PIERRE LARBIER /
05.01.2012
Originally featured on BroadcastEngineering.com
AVC-I
There are benefits to using an intra-based codec for broadcast contribution.

Broadcast contribution applications like newsgathering, event broadcasting or content exchange currently benefit from the large availability of high-speed networks. These high-bandwidth links open the way to a higher video quality and distinctive operational requirements such as lower end-to-end delays or the ability to store the content for further edition.

Because a lighter video compression is needed, the complexity of common long-GOP codecs can be avoided, and simpler methods like intra-only compression can be considered. These techniques compress pictures independently, which is highly desirable when low latency and error robustness are of major importance. Several intra-only codecs, like JPEG 2000 or MPEG-2 Intra, are today available, but they might not meet all broadcasters' needs.

AVC-I, which is simply an intra-only version of H.264/AVC compression, offers a significant bit-rate reduction over MPEG-2 Intra, while keeping the same advantages in terms of interoperability. AVC-I was standardized in 2005, but broadcast contribution products supporting it were not launched until 2011. Therefore, it may be seen as a brand new technology, and studies have to be performed to evaluate if they match currently available technologies in operational use cases.

Why intra compression?

Video compression uses spatial and temporal redundancies to reduce the bit rate needed to transmit or store video content. When exploiting temporal redundancies, predicted pixels are found in already decoded adjacent pictures, while spatial prediction is built with pixels found in the same picture. Long-GOP compression makes use of both methods, and intra-only compression is restricted to spatial prediction.

Long-GOP approaches are more efficient than intra-only compression, but they have also distinctive disadvantages:

  • Handling picture dependencies may be complex when seeking in a file. This makes editing a long-GOP file a complex task.

  • Any decoding error might spread from a picture to the following ones and span a full GOP. This means that a single transmission error can affect decoding for several hundred milliseconds of video and, therefore, be very noticeable.

  • Encoding and decoding delay might be increased using long-GOP techniques compared to intra-only because of compression tools complexity.

Another problem inherent to long-GOP compression relates to video quality that varies significantly from picture to picture. For example, Figure 1 depicts the PSNR along the sequence ParkJoy when encoding it in long-GOP and in intra-only. While the quality of the long-GOP pictures is always higher than the one of their intra-only counterparts, it varies considerably. On the other hand, the quality of consecutive intra-only coded pictures is much more stable.

Therefore, intra-only compression might be a better choice than long-GOP when:

  • Enough bandwidth is available on the network;

  • Low end-to-end latency is a decisive requirement;

  • Streams have to be edited; and

  • The application is sensitive to transmission errors.

Several intra-only codecs are currently available to broadcasters to serve the needs of contribution applications:

  • MPEG-2 Intra — This version of MPEG-2 compression is restricted to the use I-frames, removing P-frames and B-frames.

  • JPEG 2000 — This codec is a significantly more efficient successor to JPEG that was standardized in 2000.

  • VC-2 — Also known as Dirac-Pro, this codec has been designed by BBC Research and was standardized by SMPTE in 2009. Like JPEG 2000, it uses wavelet compression.

Older codecs like MPEG-2 Intra benefit from a large base of interoperable equipments but lack coding efficiency. On the other hand, more recent formats like JPEG 2000 are more efficient but are not interoperable. Consequently, there is a need for a codec that could be at the same time efficient and also ensure interoperability between equipment from various vendors.

What is AVC-I?

AVC-I designates a fully compliant variant of the H.264/AVC video codec restricted to the intra toolset. In other words, it is just plain H.264/AVC using only I-frames. But, some form of uniformity is needed in order to ensure interoperability between equipment provided by various vendors. Therefore, ISO/ITU introduced a precise definition in the form of profiles (compression toolsets) in the H.264/AVC standard.

H.264/AVC intra profiles

Provision to using only I-frame coding was introduced in the second edition of the H.264/AVC standard with the inclusion of four specific profiles: High10 Intra profile, High 4:2:2 Intra profile, High 4:4:4 Intra profile and CAVLC 4:4:4 Intra profile. They can be described as simple sets of constraints over profiles dedicated to professional applications. Table 1 gives an overview of the main limitations introduced by these profiles.

H.264/AVC Intra profiles Based on: Summary of the restrictions to the base profile
High10 Intra High 4:2:2 profile (targets mainly Contribution applications with up to 4:2:2 10-bit pixels) All pictures are IDR*
(no P or B pictures)
Limited to 4:2:0 chroma format
(no 4:2:2 chroma format)
High 4:2:2 Intra All pictures are IDR
(no P or B pictures)
High 4:4:4 Intra High 4:4:4 Predictive profile (targets mainly Archiving applications with up to 4:4:4 14-bit pixels) All pictures are IDR
(no P or B pictures)
CAVLC 4:4:4 Intra All pictures are IDR
(no P or B pictures)
Only CAVLC entropy coding

Table 1. This shows the different H.264/AVC Intra profiles. (IDR = Instantaneous Decoder Refresh, CAVLC = Context Adaptive Variable Length Coding)

Because the intra profiles are defined as reduced toolsets of commonly used H.264/AVC profiles, they don't introduce new features, technologies or even stream syntax. Therefore, AVC-I video streams can be used within systems that already support standard H.264/AVC video streams. This enables the usage of file containers like MPEG files or MXF, MPEG-2 TS or RTP, audio codecs like MPEG Audio or Dolby Digital, and many metadatastandards.

