ADS Technologies USBAV-708 INSTANT VIDEOMPX, User Manual

Page: 32 / 116

The
Guide
32
Bi
tra
te
m
od
e
Wh
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Bitrate mode
In a computer the minimum unit of storage is one bit. A bit 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.
For practical purposes the smallest unit of storage software normally works with
is actually one
byte . A byte is composed of eight bits.
To avoid confusion when working with quantities of bits and bytes the following
tables should be adhered to:
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 aim 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 following example demonstrates the bitrate calculation for encoding 60
minutes of video with 128 kbps MP3 audio for a 700MB CD-R. The algorithm is
applicable to any video or target file size.
What is the bitrate?
Bits
One bit = Smallest unit
One kilobit = 1,000 bits
One megabit = 1,000 kilobits
or 1,000,000 bits
Bytes
One byte = 8 bits
One kilobyte = 1,024 bytes
One megabyte = 1,024 kilobytes
or 1,048,576 bytes
or 8,388,608 bits