Overview of H.264

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1. Introduction
Broadcast television and home entertainment have been revolutionised by the advent of digital TV and
DVD-video. These applications and many more were made possible by the standardisation of video
compression technology. The next standard in the MPEG series, MPEG4, is enabling a new generation
of internet-based video applications whilst the ITU-T H.263 standard for video compression is now
widely used in videoconferencing systems.
MPEG4 (Visual) and H.263 are standards that are based on video compression (“video coding”)
technology from circa. 1995. The groups responsible for these standards, the Motion Picture Experts
Group and the Video Coding Experts Group (MPEG and VCEG) are in the final stages of developing
a new standard that promises to significantly outperform MPEG4 and H.263, providing better
compression of video images together with a range of features supporting high-quality, low-bitrate
streaming video. The history of the new standard, “Advanced Video Coding” (AVC), goes back at
least 7 years.
After finalising the original H.263 standard for videotelephony in 1995, the ITU-T Video Coding
Experts Group (VCEG) started work on two further development areas: a “short-term” effort to add
extra features to H.263 (resulting in Version 2 of the standard) and a “long-term” effort to develop a
new standard for low bitrate visual communications. The long-term effort led to the draft “H.26L”
standard, offering significantly better video compression efficiency than previous ITU-T standards. In
2001, the ISO Motion Picture Experts Group (MPEG) recognised the potential benefits of H.26L and
the Joint Video Team (JVT) was formed, including experts from MPEG and VCEG. JVT’s main task
is to develop the draft H.26L “model” into a full International Standard. In fact, the outcome will be
two identical) standards: ISO MPEG4 Part 10 of MPEG4 and ITU-T H.264. The “official” title of the
new standard is Advanced Video Coding (AVC); however, it is widely known by its old working title,
H.26L and by its ITU document number, H.264 [1].
2. H.264 CODEC
In common with earlier standards (such as MPEG1, MPEG2 and MPEG4), the H.264 draft standard
does not explicitly define a CODEC (enCOder / DECoder pair). Rather, the standard defines the
syntax of an encoded video bitstream together with the method of decoding this bitstream. In practice,
however, a compliant encoder and decoder are likely to include the functional elements shown in
Figure 2-1 and Figure 2-2. Whilst the functions shown in these Figures are likely to be necessary for
compliance, there is scope for considerable variation in the structure of the CODEC. The basic
functional elements (prediction, transform, quantization, entropy encoding) are little different from
previous standards (MPEG1, MPEG2, MPEG4, H.261, H.263); the important changes in H.264 occur
in the details of each functional element.
The Encoder (Figure 2-1) includes two dataflow paths, a “forward” path (left to right, shown in blue)
and a “reconstruction” path (right to left, shown in magenta). The dataflow path in the Decoder
(Figure 2-2) is shown from right to left to illustrate the similarities between Encoder and Decoder.


Fn
(current)
F'n-1
(reference)
MC
Intra
prediction
ME
Filter
Inter
Intra
T
T-1
Q
Q-1
Reorder
Entropy
encode NAL
Dn
P
uF'n
+
-
+
+
X
F'n
(reconstructed)
D'n
(1 or 2 previously
encoded frames) Choose
Intra
prediction
Figure 2-1 AVC Encoder
F'n
(reconstructed)
MC
Filter
Inter
Intra
T-1 Q-1 Reorder
Entropy
decode
NAL
P
+
+
D' X n uF'n
F'n-1
(reference)
Intra
prediction
(1 or 2 previously
encoded frames)
Figure 2-2 AVC Decoder
2.1 Encoder (forward path)
An input frame Fn is presented for encoding. The frame is processed in units of a macroblock
(corresponding to 16x16 pixels in the original image). Each macroblock is encoded in intra or inter
mode. In either case, a prediction macroblock P is formed based on a reconstructed frame. In Intra
mode, P is formed from samples in the current frame n that have previously encoded, decoded and
reconstructed (uF’n in the Figures; note that the unfiltered samples are used to form P). In Inter mode,
P is formed by motion-compensated prediction from one or more reference frame(s). In the Figures,
the reference frame is shown as the previous encoded frame F’n-1 ; however, the predicton for each
macroblock may be formed from one or two past or future frames (in time order) that have already
been encoded and reconstructed.
The prediction P is subtracted from the current macroblock to produce a residual or difference
macroblock Dn. This is transformed (using a block transform) and quantized to give X, a set of
quantized transform coefficients. These coefficients are re-ordered and entropy encoded. The entropyencoded
coefficients, together with side information required to decode the macroblock (such as the
macroblock prediction mode, quantizer step size, motion vector information describing how the
macroblock was motion-compensated, etc) form the compressed bitstream. This is passed to a
Network Abstraction Layer (NAL) for transmission or storage.
2.2 Encoder (reconstruction path)