User guide

DivX 10

DivX, LLC User Guide

BITRATE, FRAMERATE, AND FPS

The Bitrate indicates the size of a video or audio stream over time,

usually calculated in Kilobytes per second.

In a computer the minimum unit of storage is one bit, which can

represent two values, 0 or 1, and hence computers use the binary

number system (base 2) as opposed to the denary system (base 10,

0-9) that humans use. When several bits are combined they can be

used to store more complex data, similar to adding more digits in

denary in order to store larger numbers.

For practical purposes the smallest unit of storage software

normally works with is actually one byte. A byte is composed

of eight bits.

The bitrate describes how many thousands of bits per second on

average the encoder should aim to spend when encoding the video.

Given a video of any fixed duration encoding at a higher bitrate will

lead to larger file sizes (and better quality video), while conversely

encoding at a lower bitrate will lead to smaller file sizes (but lower

quality video).

A key aijm when encoding is to achieve a desired file size; after

all the prime reason for video compression is to reduce storage

requirements. Given the length of a video and a target file size, it is

possible to determine a suitable bitrate in just five simple calculations.

The bitrate calculated is the average bitrate for the video. DivX may

choose to vary the actual bitrate throughout the video as it encodes,

a technique known as variable bitrate encoding. Since not all

sequences in a video are equally complex in terms of image and

motion, it is impossible to maintain a constant bitrate throughout the

file without introducing substantial quality differences between frames.

Instead, DivX distributes the total bandwidth as appropriate with the

goal of maintaining a consistent overall quality that permits it to meet

the average bitrate. It does this via the rate control, which will be

introduced shortly when multipass mode is covered in more detail.

The Frame rate tells us how many images of a movie are displayed

for every second a movie is played. A high frame rate would therefore

have more frames per second, and a larger file size, for the same film

of a lower frame rate. Frame rate is measured in ‘frames per second’

or fps. In the NTSC system, the frame rate is usually about 29.9 fps,

and in PAL it is usually 25 fps.