VC-1

VC-1 is the informal name of the SMPTE standard 421M. On April 3, 2006, SMPTE announced the formal release of the VC-1 standard as SMPTE 421M. Its most popular implementation is Windows Media Video 9.

It is an evolution of the conventional DCT-based video codec design also found in H.261, H.263, MPEG-1, MPEG-2, and MPEG-4. It is widely characterized as an alternative to the latest ITU-T and MPEG video codec standard known as H.264/MPEG-4 AVC. VC-1 contains coding tools for interlaced video sequences as well as progressive encoding. The main goal of VC-1 development and standardization is to support the compression of interlaced content without first converting it to progressive, making it more attractive to broadcast and video industry professionals.

Although widely considered to be Microsoft's product, there are actually 15 other companies in the VC-1 patent pool (as of April 2006). As a SMPTE standard, VC-1 is open to implementation by 3rd parties which in turn have to pay licensing fees to the MPEG-LA licensing body.

Both HD DVD and Blu-ray Disc have adopted VC-1 as a mandatory codec, meaning all video playback devices will be capable of decoding and playing video-content compressed using VC-1.

The FFmpeg project is working on a free implementation of this codec.

Microsoft has designated VC-1 as the Xbox 360 video game console's official video codec, and game developers may use VC-1 for full motion video included with games.

Codec implementations

The VC-1 codec specification has so far been implemented only by Microsoft in the form of 3 codecs, each identified with a unique FourCC code.

WMV3

WMV3, better known as Windows Media Video 9 codec, served as the basis for development of the VC-1 codec specification. As a result, Simple and Main Profiles of VC-1 remained completely faithful to the existing WMV3 implementation, making WMV3 bitstreams fully VC-1 compliant from the very beginning. The WMV3 codec was designed to primarily support progressive encoding for computer displays. An interlaced encoding mode was implemented, but quickly became deprecated when Microsoft started implementing WMV Advanced Profile. Whereas WMV3 progressive encoding was implemented in the YUV 4:2:0 color space, the deprecated interlaced mode was implemented in the less common YUV 4:1:1 color space.

The Windows Media Video 9 (WMV3) codec implements the Simple and Main modes of the VC-1 codec standard, providing high-quality video for streaming and downloading. "It provides support for a wide range of bit rates, from high-definition content at one-half to one-third the bit rate of MPEG-2, to low-bit-rate Internet video delivered over a dial-up modem. This codec also supports professional-quality downloadable video with two-pass and variable bit rate (VBR) encoding. Windows Media Video 9 is already supported by a wide variety of players and devices."

A number of high definition movies and videos have been released commercially in a format dubbed WMV HD. These titles are encoded with WMV3 Main Profile @ High Level (MP@HL).

WMVA

WMVA was the original implementation of WMV Advanced Profile prior to the acceptance of the VC-1 draft by SMPTE. The codec was distributed with Windows Media Player 10 and Windows Media Format SDK 9.5 install packages. There are slight bitstream differences between WMVA and WVC1, so consequently WMVA is handled by a different DirectShow decoder than WVC1. Some 3rd party hardware and software decoders only decode WMVA based content. As of 2006, WMVA is considered a deprecated codec because it is not fully VC-1 compliant.

WVC1

WVC1, also known as Windows Media Video 9 Advanced Profile, implements the Advanced Profile of the VC-1 codec standard. It offers support for interlaced content and is transport independent. With the previous version of the Windows Media Video 9 Series codec, users could deliver progressive content at data rates as low as one-third that of the MPEG-2 codec and still get the same quality as MPEG-2. The Windows Media Video 9 Advanced Profile codec also offers this same improvement in encoding efficiency with interlaced contents.

Profiles

	Simple	Main	Advanced
Baseline intra frame compression
Variable-sized transform
16-bit transform
Overlapped transform
4 motion vector per macroblock
¼ pixel luminance motion compensation
¼ pixel chrominance motion compensation
Start codes
Extended motion vectors
Loop filter
Dynamic resolution change
Adaptive macroblock quantisation
B frames
Intensity compensation
Range adjustment
Field and frame coding modes
GOP Layer
Display metadata
	Simple	Main	Advanced

Bit rates & resolutions

Profile	Level	Maximum Bit Rate	Resolutions by Framerate
Simple	Low	96 kbit/s	176 x 144 / 15 (QCIF)
Simple	Medium	384 kbit/s	240 x 176 / 30 352 x 288 / 15 (CIF)
Main	Low	2 Mbit/s	320 x 240 / 24 (QVGA)
	Medium	10 Mbit/s	720 x 480 / 30 (480p) 720 x 576 / 25 (576p)
	High	20 Mbit/s	1920 x 1080 / 30 (1080p)
Advanced	L0	2 Mbit/s	352 x 288 / 30 (CIF)
	L1	10 Mbit/s	720 x 480 / 30 (NTSC-SD) 720 x 576 / 25 (PAL-SD)
	L2	20 Mbit/s	720 x 480 / 60 (480p) 1280 x 720 / 30 (720p)
	L3	45 Mbit/s	1920 x 1080 / 24 (1080p) 1920 x 1080 / 30 (1080i) 1280 x 720 / 60 (720p)
	L4	135 Mbit/s	1920 x 1080 / 60 (1080p) 2048 x 1536 / 24

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Video codecs