Codecs 101 for Windows Media Services

Windows Media Services

May 22, 1998

The quality, frame rate, and frame size of your Windows Media Services content is largely determined by the codec (compressor/decompressor) technology you select when creating content. The fact that Windows Media Services is codec-independent means that you can choose from a variety of codecs and use the ones that best suit your tastes and your content.

For a complete discussion of codecs, and detailed comparisons of the ones that ship with Windows Media Services, see the Content Creation Authoring Guide available for downloading from the Windows Media Services product site .

The symbol denotes a Windows Media Services ASF file or stream. Please install the player to view items marked with this graphic.

Contents:

OK, So What Is a Codec?

Codecs (compressor/decompressors) are drivers that convert from one format type to another. Typically, a codec converts between a compressed format and an uncompressed format. By using codecs for compressing audio and video data into smaller packages, network and multimedia applications provide richer and fuller content and don't consume as much hard disk space or network bandwidth as non-compressed methods.

Codec independence is one of the key benefits of Microsoft Windows Media Services. Most other, similar technologies use a proprietary codec (for example, one that can only be used by that one application; other codecs cannot be applied). Microsoft Windows Media Services offers the content author and end user a well-rounded suite of bundled and stand-alone third-party codecs for voice, music, images, and video, at a variety of bit rates. This gives you more options, more flexible, higher-quality content, and -- since the codecs can be upgraded independent of Windows Media Services -- faster innovation.

The average user doesn't even have to know what a codec is to use Windows Media Services content; they just click on Windows Media Services content and it plays. For the developer of Windows Media Services content, however, there are some things about codecs that should be learned.

Codecs Quick Reference Chart

The audio codec is shown at the top and the video codec is shown at the bottom of each cell.

Refer to the Content Creation Authoring Guide, available from the Windows Media Services download page , for a detailed discussion on the topic. This chart provides baseline, or lowest common denominator information. Windows Media Services can be greatly optimized for your particular content, and you should look at this as a starting point and adjust variables such as which codec you choose, the frame rate, and audio/video quality depending on your initial results and configuration. Also, remember that there are many things to consider when selecting a codec.

Type of Content	Modem Speed (28.8 Kbps)	ISDN Speed (56 Kbps)	Intranet Speeds (110 Kbps)
Audio Only -- Music or Voice	Voxware MetaSound Format: AC16v1 16 Kbps, 16 kHz, Mono	Voxware MetaSound Format: AC48v2 48.1 Kbps, 44 kHz, Mono	Voxware MetaSound Format: ACS96v2 96.2 Kbps, 44 kHz, Stereo
Illustrated Audio -- Audio is Voice	ACELP.net Format: 5 Kbps, 8,000 Hz, Mono Loss-Tolerant JPEG Images	ACELP.net Format: 5 Kbps, 8,000 Hz, Mono Loss-Tolerant JPEG Images	ACELP.net Format: 16 Kbps, 16,000 Hz, Mono Loss-Tolerant JPEG Images
Illustrated Audio -- Audio is Music	Voxware AC6v2 Format: 6 Kbps, 8,000 Hz, Mono Loss-Tolerant JPEG Images	Voxware AC10v1 Format: 10 Kbps, 11,000 Hz, Mono Loss-Tolerant JPEG Images	Voxware AC16v1 Format: 16 Kbps, 16,000 Hz, Mono Loss-Tolerant JPEG Images
Voice and Video Summary of Windows Media Services Codecs	ACELP.net Format: 5 Kbps, 8,000 Hz, Mono MPEG-4v2 Advanced: Image size: QCIF (176 x 144) Frame rate: 5 - 10 fps	ACELP.net Format: 5 Kbps, 8,000 Hz, Mono MPEG-4v2 Advanced: Image size: QCIF (176 x 144) Frame rate: 10 - 15 fps	ACELP.net Format: 16 Kbps, 16,000 Hz, Mono MPEG-4v2 Advanced: Image size: QCIF (176 x 144) Frame rate: 15 fps
Music and Video Summary of Windows Media Services Codecs	Voxware AC8v1 Format: 8 Kbps, 8,000 Hz, Mono MPEG-4v2 Advanced: Image size: QCIF (176 x 144) Frame rate: 5 - 10 fps	Voxware AC10v1 Format: 10 Kbps, 11,000 Hz, Mono MPEG-4v2 Advanced: Image size: QCIF (176 x 144) Frame Rate: 10 - 15 fps	Voxware AC16v1 Format: 16 Kbps, 16,000 Hz, Mono MPEG-4v2 Advanced: Image size: QCIF (176 x 144) Frame Rate: 15 fps

