About the MSAudio Codec

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Starr Andersen
Microsoft Corporation

Posted April 14, 1999

Contents
Overview
Encoding with MSAudio
Summary

Overview

MSAudio is a new audio codec created by Microsoft Research. The MS Audio codec is designed to handle all types of audio content, from low-bandwidth 8-kilohertz (kHz) speech to 48-kHz stereo music. This codec is very resistant to degradation due to packet loss because it does not use interframe memory. Its tolerance makes it excellent for use with streaming content. In addition, by using an improved encoding algorithm, this codec encodes and decodes much faster. The compression algorithm used creates audio files that need much less disk space for storage than the same content created with other codecs. Content created using the MSAudio codec is easily distributed over the Internet because the files can be downloaded more quickly. Therefore, if you are creating audio files for download, the MSAudio codec is a great choice because it provides near-CD-quality sound at half of the bandwidth required by most codecs.

During tests against comparable codecs, the MSAudio codec provided much clearer sound with greater tonal distinction than the others. The combination of superb clarity and tonal depth produces better sounding music content across all supported bandwidths. The MSAudio codec is a highly scalable codec that provides high-quality mono and stereo audio content over a wide range of bandwidths. This allows you to choose the best combination of bandwidth and sampling rates for your content. Thus, Microsoft has chosen to make it the default audio codec for Microsoft® Windows® Media Tools version 4.0, and to create a software development kit (SDK) specifically for developers who want to use the MSAudio codec to create high-quality audio files.

Encoding with MSAudio

To encode content using the MSAudio codec, you have the following options:

• Download Windows Media Tools 4.0. These tools are optimized for use with the MSAudio codec. They produce Advanced Streaming Format (ASF) streams, or .asf files.


If you use Windows Media Tools to create audio-only .asf files with the MSAudio codec, it is highly recommended that you rename these files with the Microsoft Windows Media Audio (.wma) extension. The .wma extension shows that the file is encoded using the MSAudio codec and is audio-only. Users can download .wma files for later playback rather than streaming them from your Web site. A .wma file can be distributed using Microsoft® Windows® Media Services. When a user requests a .wma file, Windows Media Services automatically creates a special type of announcement file (.wax) that directs the player to the content.
• Download the Microsoft® Windows® Media Audio Software Development Kit (SDK), and create your own encoding tools. The SDK includes a redistribution license for the MSAudio codec so that you can add the codec to your existing encoding tools. Audio files created by using the SDK use the .wma file extension by default.

When encoding content using the MSAudio codec, your input source can use any combination of a sampling rate from 8 kHz to 48 kHz, 8-bit or 16-bit data, and one or two channels (mono or stereo). However, to hear the better sound quality produced with the MSAudio codec, it is highly recommended that 8-bit sound is avoided, and that mono input is not used to produce stereo output.

The MSAudio codec is highly scalable to target network bandwidths. You can produce content for bandwidths ranging between 5 kilobits per second (Kbps) and 160 Kbps. Content encoded at 160 Kbps is virtually indistinguishable from content originating on a compact disc. This extremely high-quality content is called CD transparency. To listen to comparable samples of MSAudio content and other content encoded with other codecs, see the Microsoft® Windows® Media Technologies version 4.0 launch site.

Content created using the MSAudio codec can be packaged using Microsoft® Windows® Media Packager, a feature of Windows Media Rights Manager that assists in application of Digital Rights Management (DRM) to .asf and .wma files. A packaged file is encrypted, and cannot be played without the decryption key provided in the user license. Users obtain licenses by registering their content with the distributor or content creator. Packaging enables you to secure your content before you distribute it on the Internet. If you want to package your .asf or .wma file during content creation, you have three options:

1. Use Windows Media Tools to create the file, and then use Windows Media Packager to package the file. This gives you both a packaged and an unpackaged version of the content. Windows Media Tools can use live content, stored .wav, .mp3, or .avi files, Microsoft PowerPoint® presentations, or a combination of JPEG images with a .wav file to create the .asf file. Once the .asf file is created, Windows Media Packager adds the packaging data to the file.
2. Use Windows Media Packager to directly encode and create a packaged file. Windows Media Packager can encode a source .wav or .mp3 file into an .asf file. By default, it uses the MSAudio codec to create an audio-only .asf file that is played at 64 Kbps with a sampling rate of 44 kHz.
3. Use the attributes detailed in the Windows Media Audio SDK to implement DRM in your custom application. DRM is fully supported in the Windows Media Audio SDK. Windows Media Packager is not required.

Windows Media Audio files can also support ID3 metadata. If you encode a source .mp3 file using the MSAudio codec, any ID3 properties are included in your .wma file. Both Windows Media Packager and Windows Media Tools can import the following properties from ID3 version 1: song title, artist, album, year, album track, and genre. If you are using ID3 version 2, you can use the Windows Media Audio SDK to create custom attributes to support any additional file property information.

Summary

The MSAudio codec is a new, high-quality audio codec optimized for both voice and music. MSAudio provides the highest-quality sound available using low- to high-range bit rates. In addition to superior sound quality, MSAudio produces smaller files and encodes faster than earlier audio codecs. Files encoded with the MSAudio codec can be downloaded in half the time of comparable files, and can be stored in half the space. MSAudio comes with a variety of mono and stereo audio formats to choose from for different network bandwidths. MSAudio content can be created by using Windows Media Tools, Windows Media Packager, and a custom application created using the Windows Media Audio SDK. A new file format has been created especially for audio-only content encoded with the MSAudio codec, .wma. In addition, Windows Media Audio files can be packaged and licensed to deter content piracy.

For more information about Window Media Technologies 4.0, the MSAudio codec, and Digital Rights Management, see the Windows Media Technologies 4.0 launch site. Also, see the MSDN Online Web Workshop article, "Packaging Your Content Using Windows Media Rights Manager."