An introduction to
audio/video
compression
Dr. Malcolm Wilson
Introduction
• Review digital video.
• Need for compression.
• Lossy and lossless
compression.
• How to apply
techniques to still video
(jpeg).
Digital Video
• Samples represent pixel
values of a video frame.
• Typically
480x640x3=1M (approx)
per frame
• Sample size 8-10 bits
typically.
Video datarates
• With 8 bits samples at
25 frames per second we
have a datarate of 200
Mbits per second.
•Broadcast video (10 bit
and includes audio) 270
Mbits per second.
Why do we need
compression?
Modems
Broadband
USB 1.1
USB 2.0
IEEE 1394
Ethernet
DVD
CD-ROM
EIDE Hard
SCSI Hard
56kbit s-1
8Mbit s-1
12Mbit s-1
480Mbit s-1
400Mbit s-1
100kbit s-1
14Mbit s-1
50Mbit s-1
100MB s-1
160MB s-1
Lossless
Compression
• Achieve datarate
reduction without
discarding information.
• Run Length Coding
Eg: 5 5 5 5 5 5
May be coded 5 6
Lossless
Compression
Entropy (Huffman)
Coding
• Assign shorter codes to
the more probable
values.
• Possible when there is
statistical bias in image
data.
Lossy Compression
• Achieving datarate reduction
by discarding information.
• Missing out samples from
data stream/reducing sample
size generally noticeable.
Transforms
• Frequency transforms
(DCT) separate
frequency components.
• Transformed image
allows efficient coding of
high frequency data.
Example DCT Basis
Pictures
• Picture broken into 8x8
pixel blocks (typically).
• Any 8x8 pixel (picture)
block made by adding
different proportions of
basis pictures.
• So store the
proportions.
DCT basis functions
(Basis pictures).
Original Picture
Using one (dc) basis
picture
Using 6 basis pictures
Video compression
• Most 8 x 8 blocks do
not contain high
frequency components.
• Code high frequency
components using less
bits (introduces noise).
Video compression
• We are left with a
sequence of values.
•Zig-Zag scan.
• Many values of zero
towards the bottom right
hand corner.
• Use RLE to reduce the
datarate further.
Variable length coding
• More likely that any non-zero
value will have a run of zeros
before it.
• Non-zero value more likely
to have a low value than a
high one.
• Use Huffman coding.
Summary
• Compression is needed
to replay video from
computer peripherals and
to economise on storage.
• Lossless techniques
can allow some
compression of video.
•Lossy techniques can be
used by considering
psychological factors.
Summary
•We need to transform
the signal to achieve
unnoticable lossy
compression.
•Lossless techniques
may then be used to
achieve further
compression.
Psychological
Considerations
• Our eyes are not as
sensitive to colour detail.
• Colour information
–Less bandwidth
• Our eyes are less
sensitive to high
frequency noise.