By Jennifer Eyre, 3/14/2005
Digital video found its first big consumer market in DVD players, and has moved
on from there. Now you can buy digital set-top boxes, camcorders, personal video
recorders (PVRs), portable media players, and even digital-video-enabled cell
phones. Products that can only handle analog video will soon be extinct; they’ll
be relegated to technology museums, sitting next to vinyl records and eight-track
tape players.
The mass migration from analog to digital video has been enabled by video compression
algorithms, or “codecs” (for COmpression/DECompression). In the
March
2004 edition of Inside DSP we introduced the basics of video compression.
In this article, we take a closer look at one of the hottest new video codecs,
H.264.
H.264 was jointly developed by the Moving Picture Experts Group (MPEG, part
of ISO) and the International Telecommunication Union (ITU). H.264, published
in 2003, is the standard’s ITU name; it also goes by the somewhat lengthy
“MPEG-4 Part 10 Advanced Video Coding (AVC).”This designation distinguishes
it from MPEG-4 Part 2 (often referred to as MPEG-4), a successor to MPEG-2
that has had limited success in the market.
H.264 is joining a field of established video codecs. The most popular of these
is MPEG-2, which is used in all current DVD players. Windows Media 9 and DivX
are widely used in streaming video applications (i.e., applications where compressed
video “streams” over the Internet and is played back in real time
rather than being stored first).
One key attribute of a video compression application is the bit rate of the
compressed video stream. Codecs that target specific applications are designed
to stay within the bit rate constraints of these applications, while offering
acceptable video quality. For example, DVDs use 6-8 Mbps with MPEG-2; video
conferencing applications require 50-300 kbps using H.263 (a video conferencing
codec). Streaming video applications typically require 50 to 500 kbps, but can
exceed 1 Mbps.
Emerging digital video applications such as HDTV and HD-DVD can easily demand
a staggering 20-40 Mbps using MPEG-2. Such high bit rates translate into huge
storage requirements for HD-DVDs, and a limited number of channels for HDTV.
Thus, a key motivation for developing a new codec is to lower the bit rate while
preserving (or even improving) video quality relative to MPEG-2.This was the
motivation that led to the development of H.264.
As an example of the improvement offered by H.264, Figure 1 shows the same
video frame encoded using MPEG-2 and H.264 at the same bit rate.
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