OTHER TECH

Not All Bits Are Created Equal

Counting mutes was interesting. But there was a more pressing issue at hand - the significant difference in sound quality between the two systems. Each of us was shocked when we switched vehicles on the morning of the second day.

Since the satellite signal can't penetrate solid rock, tunnels provide the toughest test of receivability.

Within a block, we were on the walkie-talkies asking, "Can you believe this?" We'd expected the systems to sound similar, but Sirius's sound quality was clearly inferior to XM's. True, the playback systems and the car cabin's acoustics can have a big influence on sound. But after listening to FM and CDs to get a better sense of the sound characteristics of each car system, we confidently concluded that what we were hearing was part of the encoded signal itself. All of the Sirius channels we listened to exhibited the kind of artifacts compressed-data audio codecs generate when low bit rates are used. Most noticeable was a mid- to high-frequency swirling effect that was most evident on slow to moderately fast music with a clear high end - acoustic guitars, cymbals, and strings were the most problematic. For example, on The Calling's "Wherever You Will Go" (on US-1, a pop channel), the swirling artifacts caused the sound of the cymbals and acoustic guitars to blur together. Similarly, the rapid notes played by the strings in the finale of Bartok's Concerto for Orchestra (on the Symphony Hall channel) were rendered indistinguishable. Phasing effects caused the solo part on a horn concerto (composer unknown because the Kenwood head unit doesn't display composer information on the classical channels) to become disturbingly distorted. This phasing was also apparent on pianos. Sustained notes in particular had a fluttering, unstable quality. Musical notes often seemed to lose their attack. This was especially evident in Sting's "If I Ever Lose My Faith in You" (the Bridge channel), where the sharp snap of the percussion and bass all but disappeared. We had similar problems with the speech channels, which Sirius says are coded at a lower bit rate than the music channels. And these problems were audible both while we were stopped and while driving. We could even hear them when we were standing outside of the vehicle in a parking lot with the system playing at normal volume and the windows rolled down. The sound quality on XM's music channels, on the other hand, was quite good - in fact, surprisingly good considering the low bit rates that both XM and Sirius use to transmit each channel. Response at the top of the audio range was limited, as it was with Sirius. But the "swirling" effects were much more subdued here than they were with Sirius, making them much easier on the ears. For instance, "Eighteen" by the Pat Metheny Group sounded sharp and vivid on XM's Watercolors jazz channel. Given the bit rates involved, it wouldn't be fair to expect CD-quality sound, but XM is a big improvement over what you're used to hearing on FM radio.