Apple's biggest rival when it launches its $10-a-month streaming music service on Tuesday might not be Spotify or Tidal, but piracy.

About a fifth of Internet users around the world continue to regularly access sites offering copyright infringing music, according to the International Federation of the ­Phonographic Industry.

In the U.S. alone, 20 million people still get music through peer-to-peer file-sharing networks, according to research firm MusicWatch. And newer methods have emerged, such as mobile apps and software that rips audio from YouTube.

Comparatively, just 7.7 million Americans paid for a music subscription service last year.

"It's a tremendous problem," said analyst Russ Crupnick of MusicWatch. "The good news is some of the very traditional ways of stealing are down pretty dramatically."

The rise of convenient, licensed streaming has helped cut U.S. file-sharing rates in half in the last decade. Anti-piracy efforts of the Recording Industry Association of America — representing Universal Music Group, Sony Music Entertainment, Warner Music Group and others — have also contributed to the drop off.

But the music industry is still trying to recover from piracy's heyday. Last year, total music industry revenue was about $15 billion worldwide, well below the 1999 peak of $38 billion.

Free downloads have maintained their allure for people who want to build music collections and refuse to go to iTunes, Amazon or Wal-Mart.

Part of the problem is getting people who grew up in the age of Napster, LimeWire and Kazaa to pay anything for music, industry experts say. Many young people don't see anything wrong with downloading from unauthorized sites or ripping from YouTube.

"We now have a generation of people for whom the value proposition of music has changed," said Larry Rosin, co-founder and president of Edison Research. "A lot of people saw this as revenge for being ripped off by the industry for years."

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