He and bassist Mike Marcinko have been pushing local music's outer limits at the helm of A.T.S. for 20 years now, but guitarist Evan Knauer talks with the enthusiasm of a high-school kid who's just come back from his first rock-band practice when the conversation turns to how they went about composing the music on "A.T.S. House Music: Cellar Steps Series Volume One" in Knauer's basement.

		A.T.S. 20th Anniversary Show Where: The 31st Street Pub. When: 10 p.m. Saturday. Tickets: $5; 412-391-8334.

"Lately, at rehearsals," he explains. "And when I say lately, I mean for the last seven years, [longtime drummer] Kip [Ruefle] and Mike come in on Monday, have some coffee and just play. And I record it on my four-track. Then, during the rest of the week, I add guitar and vocals and I get to make up lyrics. But it's really driven by the rhythm. Whatever Kip makes up, Mike usually has something to add to it. So my songwriting work, the load has dropped immensely. The rhythm's already there. I just need to attach a melody to it and find one of my short stories or Kevin Forsythe's short stories and take lyrics right out of there."

He laughs, then adds, "It's almost foolproof. And I know it's good if I laugh at it or if somebody else goes 'Ah-ha-ha-ha.' That's our criteria right there."

So does he ever just sit down, guitar in hand, and write a song for A.T.S.?

"I do," he says. "But now, it's always at the 4-track with a playback of Kip and Mike playing drums and bass. And it's so easy. When you have that, it's like you've got the skeleton. Melodies seem to come so easy when you already have a drummer making this stuff up. And Kip has just launched into a super-creative period of his life. He comes down every week and I have everything ready. This is something I look forward to. And I don't even practice anymore. I sit and watch them with my hand on the 'record' button for about an hour. And then, I join in and we go over some songs for an hour or something like that. But often times, I don't. And that's been such a luxury."

The 13 tracks of free-form post-funk art-punk improv captured with a twisted smile on A.T.S.'s new CD have all been featured on the "First Takes" series the band has been selling at shows on CD-Rs, including such obvious highlights as "Porcelain Bus," the rubbery math-funk workout "Fiver and a Single" and, of course, "Mike Watt," their epic, decidedly Watt-esque tribute to the former Minuteman (with Knauer's vocals running through some pretty great effects).

"The reason why we called them 'First Takes,' " Knauer says, "is 'cause for Mike and Kip, that's literally the first time they ever played that stuff."

At this point, Knauer says, they've got another three or four "First Takes" collections sitting in his basement. And they're writing new material every week.

"I have a wall of tapes that are dated," he says. "I very intricately document it. Each song has a little file. And I amuse myself for hours. Nobody listens to it but me and Mike and Kip and maybe 10 other people."

They're taking it public this Saturday night, though, at the 31st Street Pub, with a record release show that's also doubling as an anniversary party to honor their 20th year of making music on the edge. And anyone who's ever played in A.T.S. is joining in the fun, including the other original members, guitar-playing frontman Josh Arnson and drummer Steve Heineman. This marks the first time the originals have been on stage together since that Decade show in June of 1990 they billed as "The Last Mosh for Josh."

"We got the whole night," Knauer says. "And every musician who ever recorded with us or played a bunch of shows with us is taking part. And it's quite a long list." That list includes Alexei Plotnicov, Tim Pollock, Lisa Miles, Steve Seel, Minette Seate, Erin Snyder, Heather Clark, Uptown Steve Browne and "everyone in Watershed," to name a few. And Heineman and Daryl Fleming will be driving there directly from the Quiet Storm, where they'll be doing an acoustic set of Heineman material at 9.

The Pub show starts at 10 p.m. with Knauer, Ruefle and Marcinko doing new material and then they'll work back through the years to 1985, reviving songs that once led Magnet magazine to hail the group as "Phish for smart, angry kids with Attention Deficit Disorder," while in Option, Fred Mills went with "Traces of the Minutemen's jazz-punk/rock-funk fusion and a sonic kinship to the Meat Puppets shows that 99 percent of the nouveau hippie bands from Blues Traveler to Phish are downright unimaginative."

"I actually forgot how many songs we have," says Knauer. "Personally, I like the stuff we're doing now better. We tried to be as sophisticated as we could, but we were a sex-drugs-and-rock-'n'-roll band when we started. We really dug all that. But I think we're getting into I wouldn't call it a mature phase, but maybe it is. It's way more off-time. Those songs on that CD, we thought we could sell. Those are our most commercial offerings. We're gonna have some CDs coming out in the next year or two that I think will have some really odd, totally uncommercial type music on them."