Ed Sheeran is preparing for a blockbuster tour and album release amid regular trips to Manhattan's federal court, where he's defending his songwriting in a closely watched copyright case.

The trial centers on whether the British pop phenom plagiarized Marvin Gaye's 1973 soul classic "Let's Get It On" in his 2014 hit "Thinking Out Loud."

The heirs of Ed Townsend, who co-wrote Gaye's smash, are behind the civil suit and allege "striking similarities and overt common elements" between the songs.

It's the latest in a series of high-profile music copyright cases that have the industry on edge, leaving some songwriters paranoid about their own creative processes and vulnerability to litigation.

Sheeran, 32, has spent days testifying with guitar in hand, playing demos for the court to prove the 1-3-4-5 chord progression that's primarily in question is a basic building block of pop music that can't be owned.

His legal team argues that Gaye and Townsend were far from the first to record it, citing, for example, a number of Van Morrison songs that use the sequence and that were released prior to "Let's Get It On."

It's a notion echoed by forensic musicologist Joe Bennett.

"The world I want to live in is one where nobody sues anyone for a one- or two-bar melodic or harmonic similarity, because those similarities can so easily occur through coincidence," the professor at Massachusetts' Berklee College of Music told AFP.

"They shouldn't be protectable by copyright."

The case hinges on the actual composition of the songs rather than the recorded versions -- think sheet music, not vibes.

In theory, that's a specificity that could help Sheeran's case, but once a music copyright suit reaches the point of jury trial, anything can happen.

Winning demands significant funding and resources, and defendants are beholden to the volatility of opinions from jury members who almost certainly don't have a background in musicology.

Both sides have hired expert witnesses to explain the technical details to jurors, but, of course, their conclusions differ significantly.

"If you play music to a jury, it could go either way," Bennett said.