The opening bass line sounded, and the strange musical bedfellows started playing strong, live - and at the same time. He delivered the swaggering boasts of his hit Dirt Off Your Shoulder, they unleashed the jackhammer guitars of their metal-edged Lying From You, and the crowd, a mixed bag of surly suburban youth and nodding hip-hop heads, lost its collective nut.

This genre-crashing slugfest between Jay-Z and LP, recorded in July, can be seen and heard on Collision Course, the first major-label release of a high-profile ``mash-up'' - that is, the suddenly red-hot art of taking two existing songs and making them one.

Collision Course, which includes an EP of studio-recorded mashes between the rapper and the rockers plus a DVD of the live Roxy battle, debuted at No1 in the United States.

Dance club DJs have been remixing songs - adding fast beats to a ballad, for instance - for a long time, and sampling certainly isn't new to hip-hop. But full-fledged mash-ups, two songs at once, didn't become the craze of the music biz until recently.

A year ago, a Los Angeles producer named DJ Danger Mouse (aka Brian Burton) had the inspired idea - and music-making software called ACID Pro to back it up - to blend an a cappella version of Jay-Z's The Black Album with the music of The Beatles (White Album).

The mad scientist called his layering experiment The Grey Album, and the fiendish crossbreeding of tunes such as Jay-Z's What More Can I Say and the Fab Four's While My Guitar Gently Weeps - his rhymes and their piano melody give way to their chorus and some booming hip-hop beats - became the year's must-have download.

Bootleg copies of The Grey Album - which is more amusing than flat-out entertaining, and technology-wise is already sounding dated - were making the rounds until Jay-Z's lawyers issued a cease-and-desist order. As is the case with sampling, if mashers want to make money from their efforts, at least on the up-and-up, permission is needed for music used.

Of course, pesky legalities aren't stopping ``collagists,'' as they call themselves, from smooshing for fun and launching their efforts in cyberspace. With the right software, mash-ups can be made by measuring the beats per minute in one track and then splicing in vocals and choruses from another track. There's often significant digital manipulation of the tracks, as well - slowing down a song, cutting snippets of music here and adding them there. It took Danger Mouse three weeks to make Jay play nice with John, Paul, George and Ringo.

But the payoff can often be enlightening, and the Internet is loaded with such unlikely pairings as Destiny's Child's Independent Women and Fugazi's Waiting Room; the Cars' Drive and D12's 40 Oz.; and a beast called Hey Ladies Night, a kitchen-sink showdown between the Beastie Boys and Kool & the Gang, with Cyndi Lauper as referee.

In making With a Little 1999, New York's DJ Reset (known to his family as Jeremy Brown) went through ``about a thousand'' a cappella tracks before he found the perfect one to merge with Wings' With a Little Luck: Prince's 1999.

``Any one of your readers could make a mash-up using audio editing software that's free on the Internet,'' says the 30-year-old Reset, who, as a University of Massachusetts student, studied with legendary jazz drummer Max Roach. ``It's all trial and error.''

Reset often adds his own music - drums, bass, piano - to his creations, some of the wildest of which can be found at www.djreset.com. There's Eminem vs the White Stripes, the Beatles vs Slick Rick, and his new mash-up, Frontin' on Debra, a merging of Beck, Jay-Z and Pharrell Williams.

``There's been a positive response to this new twist on music,'' says Patrick Ferrise, music director at WHFS, the Washington radio station that recently added the Mash-Up Radio show to its line-up. ``The momentum has been gaining over the last year.''

  

Collision Course actually represents the next evolution of mash-ups. Instead of a deejay mixing up an odd coupling, it's the odd couple themselves who are doing the mashing. Jay-Z and Linkin Park rehearsed for days before their live show. They also recorded a few new raps and vocals in the studio to ease the transitions on the EP cuts.

Whether that's considered cheating or not, the pairing offers a few nice surprises. Jay-Z and Linkin Park certainly aren't the toughest acts to mash - he has always loved a good rock hook; they incorporate hip-hop into many of their songs - the rapper's percolating wit lightens up the rockers' odes to gloom, and in several places the band's thunder gives the MC's poetry enhanced oomph.

It's also quite the showcase for the supposedly retired rapper born Shawn Carter, slated to take over as president and CEO of Def Jam Recordings.

``I might have to bring out the young Jay-Z,'' he says on the DVD as he's getting ready to re-record a rap from his Jigga What to weave into Linkin Park's fastball of rage Faint. He nails it, of course, and the result is a head-spinning wonder and ample proof that there isn't a beat in any genre that Jay-Z can't manipulate to suit his needs.

The CD and DVD's finale is the powerhouse Points of Authority/99 Problems/One Step Closer, Jay-Z's flammable self-portrait from 2003's The Black Album sandwiched by two thrashing workouts from Linkin Park's 2000 Hybrid Theory. Their rage against the machine, his rage against a racist cop - wow! It's a showstopper, especially when Linkin Park's Mike Shinoda plays the part of the officer who busts the rapper for ``doing 55 in a 54.''

The downside to Collision Course is that the shaggy metalists often seem so in awe of the MC that they allow his songs to dominate. There's not a whole lot of mashing on Big Pimpin'/Papercut and I.Z.Z.O./In the End, with Chester Bennington and Shinoda just shouting out backup. Yes, those are great Jay-Z songs, but that's not the point of the exercise. Nevertheless, Collision Course ultimately gives you a warm, chummy feeling - like a good buddy movie.

Walking away from the Roxy with a victorious strut, Jay-Z sums up his collaboration with Linkin Park with a smile. ``Mashing of the cultures,'' he says proudly. And there's certainly nothing wrong with that.