Ed Sheeran's Shape of You was written in just 90 mins

Getty Images Ed Sheeran, Liam Payne, Pink, Rita Ora, Zara Larsson and Clean Bandit

The co-writer of Ed Sheeran's Shape Of You has revealed the song was written in just 90 minutes.

Steve Mac says it was the first thing to come from a writing session with Ed, and so far it's the biggest song of 2017, selling more than three million units.

"Ed is one of the best writers I've worked with," Steve tells Newsbeat.

"He has a brilliant flow, the way he creates his rap and then adds a melody to it."

Now Steve has received The American Society of Composers, Authors and Publishers (ASCAP) Founders Award for his contribution to music.

The songwriter, who has a career spanning nearly three decades, says Shape of You has changed his life and describes Ed as "extraordinary".

"Ninety minutes sounds quick but it's 27 years of making music and understanding what not to do in songs, as opposed as what to do.

"Also, the fact it was Ed Sheeran helps."

After nearly three decades working in the music industry, Steve has arguably had his most successful year to date

Steve Mac's 2017 writing credits include: Ed Sheeran - Shape Of You, Liam Payne - Strip that Down, Rita Ora - Your Song, Clean Bandit & Zara Larsson - Symphony, and Pink - What About Us — After nearly three decades working in the music industry, Steve has arguably had his most successful year to date

Over the years, Steve has worked with JLS, The Vamps, The Wanted, and Jess Glynne. He's also responsible for much of Westlife's back catalogue.

"Flying Without Wings was a big moment," he admits.

"To see that get to number one meant a lot for myself and [co-writer] Wayne Hector. We worked hard on that song."

To avoid writer's block, Steve says he works five days a week - with or without an artist.

"I go in every day and I never work weekends. You have to see everything as a positive.

"Even if the song doesn't work that day, you may have found a new drum sound or a new melody to that song - never give up."

Watch a Newsbeat YouTube video explaining why pop music can be good for you.

Despite working in the industry for 27 years, he says song writing doesn't get any easier.

"We're hearing music much quicker now and I'm listening to more music than I ever did before."

He says people want their information far quicker in songs and it's "a shame" that introductions to song are not as long as they used to be.

The average intro time has dropped from more than 20 seconds to five since the mid-1980s, recent research found.

Unlike many artists, Steve says most of his lyrics stay in his head: "I have nothing prepared when I go into the studio. We sit down, we start and something comes out.

"It's that simple."

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