Making music while in prison is nothing new. Mac Dre, T.I., former Dipset member Hell Rell and a few others have managed to drop bars from behind bars. Vybz Kartel, the dancehall legend who was recently released from prison after serving 13 years of a 35-to-life sentence, revealed in a recent interview that he kept his career going by finding novel ways to record music while locked up.

For an episode of the Juan Ep Is Life podcast, Vybz Kartel hopped on a video call with hosts Cipha Sounds and Peter Rosenberg to talk about the first thing he did when he was released from prison (have sex with his fiancée), how he’s been spending his time (eating healthy, working out, managing his Graves’ Disease and making music), and what he’s planning for the immediate future (a series of performances around the world, including a massive one in Jamaica on Dec. 31). But the most surprising part of the interview came about 25 minutes into the episode when they begin talking about the massive amount of music Kartel released while locked up.

“We’re not trying to snitch on you, we’re just so curious, we’re fans,” said Cipha Sounds after Rosenberg mentioned that a number of his most successful releases came during his time away. Kartel said when he first went to prison he was able to continue his usual clip of releases because of how much music he had recorded beforehand. “In the first year, we had songs unreleased,” said Kartel. “But then the songs ran out and we had to do what we had to do. That’s when I started recording behind bars.” He said he used to record his vocals on an Apple iPhone because the handheld had “an amazing sound quality.”

To try and get the best sound quality possible while recording, Kartel said he would use the mattress from his bed to imitate the padding found in a studio’s recording booth. “I would have the phone, like, 3 feet from my face and I would wrap the mattress around my head and record. It was crazy.” Afterward, he would make a demo using the vocals from the iPhone and a beat playing on an iPad. He would then send both to an engineer who would take all the components and turn it into a full song.

It’s all very impressive. But what’s most amazing about that process is Kartel says it’s the same one used to create what is considered by many to be his biggest song: “Fever.” Released in 2016 as the single for Kartel’s King of Dancehall album, “Fever” became an instant hit and is now his most-streamed song, notching more than 100 million streams on Spotify. Kartel said he would record mostly at night or wait until the guards were on their lunch break to record his vocals, always keeping an eye out for any that were on patrol. “I’d still have to be watching,” he remembers. “So I’d [sing], ‘fever!’ and then I’d have to get up and look. It was crazy. It was line by line!”