Heavenly Music (continued 2/4)

The amount of processing power and data storage needed to handle 100 channels of compressed audio is staggering. XM's headquarters, which contains 82 studios with all the latest equipment, is networked top- to-bottom with fibre-optic cable to ensure that the gargantuan loads of audio data can zip from servers to studios to satellites effortlessly. When audio is not being beamed out live, it is kept on 400 workstations that together store 1.5m songs or 50 terabytes (ie, 50m megabytes) of data—more than four times that held in the Library of Congress. The firm's operations centre puts the Pentagon's early-warning system to shame, with banks of computers scrutinising the output of each channel, while the positions of the satellites and overall reception are tracked on three massive overhead screens. The military analogy is fitting: XM's senior vice-president of engineering and operations is a retired air-force brigadier-general.

Though the studios are impressive, the most critical component for digital radio is in the receiver. At its core is a set of chips whose job it is to reassemble multiple digital signals arriving at varying times from alternate directions. This chipset then decompresses the stream into clean, crisp audio. More than any other factor, the chipset defines the audio experience. A poorly designed chipset will corrupt the sound with pops, clicks or dead air.

Working with Germany's Fraunhofer Institute (the birthplace of MP3) and STMicroelectronics, XM essentially re-engineered an existing chipset. Supplied by SGS-Thomson, the earlier chipset had been used by WorldSpace since 1997, but could not get mobile reception. Among other things, it lacked a memory “buffer” which banks audio in a separate storage area that can be drawn on if line-of-sight reception is blocked.

A closer look at XM's chipset reveals two unique integrated circuits, each assigned to different tasks. The first scrutinises multiple streams of data arriving from satellites and ground repeaters (which help boost weak signals), then decides how best to reassemble them. The second circuit handles decompression and encryption. Relying on subscription-paying customers, XM, Sirius and Global Radio scramble their signals so that they cannot be heard free of charge. The second circuit also buffers four seconds' worth of incoming data, so that tunnels, underpasses or other blind spots do not hinder reception.

By contrast, the Sirius chipset, which is manufactured by Agere Systems (formerly part of Lucent Technologies), encompasses eight integrated circuits. Pundits surmise that getting such a complex device to work properly has been one of the reasons why Sirius had to delay its launch. Eight months behind schedule, Sirius finally got its system in orbit on February 14th.

Sirius has had other difficulties. Instead of “hovering” two geostationary satellites over the equator due south of its American market like XM, Sirius chose to fly three satellites in an orbit that is 29,000 miles above the earth and inclined at an angle to the equator. This was supposed to make things easier. Each satellite covers the continental United States for only 16 hours each day; but with two satellites in sight at any one time, the elliptical orbit places the signal's emitter more directly overhead. In theory, this should improve reception a lot. However, it also requires the Sirius satellites to “hand-off” signals from one to another as they move out of range. Some engineers believe that passing the signal between the satellites, coupled with data arriving from repeater stations on the ground, could create anomalies in reception. To be fair, Sirius argues, with some justification, that its system should eventually provide superior sound. That is because, instead of limiting broadcasts to fixed bit rates, Sirius continually fine-tunes its audio quality—a practice called “statistical multiplexing”.

Global Radio has taken yet another approach. Eschewing the “hard-wired” design that is difficult to upgrade, Global Radio has set out to create a software-driven chipset that is based on a generic digital signal processor (DSP). In doing so, Global Radio can upgrade the software or replace the hardware whenever newer and better technology become available, allowing its chipset to improve steadily with time. In addition, being able to buy DSPs off the shelf has saved the company a fortune in not having to create a custom-built chipset from scratch.

The company's co-founder and chief executive, Paul Heinerscheid, talks of adding a GPS (global positioning system) feature to the receivers. With Global Radio's transmissions and GPS signals being neighbours in the radio spectrum, it ought not to be too difficult to make a dual-function chipset for an all-purpose, music-everywhere, never-get-lost gizmo. Global Radio needs $1.3 billion to achieve its ends. Assuming the money can be raised, it is going to take a further two years before it is broadcasting from space.

Room for all?
Operators of traditional radio stations in America claim not to be unduly worried by the satellite invasion—or at least, not by the prospect of competing with hundreds of specialist channels or even a national broadcaster. They point to the 51 different types of radio formats they beam out to 200m potential listeners. With XM and Sirius targeting only 4% of the market between them, radio operators say there are plenty of advertising revenues to go round. But that assumes the 11% annual growth in advertising revenue that the terrestrial stations enjoyed in the latter half of the 1990s will return. It also assumes that satellite radio goes ahead successfully as planned.

The one thing that gives local broadcasters sleepless nights is the network of repeater stations that the satellite companies are setting up in dozens of cities—to capture the incoming signal from orbit and retransmit it to “dark” areas that are hard to serve by line-of-sight transmission from satellites. The worry is that if satellite radio founders, the 1,500 or so repeaters perched on hill tops and tall buildings could easily be rerigged to broadcast local content. Objectors point to Boston, where XM Radio has set up 66 repeaters, giving it the option of establishing that many independent radio stations should it desire.