Imagine telling people in 1968 that they could take their collections of vinyl 45s, add about 10,000 songs to them and fit them into something the size of a cigarette pack.

They would have thought you were hallucinating.

Now, through the wonders of technology and cheap overseas labor, it's possible, and while the music may or may not be better, music listeners in this millennium have it all over their parents in terms of portability.

Today, iPods and other MP3 players are not only changing the way we listen to music but how we chart it, which, in turn, affects what we listen to, as most fans want to hear what's hot.

The trend recently was reflected in a change Billboard magazine made to the way it selects its Hot 100, now factoring legal downloads of singles into its formula for picking the top tunes.

Back in the days of the spindle and the 45, it was easy to keep track of which songs were the most popular in the country by tracking their sales.

Not so in the digital age. In fact, Mario's "Let Me Love You," the No. 1 single in the country, has been selling a mere 500 CD copies a week.

With the decline of the CD single, Billboard has been charting singles mostly based on airplay -- in effect, guessing which songs people like by how many spins they get on radio.

Now the trade magazine is choosing its top singles on a combination of airplay, the small amount of in-store singles still sold and the number of legal downloads, which accounted for more than 140 million tracks sold in 2004.

Geoff Mayfield, director of the Billboard charts, says the trade publication has been watching the popularity of sites like iTunes, eager to use their figures in the tally.

"So few hits get released as singles that most stores have abandoned having a singles section. ... As fewer came into the market, not only had stores found it tough to justify having a singles section, the consumer had learned to not look for them," Mayfield said.

"When iTunes, Napster and others began to sell music, you had a wide variety of songs that you could buy and a lot of hits were available."

By the end of 2003, sales of downloaded singles exceeded those of CD singles, hitting a landmark of 1 million per week. That number grew steadily through 2004, reaching 3 million by the summer, then up to 4 million by the week before Christmas.

Then Santa came.

"There was an amazing growth thanks to all the people who got MP3 players under their Christmas tree," Mayfield said.

There are now more than 10 million iPods in circulation, 8.2 million of them sold last year. The week after Christmas digital singles hit 6.7 million downloads and the number has hovered around 5 million a week in early 2005. "Let Me Love You" has been selling about 17,000 downloads a week.

Billboard added a Hot Digital Tracks chart in 2003, and made it Hot Digital Songs this year. The impact of figuring the digitals into the top singles is that a song such as Green Day's "Boulevard of Broken Dreams" is suddenly on the fast track up the singles charts.

R&B and hip-hop tracks have generally dominated the singles chart, due to those radio stations, like KISS-FM, spinning their hits more frequently than rock or country stations do.

"Boulevard of Broken Dreams," a rock song that wouldn't normally get kind of play given to stars like Usher, topped the Hot Digital Songs chart with 40,000 copies downloaded last week and broke into the Top 5 on the Hot 100 once the digitals were factored in. (Of course, the digital sales don't represent the full popularity of "Boulevard," or any other song, because it doesn't take into account the illegal swapping).

One might think that downloading singles would sap the CD market, but that's not the case. Mayfield thinks that having more songs available online for people to sample may have helped spike CD sales (2.3 percent) in 2004, the first year they've increased since 2000.

"How do you decide if it's worth purchasing an album?" Mayfield said. "This way not only are you armed with the hit or two that you've heard on the radio, but you can go to one of these e-merchants and buy two or three other songs and you have an idea if it's worth buying the album. Making more songs available to them on the computer may have encouraged them to buy more albums in the stores."

Jennifer Dionisio, a student at the University of Pittsburgh who recently got an iPod, said: "It hasn't changed the way I buy music at all. I've been a bit of a downloader/burn-it-off-a-friend kind of person for a couple years now. The iPod has actually made me more excited at the possibility to go out and buy new things to put on it.

"The reason I bought it was because I have a ton of CDs and I take terrible care of them. I've bought some of these same albums over and over and I want them in a format that I can't scratch up."

Dionisio says in the past she used shareware sites like Kazaa to download songs for free from other listeners, but, now, "the guilt factor has finally caught up with me. It's not like I listen to bands who make unimaginable amounts of money."

Now that people are downloading singles in chartable numbers, it brings us to the inevitable next step: the rise of the digital album. Will the next album format be no format? No cover art, no liner notes, no plastic at all?

Not so fast. In 2004, 5.5 million albums were downloaded through e-merchants, accounting for less than 1 percent of CDs sold. Billboard considers it too small to factor, but Nielsen did launch a digital albums chart and Billboard is keeping an eye on it. In the meantime, the current generation of music buyers isn't ready to give up the physical product altogether.

Jaclynn Hotek, another Pitt student who recently got an MP3 player, said:says, "It's nice to have a hard copy of a CD if it's really good. I still purchase CDs if I feel the artist or album was worthy of the $17 I'd shell out. I think it'll probably continue much like that for me."