AVC-I video quality

Many academic papers have compared the coding efficiency of H.264/AVC in intra-only mode versus other intra codecs. But, those performance comparisons are carried out using objective metrics like PSNR or SSIM. (Structural SIMilarity, when referred to as SSIM Index, is based on measuring three components — luminance similarity, contrast similarity and structural similarity — and combining them into a result value.) It is important to realize that PSNR or SSIM may not reflect actual visual perception. Consequently, studies published to date do not necessarily reflect the visual experience of a given codec in the context of broadcast contribution.

For this reason, we have performed a visual evaluation of various intra codecs intended for broadcast contribution applications. The tests involved a range of products that could encode and decode AVC-I and MPEG-2 Intra up to 150Mb/s, across multiple vendors, and reference software. This investigation was done by expert viewers on a large set of test sequences representative of high-definition broadcast contribution content, mostly interlaced.

The outcome of this evaluation is that two codecs are most suitable for high bit-rate intra uses — AVC-I and JPEG 2000. The detail level appears to be about the same with both codecs on bit rates ranging from 50Mb/s to 150Mb/s. This confirms that the coding efficiency of AVC-I and JPEG 2000 is close. However, coding artifacts are different.

AVC-I and JPEG-2000 artifacts

Below 100Mb/s, a problematic defect was observed similarly on both codecs: Pictures can exhibit an annoying flicker. This issue is caused by a temporal instability in the coding decisions, amplified by noise. It seems to appear below 85Mb/s with JPEG 2000 and below 75Mb/s with AVC-I. And, it worsens as the bit rate decreases. At 50Mb/s and below, the flicker is extremely problematic, and it was felt that the video quality was too low for high-quality broadcast contribution applications, even when the source is downscaled to 1440 × 1080 or 960 × 720.

Around 100Mb/s, both codecs perform well, even on challenging content. Pictures are flicker-free, and coding artifacts are difficult to notice. However, noise or film-grain looks low-pass filtered, and its structure sometimes seems slightly modified. Even so, it wasn't felt this was an important issue.

All those defects are less visible as the bit rate is increased. But, while AVC-I picture quality raises uniformly, some JPEG 2000 products may still exhibit blurriness artifacts, even at 180Mb/s. Using available JPEG 2000 contribution pairs, a bit rate at which compression is visually lossless on all high-definition broadcast content was not found. On the other hand, some encoders appeared visually lossless at 150Mb/s, even when encoding grainy content like movies.

Bit rates in contribution

The subjective analysis of an actual AVC-I implementation on various broadcast contribution content allows us to categorize its usage according to the available transmission bandwidth. On page 48, Table 2 presents findings on 1080i25 and 720p50 high-definition formats.

Bit rate in AVC-I Remarks
≤ 50Mb/s Video quality is too low for high-quality broadcast contribution applications
50Mb/s - 75Mb/s Acceptable on low-noise sources but poor on most sequences
75Mb/s - 90Mb/s Acceptable
90Mb/s - 110Mb/s Good
110Mb/s - 150Mb/s Excellent
≥ 150Mb/s Visually lossless

Table 2. This shows AVC-I bit rate versus quality for 1080i25 to 720p50 content.

Because AVC-I does not make use of temporal redundancies, 30Hz content (1080i30 or 720p60) is more difficult to encode than 25Hz material. Additionally, to achieve the same perceived video quality level, bit rates have to be raised by 20 percent.

Conclusion

The availability of high speed networks for contribution applications enables broadcasters to use intra-only video compression codecs instead of the more traditional long-GOP formats. This allows them to benefit from distinctive advantages like: low encoding and decoding delays; more constant video quality; easy edit ability when the content is stored; and lower sensitivity to transmission errors. However, currently available intra-only video codecs require one to choose between interoperability and coding efficiency.

AVC-I, being just the restriction of standard H.264/AVC to intra-only coding, avoids making difficult compromises. It is more efficient than other available intra-only codecs, but, more importantly, it benefits from the strong standardization efforts that permitted H.264/AVC to replace MPEG-2 in many broadcastapplications.

Finally, a subjective study across a range of products from multiple vendors identified specific coding artifacts that may occur and confirmed the visual superiority of AVC-I versus MPEG-2 and JPEG 2000, when measured at high bit rates.

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Pierre Larbier is CTO for ATEME.



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