Could the Bin Laden video be a fake?

Does the video help the case against Bin Laden?

Washington calls it the "smoking gun" that puts Bin Laden's guilt beyond doubt, but many in the Arab world believe the home video of the al-Qaeda chief is a fake. Could it be?

The sound is muffled, the image at times blurred and juddery, but the dialogue - if genuine - is damning.

Osama Bin Laden, or someone who looks a lot like him, talks on camera about his prior knowledge of the 11 September attacks on the World Trade Center.

But while Washington's allies have been quick to judge the tape as evidence of Bin Laden's guilt, many in the Arab world doubt its authenticity.

There have been:

Claims that the Bin Laden figure is a look-alike

Suspicions that Bin Laden's voice has been dubbed over the original soundtrack

Allegations that Bin Laden was not discussing the WTC attacks, but an assault on US military targets some years ago

Rudimentary film editing - overlaying a new soundtrack, changing the order of the film - is open to anyone with a camcorder or some basic PC software.

It would be much more convincing if he was face-on to the camera

Nikki Nahal

Experts in the West seem in no doubt that the tape is genuine. But if it were fake, could it look convincing?

A lot depends of whether lip movements on film match the words that are being spoken. In this case there is synchronisation between the voice and the lips, says linguist John Gibbons, of the University of Sydney.

But the view of Bin Laden is not that clear, says video picture editor Nikki Nahal.

"The more the person's head is facing away from the camera, the easier it is to con it. A lot of the picture of [Bin Laden] is side on and that does open the possibility that someone else's voice could be put on top," she says.

"It would be much more convincing if he was face-on to the camera, a lot closer and the sound quality was better."

Sound authentic

But even then, it's not that simple, says Dr Peter French, a forensic expert specialising in audio, speech and language.

His lips are synchronised, but that's not conclusive proof

"You've got to make the conversation sound right. It's not just a case of getting the right words in the right order, but you have to think about the rhythm, the intonation and the pitch drift from the start to the end of a sentence," says Dr French.

"It is possible to manipulate the pitch on some advanced computer software, but that's difficult to make convincing."

Could such edits be detected on a third or fourth generation copy of a video tape?

In the days before widespread digital technology, it was easy, says Dr French.

Digital makes it difficult

"You would look for 'switching transients' - surges of electrical energy transmitted from the heads of the tape recorder when it was switched to on or off.

"These could be detected on screen or by putting the tape under a microscope."

But today, using digital equipment, "it's possible to edit or fabricate in ways that completely defy forensic detection," says Dr French.

So, if there was a section where the fakers couldn't get Bin Laden's lips to match the soundtrack they could cut and paste in a bit of film of someone else in the room. And no-one could be sure whether the edit was made by manipulating the video.

But both Dr French and Ms Nahal agree that, unlike in stills photography, it's impossible to convincingly alter the picture by adding new elements or playing with the images.

"I'd be able to spot something like that in the first frame," says Ms Nahal.