SO what is H.264 and what are the advantages and
disadvantages of this new compression technology? Essentially, H.264 is a new
video compression scheme which is set to become the worldwide digital video
standard for consumer electronics and personal computers. H.264 has already
been selected as a key compression scheme (codec) for the new optical disc
formats, such as Blu-ray disc.
The intent of the H.264 standard project was to create a
standard capable of providing good video quality at substantially lower bit
rates than previous standards (e.g. half or less the bit rate of MPEG-2, H.263,
or MPEG-4 Part 2), without increasing the complexity of design so much that it
would be impractical or excessively expensive to implement.
“H.264 has been adopted by the Motion Picture Experts
Group (MPEG) to be a key video compression scheme in the MPEG-4 format for
digital media exchange”
An additional goal was to provide enough flexibility to
allow the standard to be applied to a wide variety of applications on a wide
variety of networks and systems, including low and high bit rates, low and high
resolution video, broadcast, DVD storage, RTP/IP packet networks, and ITU-T
multimedia telephony systems.
H.264 has been adopted by the Motion Picture Experts
Group (MPEG) to be a key video compression scheme in the MPEG-4 format for
digital media exchange. H.264 is sometimes referred to as “MPEG-4 Part 10”
(part of the MPEG-4 specification), or as “AVC” (MPEG-4’s Advanced Video
Coding). This new compression scheme has been developed in response to
technical factors and the needs of an evolving market:
* MPEG-2 and other older video codecs are relatively
inefficient.
* Much greater computational resources are available
today.
* High Definition video is becoming pervasive, and there
is a strong need to store and transmit more efficiently the higher quantity HD
data (about 6 times more than Standard Definition video).
H.264 clearly has a bright future, mostly because it
offers much better compression efficiency than previous compression schemes.
The improved efficiency translates into three main benefits, or a combination
of them:
* Higher video quality at a given bit-rate; reduction in
artifacts such as blockiness, color bands, etc
* Higher resolution; as the video world transitions to
High Definition, a mechanism is needed to deliver it. The new Foxtel HD
transmission is H.264
* Lower storage requirements; lower storage requirements
will allow for large amounts of content to be delivered on a single disc.
It is likely that future delivery of Digital TV signals
(both in SD and HD) will use H.264. For SD, the same content at a given quality
can be delivered with a lower bit-rate (allowing for more channels to be
transmitted on the same medium), or higher quality and/or higher resolution can
be delivered at the same bit-rate. Also, many CCTV suppliers are now showing
their new systems with H.264. Future Digital TV delivery vehicles include:
* Satellite
* Cable
* IPTV (over cable or DSL)
* Over-the-Air broadcast
* CCTV systems.
Some of the above are already turning to H.264 as a
standard; worldwide, more are likely to announce that they are following
shortly. High-Definition Optical Discs High-definition video is gaining in
popularity, aided by the falling cost of HD television sets. A key deployment
vehicle for High Definition content is likely to be optical discs carrying this
content. Blu-ray Disc format is currently proposed. This disc format has chose
to adopt H.264 as one of the key means of storing the HD video content. The
high bit-rates that are used to encode the video on HD-discs will be
particularly challenging today’s PCs; we will examine this further after we
compare MPEG-2 and H.264)
Differences between H.264 and MPEG-2 video decoding
MPEG-2 is today’s dominant video compression scheme and
it’s used to encode video on DVDs, to stream internet video and is the basis
for most worldwide digital television (over-the air, cable and satellite).
While MPEG-2 is a video-only format, MPEG-4 is a more generic media exchange
format, with H.264 as one of several video compression schemes offered by
MPEG-4.
There are numerous differences between these compression
schemes, but a key point is that H.264 has been developed to deliver much higher
compression ratios than MPEG-2. However, this greater degree of compression (up
to 2-3 times more efficient than MPEG-2) comes at the expense of much higher
computational requirements. This additional computational complexity is
widespread in the overall decoding process, but three key techniques areas
stand out in adding to the new overhead: Entropy encoding, smaller block size
and In-loop deblocking.
* Entropy encoding
Entropy encoding is a technique used to store large
amounts of data by examining the frequency of patterns within it and encoding
this in another, smaller, form. H.264 allows for a variety of entropy encoding
schemes, compared to the fixed scheme employed by MPEG-2. In particular, the
new CABAC (Context-based Adaptive Binary Arithmetic Coding) scheme adds 5-20%
of compression efficiency but is much more computationally demanding than
MPEG-2’s entropy encoding.
* Smaller block size
MPEG-2, H.264, and other most other codecs treat portions
of the video image in blocks, often processed in isolation from each another.
Independently of the number of video pixels in the image, the number of blocks
has an effect of the computational requirements.
While MPEG-2 has a fixed block size of 16 pixels on a
side (referred as 16×16), H.264 permits the simultaneous mixing of different
block sizes (down to 4×4 pixels). This permits the codec to accurately define
fine detail (with more, smaller blocks) while not having to ‘waste’ small
blocks on coarse detail. In this way, for example, patches of blue sky in a
video image can use large blocks, while the finer details of a forest in the
frame could be encoded with smaller blocks.
* In-loop deblocking
When the bit-rate of an MPEG-2 stream is low, the blocks
(and specifically, the boundaries between them) can be very visible and can
clearly detract from the visual quality of the video. “De-blocking” is a
post-processing step that adaptively smoothes the edges between adjacent
blocks. De-blocking is computationally “expensive”.
In the past, de-blocking has been an optional step in
decoding, only enabled when it was possible for the playback device (such as a
PC) to perform it in real time. ATI has offered de-blocking capability for
playback of video for some time. In H.264, however, In-loop deblocking is introduced.
The “in-loop” refers to when previously ‘de-blocked’ image data, in addition to
being displayed, is actually used as part of the decoding of future frames; it
is in the decoding ‘loop’. Because of this, the de-blocking is no longer
optional. It adds to the quality of the decoded video, but also adds
significantly to the computational overhead of H.264 decode.
In the coming months the CCTV industry will see a
significant increase in H.264 compression technology with most CCTV
manufacturers.
Acknowledgment: ATI Technologies Inc.
* Les Simmonds is an independent CCTV consultant. Email: les@cctvconsultants.com.au
