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05.22.02 Satcasters Battle Over Codecs
XM, Sirius Each Claim the Better Codec, While Ibiquity's PAC Gets Slightly Crunched in the Shuffle


by Leslie Stimson


It was the battle of audio compression algorithms this spring, with each of the satellite radio companies claiming its coding is better.

XM Satellite Radio revealed that its audio codec of choice now is CT-aacPlus, rather than the Perceptual Audio Codec, the earlier choice for which it still has a license agreement with Ibiquity Digital Corp.

By contrast, both Ibiquity and XM's competitor Sirius Satellite Radio use PAC.

Some observers said XM may be worried about sound quality now that listener comparisons to Sirius Satellite Radio are possible.

"If it's perceived that the difference between them sound-quality-wise is significant, that's not good," said one source.

XM claims superior sound quality using CT-aacPlus, a combination of Advanced Audio Coding - the work of AT&T, Dolby, Fraunhofer and Sony - and Spectral Band Replication from Coding Technologies.

Digital Radio Mondiale also has adopted CT-aacPlus as its codec, according to XM. DRM is developing digital technology for shortwave, medium- and long-wave service overseas.

XM's use of CT-aacPlus is the first commercial implementation of this codec.

Third-party testing of CT-aacPlus by the BBC, Deutsche Telekom and Robert Bosch Gmbh found it to be more than 30 percent more efficient than AAC at providing superior sound quality at satellite bit rates, XM said.

XM is enhancing the effect of the codec with Neural Audio, preprocessing software that uses algorithms based on models of the brain's perception of sound. Neural Audio also is the name of the Seattle-based research lab where the product got its start.

Tony Masiello, XM vice president of operations, said Neural Audio helps the encoder make a better decision about which audio is perceived by the human ear and which is not.

"That's how codecs work. They achieve their reduction by deciding which audio elements are not perceived, therefore they're not encoded."

Masiello said, "The neural preprocessor takes audio from the studio and processes it before it goes into the encoder on a channel-by-channel basis."

"We take the studio signal, feed it into the neural engine, then take the same audio and go through the encoder and (satellite) uplink. Then we take the decoded output of a radio, put that into the neural engine as well and the neural engine learns the differences between the original and the audio that has gone through the coding process, and compensates for the differences."

There's a neural engine for each of the 100 channels, he said. A white paper released by XM states that Neural's process analyzes and adjusts the audio signal every 22.7 microseconds.

One broadcast source whose company works with codecs characterized the neural concept as "marketing hype" and said the way XM and Sirius compress sound has little relevance for applications to a typical station. He declined to be named.

XM said its use of CT-aacPlus and the Neural Audio preprocessing algorithm means it doesn't need to use "traditional statistical multiplexing."

Sirius, however, is using statistical multiplexing.


No 'tit for tat'

A spokesman said Sirius didn't want to get into a "tit-for-tat discussion" of the merits of both Sirius and XM's technologies, but said Sirius is pleased with both PAC and statistical multiplexing.

"We have had positive customer feedback. There was a reason that changes were made on the other side."

Mark Kalman, vice president of Sirius' national broadcast studio, said statistical multiplexing uses a variable bitrate encoder, unlike AAC or MP3, for example, which employ fixed bitrates.

Statistical multiplexing, he said, is efficient and doesn't waste bits; it takes only those it needs to encode the audio at that instant. Statistical multiplexing puts the unused bits in a common bit pool for the other channels, giving Sirius the ability to change the bitrate for each channel constantly, and devote more to some and less to others.

Kalman said DirecTV and phone companies traditionally use statistical multiplexing to increase the number of channels they can put in a set amount of bandwidth; but Sirius is not using the technology that way.

"We have a basic quality (bit rate) target; statistical multiplexing improves the quality above that target, and most of the time, we're above that."

Ibiquity has said its IBOC technology will allow stations to use 96 kilobits per second on FM, and that the satellite companies are using less, around 64 kbps. Sirius said it doesn't comment on its bit rates.

"These algorithms are so advanced and so efficient that what people know in the marketplace, like Windows Media, Real Audio and MP3, are not in the same league as these coders," said Kalman.


PAC or no?

The selection of CT-aacPlus means that XM will not use Ibiquity's Perceptual Audio Codec. An XM spokesman said, "PAC is a fine technology. But we felt CT-aacPlus, enhanced by the Neural Audio preprocessing technology, was the best for our platform."

Ibiquity Digital Corp. President/CEO Robert Struble said XM's decision was a disappointment, but would have small impact on Ibiquity's revenue. XM paid Ibiquity an undisclosed amount for licensing PAC.

Sirius is using Version 4 of Ibiquity's PAC and is happy with its performance.

Sources close to the issue said XM had an older version of PAC, Version 3, and that there was a big difference in how the versions performed. "Version 3 had some severe limitations," one source said.

Version 4 is what Ibiquity uses in its in-band, on-channel terrestrial digital radio system, Struble said.

Accounts differed about whether the codec revelation was a surprise to Ibiquity. Certainly it did not come as welcome news, as the company hopes to license PAC for different applications. No sources who spoke with Radio World saw XM's decision as a blow to IBOC, although some felt it was a blow to Ibiquity's business plan to market PAC for other applications.

"You don't want to have folks saying, 'How come these people are saying one of the things you want to sell is no good?" said one source.

But observers contacted for this story agreed that the bottom line for digital radio is not about who has the best codec. It's about the programming, they said, and whether consumers will feel it's a good deal to pay more in the future for analog