AVC/H.264 encoding
May 1, 2009 12:00 PM, By Pierre Larbier
Operators can realize 50 percent efficiency gains over MPEG-2.
Using 4:2:2 10-bit encoding has become the de facto standard for professional video because it is captured and transmitted over SDI, allowing the entire production chain to use at least a 10-bit signal. In broadcast contribution, however, encoders and decoders are still limited to 8-bit signals. As a result, picture information can get lost, and quality can suffer when transmitting video.
This article demonstrates the advantages of processing video in its native SDI format using AVC/H.264 4:2:2 10-bit encoding. AVC/H.264 has historically been used at low bit rates for distribution applications. The 50 percent efficiency gains over MPEG-2 allow increased channel density, wider distance reach and reduced transmission costs. However, there is a growing need for production and contribution applications with higher standards of video quality.
Introduction
HD contribution applications are varied but share some common characteristics, including bit rates from 20Mb/s to 60Mb/s and above; low to moderate end-to-end latency, typically less than one second down to 250ms; and the need to decode and re-encode video several times before reaching the viewer.
Figure 1. Architecture with distribution encoder/decoder
Click to enlarge
For more than 10 years, production and contribution applications have used the MPEG-2 4:2:2 profile, but early design stages showed the potential for AVC/H.264 to be an improvement over MPEG-2. All MPEG-2 features were included, with the notable exception of an easy transrating process. Still, the majority of today's AVC/H.264 encoders and decoders are limited to relatively low bit rates and lack specific tools mandated by production and contribution applications.
As illustrated in Figure 1, most current AVC/H.264 broadcast contribution systems are based on existing distribution encoders and decoders.
Handling only High Profile requires downscaling to 4:2:0 8-bit before encoding, and upscaling back to 4:2:2 10-bit after decoding. This approach is also limited to less than 30Mb/s, which impedes the highest video quality applications in HD.
As technology matures, many products are now implementing the High 4:2:2 Profile, which is a superset of High Profile with two new tools that avoid the downscale/upscale stages shown in Figure 1, and feature 4:2:2 processing and up to 10-bit pixel bit depth handling.
To show actual data versus theoretical upper bounds that could never be obtained in real time, three products were used to gather results: a contribution encoder for AVC/H.264 4:2:2 8-bit measurements, a real-time HD encoder for AVC/H.264 4:2:2 10-bit measurements and a software file encoder for MPEG-2 measurements.
Using PSNR metrics to evaluate quality
Peak signal-to-noise ratio (PSNR) measures the difference between the source and the decoded pictures of a video sequence.
A known problem with PSNR is a lack of correlation with the human visual impression. For example, the same PSNR of 30dB for one sequence could look very good, while another could look very poor. Thus, two PSNR measurements on two different sequences are almost meaningless when measuring video quality.
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