One of the best-known audio and video streaming techniques is the standard
called MPEG (initiated by the Motion Picture Experts Group in the late 1980s).
This section focuses on the video part of the MPEG video standards.
MPEG's basic principle is to compare two compressed images to be transmitted
over the network. The first compressed image is used as a reference frame, and
only parts of the following images that differ from the reference image are
sent. The network viewing station then reconstructs all images based on the
reference image and the "difference data".
Despite higher complexity, applying MPEG video compression leads to lower
data volumes being transmitted across the network than is the case with Motion
JPEG. This is illustrated below where only information about the differences in
the second and third frames is transmitted.
Naturally, MPEG is far more complex than indicated above, often involving
additional techniques or tools for parameters such as prediction of motion in a
scene and identifying objects. There are a number of different MPEG standards:
||MPEG-1 was released in 1993 and intended for
storing digital video onto CDs. Therefore, most MPEG-1 encoders and decoders
are designed for a target bit-rate of about 1.5Mbit/s at CIF resolution. For
MPEG-1, the focus is on keeping the bit-rate relatively constant at the
expense of a varying image quality, typically comparable to VHS video quality.
The frame rate in MPEG-1 is locked at 25 (PAL)/30 (NTSC) fps.
||MPEG-2 was approved in 1994 as a standard and
was designed for high quality digital video (DVD), digital high- definition TV
(HDTV), interactive storage media (ISM), digital broadcast video (DBV), and
cable TV (CATV). The MPEG-2 project focused on extending the MPEG-1
compression technique to cover larger pictures and higher quality at the
expense of a lower compression ratio and higher bit-rate. The frame rate is
locked at 25 (PAL)/30 (NTSC) fps, just as in MPEG-1.
||MPEG-4 is a major development from MPEG-2.
There are many more tools in MPEG-4 to lower the bit-rate needed to achieve a
certain image quality for a certain application or image scene. Furthermore,
the frame rate is not locked at 25/30 fps. However, most of the tools used to
lower the bit-rate are today only relevant for non real-time applications.
This is because some of the new tools require so much processing power that
the total time for encoding and decoding (i.e. the latency) makes them
impractical for applications other than studio movie encoding, animated movie
encoding, and the like. In fact, most of the tools in MPEG-4 that can be used
in a real-time application are the same tools that are available in MPEG-1 and
The key consideration is to select a widely used video compression standard
that ensures high image quality, such as Motion JPEG or MPEG-4.
MPEG-4 (Part 10)
The two groups behind H.263 and MPEG-4 joined together to form the next
generation video compression standard: AVC for Advanced Video Coding, also
called H.264 or MPEG-4 Part 10. The intent is to achieve very high data
compression. This standard would be capable of providing good video quality at
bit rates that are substantially lower than what previous standards would need,
and to do so without so much of an increase in complexity as to make the design
impractical or expensive to implement.
Advantages and disadvantages of Motion JPEG, MPEG-2 and MPEG-4
Due to its simplicity, the widely used Motion JPEG--a standard in many
systems--is often a good choice. There is limited delay between image capturing
in a camera, encoding, transfer over the network, decoding, and finally display
at the viewing station. In other words, Motion JPEG provides low latency due to
its simplicity (image compression and complete individual images), and is
therefore also suitable for image processing, such as in video motion detection
or object tracking. Any practical image resolution, from mobile phone display
size (QVGA) up to full video (4CIF) image size and above (megapixel), is
available in Motion JPEG. The system guarantees image quality regardless of
movement or image complexity, while offering the flexibility to select either
high image quality (low compression) or lower image quality (high compression)
with the benefit of lower image file sizes, thus lower bit-rate and bandwidth
usage. The frame rate can easily be adjusted to limit bandwidth usage, without
loss of image quality.
However, Motion JPEG generates a relatively large volume of image data to be
sent across the network. In this respect, MPEG has the advantage of sending a
lower volume of data per time unit across the network (bit-rate) compared with
Motion JPEG, except at low frame rates as described below. If the available
network bandwidth is limited, or if video is to be recorded at a high frame rate
and there are storage space restraints, MPEG may be the preferred option. It
provides a relatively high image quality at a lower bit-rate (bandwidth usage).
Still, the lower bandwidth demands come at the cost of higher complexity in
encoding and decoding, which in turn contributes to a higher latency when
compared with Motion JPEG.
One other item to keep in mind: Both MPEG-2 and MPEG-4 are subject to
graph at right shows how bandwidth use between Motion JPEG and MPEG-4 compares
at a given image scene with motion. It is clear that at lower frame rates,
MPEG-4 compression cannot make use of similarities between neighboring frames to
a high degree, and due to the overhead generated by the MPEG-4 streaming format,
the bandwidth consumption is similar to Motion JPEG. At higher frame rates,
MPEG-4 requires much less bandwidth than Motion JPEG.
About Integrated Intelligence MPEG-4 support
Most network video products feature advanced real-time video encoding
that can deliver simultaneous MPEG-4 and Motion JPEG streams. This gives users
the flexibility to maximize image quality for recording and reduce bandwidth
needs for live viewing.
Implementation of the MPEG-4 image compression standard follows the
ISO/IEC 14496-2 standard (also known as MPEG-4 Visual or MPEG-4 Part 2). Network video products support the Advanced Simple Profile (ASP) up to level 5
and the possibility for Simple Profile (SP). With a wide range of settings, it
is possible to configure the streams to be optimized for both bandwidth and
quality. The built-in Media Control (AMC) software, with MPEG-4 decoder
included, makes viewing of streams and integration into applications easy.
Furthermore, multicasting support enables an unlimited number of viewers
without sacrificing network system performance.