EXFO EXpert IPTV Test Tools (FTB-200v2) - Manual (Page 55)

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A.1.2.2 Performance of Video Coders
There are many standardized video coding algorithms, such as ITU‐T H.261, H.263,
H.264, ISO/IEC MPEG‐1, MPEG‐2 and MPEG‐4, AVS, VC‐1, etc. These standards do not explicitly
define codecs; they only define the syntax of an encoded video bitstream together with the
methods of decoding the bitstream. The consequence is that there might be significant quality
differences between codecs conforming to the same standard [16]. The Windows Media 9
(VC‐1) of Microsoft and RealVideo of RealNetworks are widely used proprietary video coding
algorithms for video streaming applications.
As a rule of thumb, ITU‐T H.263 reduces the coding bitrate by about 50% against H.261 for
comparable video quality. ITU‐T H.264/AVC significantly outperforms all other listed
standards. Average bit‐savings of more than 60% relative to MPEG‐2 are reported. H.264/AVC
Main Profile provides more than 1/3 bitrate saving relative to its competitors, MPEG‐4
Advanced Simple Profile (ASP) and H.263 Conversational High Compression (CHC) Profiles [25].
Reports show that WMV 9 (VC‐1) achieved similar quality to MPEG‐2 and MPEG‐4 with only
1/3
and 1/2 of the bitrate, respectively [21]. A paper by Bennet and Bock [13] concludes that
comparing H.264/AVC and VC‐1, there is very little performance difference between them.
A.1.2.3 Effective Coding Bitrate
The bitrate used in estimating the mean squared error as referenced in the beginning
of section 2.2.1 can be adjusted to an effective bitrate based on performance of a video codec
described above
. The effective bitrate also needs to be adjusted based on the frame
resolution and frame rate of the video signal. Videoconferencing applications generally
support low to medium bitrates and frame resolution. The most commonly used resolutions
are QCIF (176x144) and CIF (352x288) at 10 to 30 frames per second. Entertainment‐quality
IPTV applications generally support video encoded with SD resolutions of 720x486 (30 Hz) or
720x576 (25 Hz), or HD resolutions of 1280x720 (720p) or 1920x1080 (1080i) at an average
bitrate of 3 Mbits/sec or higher.
Typically, the number of bits spent on coding a P‐frame is about 20% of the number required
for an I‐frame, whereas a B‐frame takes about 5% of the bitrate consumption of the I‐frame.
This implies that group of pictures (GOP) structure has a big impact on quality of encoded