Practical Activity 1 : Compression Methods – MPEG
The Moving Picture Experts Group ( MPEG ) is a group of
experts that was formed by the International Organisation for Standardisation (
ISO) and the International Electro technical Commission ( IEC) in 1988 and
given the task to set the standards for audio and video compression. MPEGS
official designation within these standards is ISO/IEC JTC 1/SC29/WG11 which is
the coding of audio, picture, multimedia and hypermedia information. MPEG
usually hold four meeting per year in which approx 400 experts from various
countries attend who are accredited by an appropriate National Standard body.
These are mainly experts who have a stake in the digital audio, video and
multimedia industry. The convevor of the MPEG group is Leonardo Chiariglone
also kown as the father of MPEG started the group with about 15 experts on
Below are the main MPEG standards currently in use
MPEG-1: MPEG-1 Provides a video resolution of 352-by-240 at 30 frames
per second .This produces video quality slightly below the quality of a
VCR.MPEG-2: Offers resolutions of
720×480 and 1280×720 at 60 fps. This frame rate is sufficient for all the
major TV standards including high
definition TV. MPEG-2 is used by DVD-ROMs. MPEG-2 can compress a 2 hour
video into a few gigabytes. MPEG-3: Initially designed for
HDTV MPEG-3 was abandoned in place of using MPEG-2 for HDTV.MPEG-4: A graphics and video
compression algorithm standard that is based on MPEG-1 and MPEG-2 and
Apple QuickTime technology. MPEG-4 files are designed to transmit video
and images over a narrower bandwidth and can mix video with text, graphics
and 2-D and 3-D animation layers. MPEG-4 was standardized by the ISO/IEC in
October 1998.MPEG-7: MPEG-7 provides a tool set for completely
describing multimedia content. MPEG-7 is designed to be generic and not
targeted to a specific application.MPEG-21: MPEG-21 describes a
standard that defines the description of content and also processes for
accessing, searching, storing and protecting the copyrights of content. MPEG-21
toward resources that are offered, modified, sold, consumed, delivered,
regulated or facilitated, including audio, video and images.
is designed for video and audio files and its purpose is to compress these
files primarily for two reasons
To save storage space ( a compressed audio or video
file can be drastically smaller than its original size therefore saving a great
deal of storage space on a device)
To save on bandwidth in communication which saves
time when sending data ( the smaller the size of a frame or packet this saves
on transmission time)
MPEG has gained a wide audience across the globe
rapidly. It has been adopted by leading multimedia technology firms like
Philips, Samsung, Intel, and Sony for their products.
The National Academy of Television Arts and
Sciences (NATAS) awarded its 1995-1996 Engineering Emmy for Outstanding
Achievement in Technological Development to ISO/IEC for the development of the
MPEG and JPEG standards. It was a recognition of the MPEG compression
technology as a cost-effective means of delivering high quality audio and video
programming to consumers by reducing the bandwidth necessary to carry the
signal to homes and businesses across the globe.
As below we
can see the relevance and domination of the MPEG format today:
Today’s computer systems are already supporting MPEG-1 audio and
video. MPEG encoded files are available from a growing number of
platforms, including CD-ROM and the Internet.MPEG-2 is the video standard for DVD players.For DVD multichannel audio, MPEG-2 is the standard audio format for
PAL/SECAM countries, and an option for NTSC countries.An increasing number of broadcasting applications are based on MPEG
technology, e.g. DSS (Digital Satellite System), DAB (Digital Audio
Broadcast), DVB (Digital Video Broadcast), ADR (Astra Digital Radio),
Satellite feeds to cable networks, etc.MPEG is used over wan connections to provide very high quality
video conferencing and general streaming for websites such as you tube and
social media platforms.
and purposes :
MPEG-1: Designed to produce
reasonable quality images and sound at low bit rates. MPEG-1 was designed to
fit the bandwidth of CD-ROM and Video-CD and to run at at 352×240 pixels NTSC at 1.5 megabits
(Mbits) per second, a quality level about on par with VHS. MPEG-1 can also be
encoded at bit rates as high as 4-5Mbits/sec.
MPEG-1 is also used to transmit video over telephone networks such as
Asymmetrical Digital Subscriber Lines ( DSL) for video conferencing .
