Best Practices for Windows Media Encoding

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:

http://download.microsoft.com/download/4/b/0/4b0596b4-3601-4b78-9ed0-b6f5840eae22/WMAPro-vs-HE_AAC-v2%20final.pdf.

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.

Windows Media Audio 9.2 Lossless
The WMA 9.2 Lossless codec is, as the name implies, a lossless audio codec. A lossless audio codec’s output is bit-for-bit identical to its input. Essentially, it’s a more efficient alternative to PCM (uncompressed) encoding, and functionally equivalent to Zipping up a .WAV file.

The flip side of lossless encoding is that there’s no bit rate control possible—each second of audio takes as many or as few bits as it needs. Hence the codec is only available in Quality VBR mode. Perfect silence takes up very little bandwidth, while white noise takes up as much as uncompressed. Typical savings are around 2:1 for music and 4:1 for TV/movie soundtracks.