quality during conversion.
In the first pass of two-pass encoding, the input data from the source clip is analyzed and stored in a log file. In the second pass, the collected data from the first pass is used to achieve the best encoding quality. In video encoding, two-pass encoding is usually controlled by the average bitrate setting or by the bitrate range setting (minimal and maximal allowed bitrate) or by the target video file size setting. The best way to understand why this is used is to think of a movie — when there are shots that are totally, absolutely black, like scene changes, normal 1-pass CBR encoding uses the exact same amount of data to that part as it uses for complex action scene. But by using VBR and multi-pass, encoder “knows” that this piece is OK with lower bitrate and that bitrate can be then used for more complex scenes, thus creating better quality for those scenes that require more bitrate.
Two-pass encoding is almost twice slower than one-pass coding. So if you are short of time select one-pass solution.
To turn on two pass encoding via API, just include the following in your format request:
<?xml version="1.0"?> <query> <format> <two_pass>yes</two_pass> </format> </query>
Please note, the default for this value is no.
Via UI, please select Two Pass under general settings: