Lady Gaga's friend and collaborator Zedd claims her voice is not always perfect.

The Born This Way singer has been working on her fourth album ARTPOP for over a year. Although it is due to hit stores at some point in 2013, Zedd has let slip that the star still has a lot of work to do.

The dance music star admits he was initially nervous about being candid with Gaga.

"After the first time we worked in the studio, I just went back and kind of thought about it and I decided that I would just be honest," Zedd told MTV.

"It took a day or two to kind of getting used to that, but now it's totally fine. I can say, 'That was not good. You can do better.' And when it's good I'll say, 'It's great.'

"I try to treat everyone the exact same way, because I think that will lead to the best results.

"But it was definitely hard to get used to sitting with someone who's probably the biggest singer on the planet and say, 'You know, I think you could do better.' I would always say that, and that's nothing mean. But then you say that to someone that sold millions of records and you sold a couple thousand and, you know, I had to get used to it."

As for when the album will be ready, Zedd admitted the tracks they have been working on are "still all really rough".

The Bad Romance singer is still completing her Born This Way Ball world tour, which has just arrived in the US.

"The problem with her is she's a lot busier than I am, so it's hard to find time to work on something together," he explained.

"So we've been working on it for over a year now. There's still a lot of work."

It's been a hectic time for Gaga lately as she performed at a ball to mark US President Barack Obama's inauguration on Tuesday. She tweeted some photographs of herself in a flamboyant white outfit, with legendary crooner Tony Bennett joining her on stage at the end of her set.