AVC/H.264 encoding
May 1, 2009 12:00 PM, By Pierre Larbier
Operators can realize 50 percent efficiency gains over MPEG-2.
AVC/H.264 for contribution applications
The AVC/H.264 High 4:2:2 Profile enables high maximum bit rates for the video coding layer:
- 40Mb/s for 525i and 576i (Level 3)
- 200Mb/s for 720p and 1080i25/30 (Level 4.1)
- 200Mb/s for 1080p50/60 (Level 4.2)
HD encoding at about 50Mb/s provides quasi-transparency for the vast majority of broadcast content. However, measurements show that up to 150Mb/s (35Mb/s in SD) might be needed to achieve 43dB, which is a common definition of “true” transparency. The High 4:2:2 Profile bit rate capabilities can cover the full range of production and contribution applications, including those that require advanced archiving and mezzanine format support.
MPEG-2 versus AVC/H.264
Figure 9. AVC/H.264 H422P compared to MPEG-2 H422P
Click to enlarge
Today, HD contribution is mostly performed with MPEG-2 using 422P@HL. This profile offers 4:2:2 processing but is limited to 8-bit pixel component bit depth. As illustrated by Figure 9, AVC/H.264 High 4:2:2 Profile offers important savings when compared to MPEG-2, even at the highest bit rates.
These HD examples allow us to draw some conclusions verified by subjective measurements:
- AVC/H.264 offers a bit rate gain of roughly 50 percent, below 15Mb/s.
- Above 30Mb/s, AVC/H.264 produces results comparable in quality to MPEG-2 with a 20Mb/s increase. For instance, MPEG-2 quality at 60Mb/s is achieved by AVC/H.264 at only 40Mb/s or less. At very high bit rates, this rate saving can sometimes be even greater.
- Above the 50Mb/s mark, the quality provided by AVC/H.264 increases linearly with the rate, showing that most of the encoder “effort” is spent coding nonredundant information like noise. Because the human eye is not very sensitive to noise fidelity, most sequences look quasi-transparent above this rate.
Summary
There are significant advantages to using the AVC/H.264 High 4:2:2 Profile. Using 4:2:2 10-bit coding provides the most compelling solution for production and contribution applications.
Encoding with 4:2:2, 10-bit or a combination of the two will always present a gain over High Profile because all subjective and objective measurements exhibit a quality increase for the same bit rate.
In addition, the AVC/H.264 High 4:2:2 Profile offers important rate savings over MPEG-2 even at the highest bit rates, allowing the user to significantly lower transmission costs, keeping the same visual quality, or to greatly improve the video quality using existing transmission links.
This year will be a turning point for contribution applications as encoders and decoders exploit the full potential of High 4:2:2 Profile. Furthermore, relying on a highly standardized bit stream syntax guarantees products from different manufacturers to be easily interoperable.
Pierre Larbier is CTO at ATEME.
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