Specifications for heart-rate monitors used by atheletes

I am curious about the structure of heart-rate monitors used by atheletes (a strap on chest with a transmitter & a receiver in a handheld computer). The ones that Polar or Sigma sports produce.

Does anyone has an idea what are the specs of the receiver/transmitter? Do you think they use their own sytems or use some standard? How is the heart rate monitored by a sensor in a strap?

I hope this link might help
http://www.siliconchip.com.au/cms/A_103787/article.html