By Iain Mackenzie
Newsbeat US reporter, from court in Chicago
R Kelly denies 14 counts of making and possessing child pornography
An FBI forensic video expert has dismissed claims that a tape at the centre of the R Kelly trial could have been faked using special effects.
George Skaluae said he had examined the film and found it to show "real people in a real environment."
He was giving evidence at a courthouse in Chicago, where R Kelly is accused of filming himself having sex with an underage girl.
If convicted, the star faces up to 15 years in prison. He denies the charges.
Mr Skaluae, a forensic video examiner with the US Federal Bureau of Investigation, has previously worked as an adviser to London's Metropolitan Police.
He told the court how he was asked in 2002 to examine a VHS tape which had been retrieved by the Chicago Police Department.
Among his findings, Mr Skaluae said he found the tape to be "a copy, several generations away from the original."
He explained that the presence of multiple hidden "switch points" indicated it had been duplicated at least three times, causing a deterioration in quality.
The FBI investigator was also quizzed on whether the video could have been electronically doctored.
R Kelly's defence team has previously suggested their client may have been digitally "morphed" into the film.
Mr Skaluae said that would be "very difficult and time consuming."
"If it is not done well then it is going to be detectable.
"Sometimes we look at effects in movies and laugh and say 'that doesn't look too good'. Those may have taken months or years."
The female who appears in the video has been identified by several friends and family members during the course of the trial.
Most have placed her at between 13 and 14 years of age.
R Kelly denies 14 counts of making and possessing child pornography.
The trial continues.