How Content Developers Get Codecs

Windows Media Services uses codecs that conform to the ACM/VCM standards for compression technology. This means that when a system comes with a set of ACM/VCM codecs, Windows Media Services can take advantage of them from any application that supports them. For example, even though Adobe Premiere doesn't explicitly support Windows Media Services, it does allow a content developer to create AVI files that use the same codecs that Windows Media Services uses. Therefore, the content developer can use their favorite Windows Media Services tools to use the codecs that are already installed on the system.

When you install the Windows Media Services Tools, many codecs are automatically installed.

When considering which codecs to use, remember that when you create content with a particular codec or codecs, users will also need to have those codecs installed to be able to play back that content. Therefore, you must know how your customers will get the codecs necessary to play back content.

How to Create Content that Your Users Will Be Able to Play

You have several options:

Use codecs that are only in the core client installation.
Use codecs that are in the full client installation, but not in the core client installation.
Use codecs that aren't in either installation automatically, but that are compatible with Windows Media Services.

Use codecs that are only in the Core Client Install

The most conservative approach is to develop content using codecs that ship in all delivery mechanisms. For Windows Media Services, that means using codecs that ship in the core client installation. These codecs also ship with the full client installation, Internet Explorer 4.0, and the Windows Media Services tools. These codecs are: MPEG Layer-3, MPEG-4, ACELP.net, Voxware MetaSound, and Voxware MetaVoice.

Use codecs that are in the Full Client Install, but not in the Core Client Install

This approach works well if you plan to include a link on the HTML page on which the Windows Media Services content resides, or a page preceding the Windows Media Services content that sends them to http://www.microsoft.com/windows/mediaplayer/download/default.asp to install the full client. It also makes the Windows Media Player work from any browser (unlike using codebase, which works from Internet Explorer clients only).

It's important to note that the end user will not necessarily know which codec they need. All content providers using VDOnet, Voxware, or Duck codecs must provide a link to install the full player or use the IFRAME code we supply below to make sure that they have the right codecs for the content.

Use codecs that aren't in either installation but that are compatible with Windows Media Services

If a codec manufacturer comes out with an ACM/VCM-compliant codec that suits your content well, you can go ahead and use it even if it doesn't ship in any of the existing client installs. With NetShow 2.x you would have had to use codebase in your HTML to auto-install codecs that weren't already installed on the user's system. With Windows Media Services, however, the codecs are automatically installed when the user tries to play content that was created using a codec they do not have installed on their computer.

How Users Can Get the Windows Media Services Client and Its Codecs

The best way your customers can get the Windows Media Player is to install it from the Windows Media Services downloads page at http://www.microsoft.com/windows/mediaplayer/download/Win32IE4x86.asp . These versions also contain the most codecs, so users can play back the widest range of content.

Windows Media Services and Its Loss-Tolerant JPEG Codec

Since version 1.0, Windows Media Services has supported JPEG (Joint Photographic Experts Group) images. NetShow 2.0 contains a loss-tolerant (LT) JPEG image format.

Loss of data during transmission is common on the Internet. The Windows Media Services LT JPEG codec allows JPEG images encoded using Windows Media Services tools to be rendered on the client system even when significant packet loss has occurred during transmission. The Windows Media Services JPEG codecs are non-VCM codecs. The Windows Media Services LT JPEG format is currently available only through the Windows Media Services T.A.G. Author and Windows Media Player.