As U.S. cable companies make a major push into voice over IP, they are already reaping major rewards for their earlier ventures into cable modems and digital TV service. A new In-Stat report shows North American cable companies have more than doubled their average revenue per subscriber in the past five years.

"These services have been available from cable operators for years now," said In-Stat analyst Mike Paxton, who wrote the report, "The Digital One-Two Punch: Cable Modem and Digital Cable Video Services." "The biggest roadblock is their marketing effort."

While cable companies are attracting a lot of attention now because of their movement into VoIP, the real revenue opportunities lie more in digital cable and cable modem services, Paxton says.

"Once they rolled out VoIP, they got a lot of press," he says. "But voice service is not a major contributor to ARPU. VoIP did bring some new players into the voice game, like Time Warner and Cablevision."

Paxton's study found that revenues from cable modem services "have become increasingly important to cable operators," and are up to $18 billion worldwide in 2004, from $14 billion a year earlier. Cable modem subscribership grew from 31 million at year-end 2003 to 42 million at the end of 2004.

There was a 20% increase in digital cable subscribers in 2004 to 42 million worldwide, 27 million of which are in North America.