OLN is highlighting data from Tour de France riders' heart rate monitors again in its 2006 broadcasts. They provided information exclusively from Polar Heart Rate Monitors in 2005, but this year they are showing the heart rate data from both Polar and SRM.
SRM is known more for its power meter technology. Polar has both excellent heart rate monitor technology and a unique power meter technology. The third power meter manufacturer is CycleOps, manufacturers of the PowerTap.
BikeTechReview.com has an excellent comparison of the three competing power meter technologies. SRM uses strain gauges in the crank to measure torque and pedaling cadence, and from that to calculate power. CycleOps uses strain gagues in the rear wheel hub to measure torque and rear hub angular velocity to calculate power. Polar uses a sensor on the chainstay to measure chain tension and speed to compute power.
Each technology has its advantages. SRM probably has the most accurate technology, since they measure the force applied to the crank. CycleOps' PowerTap often reports about 2% less power than SRM in the same conditions, in part because of the power lost in the translation from the crank to turning the rear wheel hub. Polar's method may not be quite as accurate as SRM's or CycleOps, but it's far simpler and cheaper because Polar doesn't require a custom crank or rear wheel hub.
The cost of these power meter systems vary widely, with the high end systems only being practical for professional use. An SRM power system for the Shimano Dura-Ace 10 drivetrain costs $3,400. I saw Floyd Landis' bike at last year's Tour de Georgia when it was equipped with a CycleOps PowerTap. A PowerTap SL adds between $1200 and $1300 to the cost of a bike. Polar's Power Option for their S-Series heart rate monitors like the Polar S-725 is the least expensive at about $350.
If you're wondering how OLN is able to pick up the data from the Polar and SRM bike computers, the manufacturers provide auxillary transmitters that are attached to the riders bikes. This increases the transmitter's range to well beyond the standard two meter (6 foot) distance.