Two more cable television companies are experimenting with allowing their respective subscribers to watch cable programs on the Internet for free.

Time Warner Cable Inc. will give about 5,000 subscribers Internet access to some television shows as part of a trial expected to start in a few months. Rival Comcast Corp. announced a similar test in July and now has 24 channels to offer cable subscribers online.

Time Warner says a dozen programmers are taking part in the test, including Time Warner’s own TNT and TBS stations as well as AMC, CBS, General Electric’s Syfy, HBO, BBC America, Discovery Communications, the Smithsonian Channel and the Sundance Channel.

Verizon is partnering with Time Warner on TV Everywhere and initially will offer Time Warner’s Turner Networks, TNT and TBS online to its cable subscribers, with more programmers joining down the road.

This is an inevitable trend. Like the music industry before it, the television industry has failed to bend the Internet to its will and now must create a business model to both leverage the online world and accommodate viewers who increasingly demand the ability to watch fresh video content online.

But as Victor Godinez of the Dallas Morning News explains in an excellent analysis piece, constructing a delivery vehicle that satisfies viewers and generates sufficient revenue won’t be easy.

The companies are experimenting with different systems for how viewers will watch the videos, such as by accessing all content through a centralized portal or by going to the Web site of each channel.

Shawn Strickland, vice president of FiOS product management for Verizon, said a centralized service makes more sense because viewers tend to follow specific shows rather than channels, and that’s what Verizon is pursuing in its negotiations with different networks.

But the sources often don’t want to surrender the advertising opportunity of having viewers go through their proprietary sites. …

There’s also some tension between the free-to-all-comers video sites that some broadcasters already support, such as Hulu.com, and the walled-garden services that Comcast and Verizon are exploring.