Descriptions of Video Formats
To get as much information as possible about the behavior of graphics cards while reproducing video, we utilized all the main video formats currently in use for our tests:
- MPEG-4 ASP/DivX/DivX HD
- MPEG-4 AVC/H.264
- WMV HD
MPEG-2/DVD is perhaps the most popular video compression format. Its area of application covers virtually everything, from the ordinary DVD to digital television systems. We mentioned some basic principles of MPEG-2 at the beginning of the article and can add that this format allows both interlaced and progressive encoding. The maximum frame resolution for DVDs is 720x576 pixels at 25fps (PAL) or 720x480 pixels at 30fps (NTSC); the maximum bit-rate is about 10Mbps.
Notwithstanding its rather old age, this format is far from obsolete and its ATSC version supports resolutions of 1280x720 at 60fps progressive and 1920x1080 at 30fps interlaced (referred to as 720p and 1080i, respectively). Today’s MPEG-2 compliant systems support bit-rates up to 80Mbps, which is quite enough to ensure a high-quality picture.
MPEG-4 ASP/DivX is about as popular as MPEG-2 today because this format provides an acceptable image quality at a rather low bit-rate, which makes it suitable for transferring video via the Internet. The format is based on a compression algorithm developed within the framework of the MPEG-4 Part 2 standard and utilizes the Advanced Simple Profile. One of the innovations of MPEG-4 ASP is the global motion compensation technique that allows to effectively compress still scenes with a moving camera. Another important innovation is the quarter-sample motion compensation that improves image sharpness by increasing the motion estimation accuracy from 1/2 to 1/4 pixel. The use of variable-size image blocks (8x8 pixels is the smallest size possible) helps increase the compression degree considerably in comparison with algorithms that work with fixed-size blocks.
The proprietary DivX and the GNU GPL XviD are the most popular of MPEG-4 ASP codecs. Less widespread are such implementations as 3ivx, QuickTime and Nero Digital. DivX’ HD capabilities are formally limited to 1280x720 resolution at 30fps and a bit-rate of 20Mbps. These are the numbers of the official High Def profile. However, we managed to encode a video clip at 1920x1080 resolution and 25fps with progressive scan and a bit-rate of about 9.5Mbps using XviD and then reproduced it normally with DivX 6.2.
MPEG-4 AVC/H.264 is an implementation of the MPEG-4 Part 10 standard which aims at achieving very high compression levels while keeping the image quality at a high level, too. Highly sophisticated, this standard contains a number of innovations it would take a whole new article to describe. Particularly, it supports motion compensation with variable-size blocks (the smallest size is 4x4 pixels), which ensures a very accurate selection of moving areas and, accordingly, a high image quality in complex scenes with lots of details. Motion compensation in H.264 may involve up to 32 reference images, as opposed to earlier formats that used 1 or 2 images only. This improves image quality and reduces the bit-rate, too. The motion estimation accuracy is increased to 1/8 pixel while the minimum size of an image block is now 4x4 pixels. Compression of similar-color areas has also been improved in comparison with MPEG-2; the entropic encoding algorithms Context-Adaptive Variable Length Coding (CAVLC) and Context-Adaptive Binary Arithmetic Coding (CABAC) are employed.
As a result, the developers achieved a very high image quality while keeping the bit-rate rather low. The downside is that H.264 is a tremendously resource-consuming standard. This format was accepted for use on new-generation video discs, HD DVD and Blu-ray. The High profile employed is employed on such discs and allows bit-rates up to 30 and 40Mbps, respectively, at a resolution of 1920x1080 progressive and a frame rate of 30fps.
The VC-1 standard is based on the Windows Media Video 9 codec (WMV3) developed by Microsoft. From a technical point of view, VC-1 belongs to the same class of DCT codecs as MPEG-1, 2 and 4. The codec developer provided an opportunity to compress interlaced video material without converting it first into a progressive format. In fact, VC-1 is a subset of WMV3 because the only thing that’s different is that it has an Advanced profile, also known as WVC1. As opposed to the Simple and Main profiles, it supports compression of interlaced video. Providing a comparable image quality, WVC1 achieves a two- or threefold reduction in video bit-rate in comparison with MPEG-2.
HD-DVD and Blu-ray use the L3 level of the WVC1 profile with a bit-rate up to 40Mbps. Resolutions of 1280x720 at 60fps progressive and 1920x1080 at 24fps progressive or 30fps interlaced are supported. With the L2 level the maximum resolution is 1280x720 pixels at 30fps with progressive scan.
WMV HD is the market name of the Window Media Video 9 codec (WMV3). All video encoded with it uses the High level of the Main WMV3 profile. The maximum video bit-rate can be as high as 20Mbps. The highest resolution is 1920x1080 pixels at 30fps with progressive scan. Although a few movies in WMV HD format were released, as separate discs or as an addition to the DVD version, this format was never meant for wide use at home. It was rather a transitional step from DVD to the true HD formats like VC-1 and H.264. A number of demo clips in this format available for download on the Internet have a non-standard resolution of 1440x1080 pixels and thus cannot be regarded as true HD content, which is oriented at displays with an aspect ratio of 16:9. It is a so-called anamorphic format and the onscreen image will only have about 800 lines after an appropriate correction of the dimensions.