By Darren Waters Technology editor, BBC News website

Mr O'Donnell was in the UK to address a conference in Brighton

For a man whose job it is to write music for one of the world's most successful video game series, Marty O'Donnell is pretty relaxed.

As audio director at Bungie, he is in charge of music and sound design for the third Halo game due in 2007. He has taken some time away from his studio at Bungie's office on the Microsoft campus in Seattle to speak at a game developers' conference in Brighton, in the UK.

The first two Halo titles have sold in excess of 12 million copies and the franchise is the closest thing to a Hollywood blockbuster that exists in the video game industry.

The new game is also a key weapon in Microsoft's armoury as it battles to become the dominant player in the $25bn gaming market.

So is Mr O'Donnell feeling any pressure?

"I do feel more pressure. But there's only so much pressure you can accept before it becomes funny.

"At the end of the day this is still just a game, just entertainment. We are not curing any diseases.

"We're just making a nice game we hope people like. I do feel that in terms of implementing a music score in a game I am learning with each title and hopefully advance the bar for each project."

Epic scope

The original Halo was released in 2001 as an exclusive title for Microsoft's Xbox games console and instantly became a hit thanks to its immersive game play and cinematic feel.

The music for the game had an epic, inspiring tone and the main theme has become iconic; an instantly recognisable tune that is to Halo what Monty Norman's theme is to the James Bond films.

The music and audio of the Halo games are widely acclaimed

The theme was written in 1999 during a weekend as Bungie prepared to unveil the game, then a planned title for the Apple Mac, at the MacWorld conference in New York.

"The nicest thing for me is coming from the commercial world I understood the importance of having a theme that was going to stick.

"In 1999 when we did the first piece of music for Halo for MacWorld, I wanted to make a big, impactful statement.

"I wanted to make sure I had a couple of nice hooky kind of themes that I could refer back to. But I did not know if they were going to work.

"But at that point there was no Master Chief yet, no star of Halo. It was sort of a concept of a place.

Sound makes it real, music makes you feel Marty O'Donnell

"I think the words were epic, mysterious, ancient, something like that. I put my own interpretation on it; epic meant big orchestra, mysterious would be some of the effects, some of the middle eastern chant and ancient was Gregorian monk type feel.

"To me now it is a matter of ' can I do it without boring people'? Can I do it without people saying 'oh that again'?

"But it is musical material I know I can dip into. I know there will be a certain part of the composition process that I will have to come up with new things. But I won't have to come up with iconic themes. I can save that for whatever the next big project is."

Final chapter

He has described the third Halo film as Bungie's Return of the King, a finale to a great trilogy.

"We all want Halo to be the great ending to an epic trilogy. We look at what Peter Jackson did with the Lord of the Rings films - which each film bettering the previous one and that is what we are aiming for.

"The tone of the music, believe it or not, has not been set yet.

"I don't approach things from very far out. I look at what is the task at hand and I write music for that.

"I look at what is in the game and score the drama of the game."

Halo 3 is scheduled for release in 2007 on the Xbox 360

Mr O'Donnell says he will not start scoring the music to the third game "until the last minute".

"I am not going to kill myself just writing a whole bunch of music for nothing.

"I'm a composer. I'll compose for what you make and I'll be part of talking about what's a good scene or level.

"It drives everyone crazy but it worked for me in the past and it works for the game in the best way. Writing music before the end just doesn't work for me."

He adds: "I'm audio director so I get to see every stage of the game anyway. I'm following, writing and thinking about music but I don't throw music onto the game until pretty much the last minute."

Halo 3 will launch on the Xbox 360 but Mr O'Donnell says he is not being distracted by the new technology.

"For too long we have let the technology side drive creativity now we have to let creativity drive technology," he says.

His background is in music for TV and film but he has found a platform in games which lets him compose music for a new medium to a new audience.

"When I talk about interactive entertainment I am interested in dramatic story telling.

"For me as musician, artist and audio guy I want to work on the kinds of interactive entertainment that are driven by storyline.

"Games are not films but music is the driver of emotion in films and probably will be in games, especially dramatic games.

"Sound makes it real, music makes you feel."

There is still no fixed release date for Halo 3 - Mr O'Donnell says the game has to pass the "Bungie test" first.

"It's about us playing the game and enjoying it. If it passes the Bungie test, if we like it, we think our fans and the world will like it.

"That's our egos speaking but it has worked so far."