“If anyone's been listening to my commentary the past year then they know I'm in favor of using replay. I think it will make tennis more interesting.” -John McEnroe

I dropped out of tennis for a while. My wife and I had our first child; I started focusing on my career; moved to a new city. When I returned to the sport I quickly realized it had jumped into the 21st Century. I'm referring to this new technology called Hawk-Eye or Player Challenge. I was watching the '07 Rogers Cup when I first witnessed this impressive device for line calling and instant replay.

It does make tennis more interesting. But how is it changing the game?

According to the hawk-eye rules: Each player receives three challenges per set. When the player challenges a point and loses, the player loses that point and one challenge. If the player wins a challenge, he wins the point and the number of challenges remains as it was. If a player has no challenges remaining and they go on a tie break, he gets one challenge.

It's a rather fair system; a player can stop play and challenge a call. And if he is wrong he loses that point anyway. What it does do is break up the pacing of the game -- a bonus for Nadal who prefers this tactic to shake his opponents. And if the replay is in the challenger's favor, then its a huge psychological boost. In this way Hawk-Eye has become another tool in a player's gear bag; another aspect of player strategy adding to the psychological aspect of the game.

Personally, I think it's some of the most interesting ruling technology to come out of professional sports. But how accurate is it?

Using multiple high-frame-rate cameras, Hawk-Eye finds the exact 3D position of the ball at a series of time intervals leading into a bounce. From these 3D positions, a trajectory of the ball is calculated. Hawk-Eye uses this trajectory to project where the ball will first make contact with the ground and then how much the ball will compress and skid once it has contacted the ground. From this, the “bounce mark” is determined up to 2-3mm.

2007 Wimbeldon: In the third game of the fourth set, with Roger Federer a break down and serving at 30-30, he did not play at a ball near the baseline. He thought that it was out, the line judge thought it was out, the umpire thought it was out and a BBC freeze frame seemed to confirm this, too.

However, after a challenge from Rafael Nadal, Hawk-Eye showed the shot in. Federer lost his temper, declaring that the machine was “killing him” and demanded that it be turned off.

One of the biggest problems with this situation was the skidding of the ball following first contact, and the use of television-quality replay affecting the decision of the audience.

Television replays look deceptive because the cameras are at the wrong angle looking down at the ball. The ball also has a lot of motion blur and the cameras do not work at a sufficiently high frame rate to capture the crucial part of when the ball first touches the ground.

What television cameras see as the ball's contact area is actually up to 10cm further than first contact when accounting for ball skid. These frame rates are more accurate when viewing at the 100 fps of Hawk-Eye cameras (as opposed to 150 fps of tv cameras).

If Hawk-Eye technology was around in the 70s would we have still been witness to the fabulous tirades of McEnroe?

Probably.