CHICAGO — An FBI forensic expert testified Thursday that the male and female in a sex tape at the heart of the R. Kelly child pornography trial weren't computer-generated or altered.

In more than an hour of highly technical analysis, George Skaluba told jurors that the video appears to depict "real people in a real environment."

The defense maintains the R&B star is not the man in the video and has repeatedly suggested that his likeness could have been computer-generated onto the 27-minute tape.

Kelly, 41, is charged with child pornography for allegedly videotaping himself having sex with a girl who prosecutors say was as young as 13. He has pleaded not guilty and faces up to 15 years in prison if convicted.

Skaluba also told jurors that even if someone wanted to create digital images, it would be costly and could take years.

"It would be very, very time consuming and very hard to do," Skaluba said.

Under cross-examination, Skaluba conceded he wasn't in a position to say if the male on the tape was actually Kelly or a look-alike.

Kelly's attorneys have also noted that the singer has a mole on his back and they claim the man on the tape does not. The alleged victim, now 23, also has denied she is on the graphic video.

The tape obtained by prosecutors is not the original, but a copy several times removed from the original, Skaluba said.

Earlier on Thursday, prosecutors said they needed more time to interview a potential defense witness before he can testify. The man told Kelly's attorneys he had information that could impeach the testimony of a prosecution witness.