Best Practices for Windows Media Encoding

Windows Media Video 9 Advanced Profile
Windows Media Video 9 Advanced Profile is a non-backwards compatible enhancement to WMV 9, focused primarily on much improved support for encoding content as interlaced, with other enhancements helpful for IPTV and HD DVD. For most web video, neither are needed.

WMV 9 AP was introduced with WMP 10 and the Format SDK 9.5 back in 2004. However, there have been a few format tweaks in the v11 implementation, meaning WMP 9 and 10 will require a codec download to play back v11-encoded WMV 9 AP.

Windows Media Video 9 Screen
The WMV 9 Screen codec is designed for efficient compression of screen recordings. It's a special-use codec—very efficient for this task, but not appropriate for encoding normal video. Screen is the most efficient with simpler, flat graphics, so you can get efficient encoding out of the "Windows Classic" theme in XP and Vista. However, richer graphics environments, especially Vista, include a lot of gradients and transparencies, and can often encode more efficiently using standard WMV 9.

For efficiency, WMV Screen needs access to the uncompressed RGB source video of the screen shots, and it doesn't work well when compressing from screen shots that have had any lossy encoding already applied to them. It's typically used in conjunction with lossless screen recording products like TechSmith's Camtasia. On a fast computer, it's possible to use Windows Media Encoder to record, or even broadcast, live screen activity with the Screen codec.

Compared to other screen capture codecs, WMV 9 Screen is unique in providing full support for 2-pass VBR and CBR encoding, making it possible to use it for real-time streaming.

Windows Media Video 9.1 Image
The WMV 9.1 Image codec is designed for video sequences made out of individual still images, including transitions. For this special class of content, Image can be quite a bit more efficient than WMV 9, but it is much less efficient for typical motion video sources.

Audio Codecs
Windows Media Audio 9.2
Windows Media Audio (WMA) 9.2 is a fully backwards-compatible upgrade to the venerable WMA standard. It's compatible back to WMP from the mid-'90s (predating ".WMA" files!), and can be played back virtually anywhere. The 9.2 version includes some minor (but welcome) performance and quality enhancements compared to the previous version.

WMA is the general, safe codec choice for any Windows Media file. It's flexible, offers high quality with sufficient bit rate, and is playable by anything that can play a .WMV file. However, there are some scenarios where Voice or WMA 10 Pro can offer better performance.

Windows Media Audio Voice 9
WMA 9 Voice is designed for low bit rate applications below 32Kbps, where WMA and WMA Pro don't perform as well. Despite its name, it does a credible job with music and other non-voice content. You're not going to dance to WMA Voice at low bit rates, but it can intelligibly compress music interludes and sound effects in otherwise voice-centric content.

WMA Voice 9 is supported in WMP 9 and higher. Note that Voice is not currently supported in all non-Windows players. Most notably, it isn't supported in the current versions of Flip4Mac or the Kinoma player for PalmOS.

WMA Voice replaced the now deprecated ACELP.net audio codec, and should be used instead.

Windows Media Audio 10 Pro
While the new video codec features of WMV are exciting, the biggest technical leap is the new Windows Media Audio 10 Professional codec (WMA Pro 10), which offers up to twice the improvement in compression efficiency compared to WMA 9. The original WMA Pro 9 has been around for several years, and it offers great audio quality and efficiency at 128Kbps and up. WMA Pro 9 supports up to 7.1 channels, up to 24-bit sampling, and up to 96KHz frequencies. But the high minimum bit rate (128Kbps) took it out of the running for most web video tasks. Thus, most streaming video projects have kept using good old WMA 9, which provides good support for lower bit rates.

With WMP 11, we've added a new frequency interpolation mode to WMA Pro, and incremented the name to WMA 10 Pro. With the new mode, we encode a "baseband" version of the audio as normal WMA 9 Pro at half the selected frequency, with additional data that tells how the higher frequencies are added. This gives a stream that's backwards-compatible with the existing decoder, but provides enhanced quality with an updated decoder (bundled with WMP 11).

WMA Pro 10 provides up to two times the efficiency of WMA 9.2, so at 64Kbps it can provide similar quality to WMA at up to 128Kbps. However, if only the old decoder is used, you only get the lower-quality baseband audio.

We feel WMA 10 Pro is the best audio codec included in a major streaming platform today. It handily beats our older codecs, as well as MP3, AAC-LC, and even the new HE AAC-LC, as demonstrated in this study.

WMA 10 Pro is appropriate to use once the majority of your customers are using WMP 11 or another player that supports the full codec. Then at the same bit rate, a WMA 10 Pro stream with the older, non-upgraded decoder can sound a little worse than a WMA 9.2 version. Because it's an enhancement of the older codec, WMA 10 Pro won't trigger a codec download—users only get the new codec if they install WMP 11, Format SDK 11, or if they're running Windows Vista.

The biggest place where WMA 10 Pro is being used today is with Verizon's V CAST mobile media service. We're also working with over a dozen additional vendors to add WMA 10 Pro support to their devices.

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