Ant McGinley, an energetic Irishman with a red beard and two U.K.-based podcasts, one on soccer, On the Left Side, and the other legal, Mondeo Law, blew into Philadelphia from Manchester, U.K., last Saturday. He taste-tested Dock Street brews and ran the Art Museum steps in his first two days.

But the rest of his trip was all about the Podcast Movement Conference, a sold-out 2,500-attendee event at the downtown Marriott that called itself "the world's largest gathering of podcasters." Attendees came from as far as Australia, China, and Brazil. Opening Tuesday, it had the feeling of an evangelical revival and a self-help group, with an emphasis on how to broaden the audience for a 15-year-old digital platform popular with 48 million weekly users.

The issue: Techie-sounding podcasts are still misunderstood by many other Americans who don't know what they are ("on-demand radio, like Netflix for audio," said convention organizer Dan Franks) or how to find them (mostly through smartphone apps).

But U.S. podcast advertising boomed last year, up 87 percent to $317 million, according to Advertising Age. Advertising and sponsorship remain the top ways to finance podcasts — with some getting listener support — and everyone from radio station groups to Google to start-ups is betting on growth.

Popular with millennials because they can be listened to "anytime, anywhere," podcasts have become a mantra for media companies seeking to tap into app-generation millennials. Podcasts are cheap to produce and offer repurposed radio content without the shock-jocking as well as  thoughtful conversations on complex topics and original content. True crime and comedy are popular genres.

Podcasts also have been helped in recent years by the ubiquity of WiFi for downloads, unlimited-data wireless plans, and storage-rich smartphones. Most podcasts are free. This can lead to poor-quality podcasts, though there also have been breakouts such as Stuff You Should Know, known as SYSK, with 500 million downloads on Apple. Podcast observers also look at smart speakers as boosting podcasts by making them convenient to listen to.

Conference attendees pointed to Netflix as the North Star of where the platform can go.

"Video got ahead of audio," Tim Murphy, senior vice president of business development at Entercom Communciations, the Bala Cynwyd-based company that now owns CBS Radio, said at the conference. "But I think the trajectory will be the same."

Tom Webster, senior vice president at the Edison Research firm in New Jersey and a lead podcasting analyst, said in one of the keynote speeches that about 48 million Americans listen to podcasts but that now the goal should be 100 million, calling it "podcast's next frontier." The challenge to those producing podcasting is to make more high-quality content to drive audience gains, he said.

Google executive Zack Reneau-Wedeen told a packed audience as an opening speaker that the internet search-engine giant is preloading Google Podcast on Android phones, opening podcasts to two billion global Android phone users. One can search for podcasts on Google Podcast over the smartphone screen.

Google is seeking to close a podcast gap with Apple, which has made podcasts available since 2005 and has a huge lead through its smartphone app and iTunes store. Listeners have downloaded 50 billion podcast episodes from Apple platforms since 2005, with 13.7 billion in 2017. That's 30 percent more than the 10.5 billion the prior year, Apple says. Apple says there are 550,000 podcasts, with 18.5 million episodes, available on Apple.

Radio is the most threatened by podcasts. But radio station groups also believe that on-demand audio could help them reach listeners by posting radio station sports-talk segments and interviews online for later listening, or developing original podcast content for listening at the gym or at home.

Gabriel Coan, a vice president at WHYY, the Philadelphia public radio station, said, "We have had a taste of high-quality audio and people want more." He also said they are cheap to produce and there is almost "endless space" for storage because of the digital.

WHYY produced 14 podcast episodes of its Cosby Unraveled on the Bill Cosby criminal trial, which were downloaded 500,000 times. Its national kids' show, Eleanor Amplified, has hit 100,000 downloads a month in its third season. And Terry Gross' Fresh Air interviews are distributed nationally as podcasts, with three million downloads a week.

Entercom, the nation's No. 2 radio station group, has acquired a 45 percent stake in New York podcasting firm Cadence 13, which both distributes and produces podcasts. Radio listeners can download Entercom's radio.com app to their smartphones for access to its stations' on-demand content, such as that on WIP in Philadelphia. "We expect consumption to keep taking off," Murphy said.

This is heady stuff for Dan Franks, one of the Podcast Movement founders. There were about 500 attendees at the first Podcast Movement Conference in 2014 in Texas. Other events were in Chicago and Anaheim, Calif. Franks, a convention organizer, said that planners had expected 2,000 attendees in Philadelphia. But the conference blew through that target, with 2,500. Some meetings were standing-room-only.

There were about 110 seminars, among them "Podcasting in Desperate Times: Strategies for Non-Political Podcasts in a Political World" and "Yes, That Marketing Advice for Your Podcast Is Still BS."

McGinley, the guy from Manchester, and Aimee Joshua, a former Comcast project manager whose podcast is Chasing Dreams, did a seminar on "Do Everything With Nothing" — in which they would launch a podcast in about 30 minutes, McGinley said.

Nicole Kirksey from Hummelstown was staying with friends for the conference. She produced a spiritual-based podcast for eight years but gave it up and now is thinking about doing one again. But she isn't sure she would like to make the time for it.

Charles Ewing, of Columbus, Ohio, was doing a podcast for his employer, Quantum Health, a competitor to Plymouth Meeting-based Accolade. But he also was thinking about personal podcasts on rap and dreadlocks. The idea, Ewing said, was "to continue the conversation."

Apple has a "top charts" list on its app. The top-five are:

Oprah's Master Class: The Podcast

Fatal Voyage: The Mysterious Death of Natalie Wood

The Wilderness

Dan Carlin's Hardcore History

The Joe Rogan Experience