MPEG-2: Designed to produce higher quality images at higher
bit rates between 3-10Mbits/sec, MPEG-2 at the full CCIR-601 resolution of
720×486 pixels. Its purpose is to provide true broadcast quality video and has
overcome MPEG-3 to be the standard for HDTV and DVD. The primary users of
MPEG-2 are broadcast and cable companies who demand broadcast quality digital
video and utilize satellite transponders and cable networks for delivery of
cable television and direct broadcast satellite.
MPEG-3: Initially intended to be the standard for HDTV
however MPEG-2 is now the preferred standard so MPEG-3 has been dropped.
MPEG-4: Initially specified for very low bit rates but now it
supports up to 4Mbps. MPEG-4 is a big progression from MPEG-1 and MPEG-2
providing additional services such as support for 2D and 3D and Coding at very
low rates (2 Kbit/s for speech, 5 Kbit/s for video) to very high ones (5 Mbit/s
for transparent quality Video, 64 Kbit/s per channel for CD quality Audio). A
very attractive addition however is the support it provides for the management
and protection of intellectual property and its ability to stream real time
MPEG-7: This version is more associated with the content
itself, to allow a user friendly interface
Advantages of using MPEG: https://bytescout.com/blog/2014/09/mp4-file-format.html
Suitable for video streaming over the internet.
Less loss of quality with higher degree of compression.
Can store data types other than video and audio, such as object
descriptors, scene descriptors and other object oriented file structures and
Separate hint track is used to encode streaming information.
MP4 files can be run virtually on
all the major media players and sound card drivers
of using MPEG: https://www.online-sciences.com/technology/video-container-formats-mpeg-mov-features-uses-cons-and-pros/
One drawback of this type of file is the software
required to run, It is very proprietary, Each vendor has a different set of
bugs, As MPEGS are so compressed,
they are not recommended for editing or archiving.
MPEG does not feature some of the
flexibilities of the other formats, making it a poor format for editing video, It is not editable, a
finished file would have to be pre-edited and then re-encoded.
How MPEG video compression works: https://bitdepth.thomasrutter.com/2010/02/02/how-jpeg-and-mpeg-picture-compression-algorithms-work/
All types of MPEG videos, work in a very similar way using block based
DCT image compression. This involves chopping up the picture into blocks,
8 pixels by 8 pixels, and compressing each block separately, and using a
mathematical algorithm called a Discrete Cosine Transformation (DCT).
Colour space conversion
If necessary, the
image is first converted into the correct colour space and resolution.
This involves separating the colour image into 3 separate images or ‘channels’:
a luma channel which carries the brightness information (like a black and white
image), and two chroma channels which carry the colour information.
Furthermore, the luma channel is usually given double or quadruple the
resolution of the colour channels; this saves some storage space already, and
works because humans are much better at seeing intricate changes in lightness
Separate into 8×8
blocks and perform Discrete Cosine Transform
Each channel of the
image is then divided into blocks of 8 pixels by 8 pixels (64 pixels per
block). To each block, a mathematical formula called a Discrete Cosine
Transform (DCT) is performed.
Quantisation is the
process of reducing the precision of a number so that it is less accurate, but
takes fewer bits to write to a device. It works by dividing a number by a
certain value, and
throwing away the remainder. When multiplying by that value
again in an attempt to get the original number, chances are that the number is
close to, but slightly different to, the number you started with. The
more you divide the number by, the greater the strength of the quantisation and
hence the greater the likely loss in precision once you multiple the number
again. The JPEG and MPEG compression can also apply different amounts of
quantisation to the different frequency coefficients within a single block, or
different amounts per block in an image. When this is done, it is all in
an attempt to get the best looking picture, as perceived by human eyes, for a
given amount of quantisation.
MPEG video encoding can also involve motion estimation, where
a block can be optimised by only encoding the difference between it and a
nearby area of some other frame. Encoding is still done per-block, and
the blocks still undergo DCT and quantisation.
Decoding the image
Decoding is a
reverse of the above process: the stream is decoded to find the coefficients
for each block, and these coefficients are multiplied by the same number they
were previously divided by, in order to reverse the quantisation.