Mid-September evening in Berlin. Grey-bodied crows straggle through the dusk, some descending to roost on the flat roof of the Palast Der Republik, formerly East Germany's parliament building. Once a flagship construction known as the "House Of A Thousand Windows" on account of the coppery reflective panels of its exterior, the Palast was found in 1990 to be contaminated with asbestos. A decade later the interior was stripped bare, leaving a husk of concrete and rusty girders. For two nights that stark industrial environment hosts Dialoge 4, latest in a series of interdisciplinary projects involving adventurous German choreographer Sasha Waltz and her company. During the second half of the programme, members of the audience are guided in groups through the vastness of the derelict space, pausing to watch dancers perform with sound installations by composer Hans Peter Kuhn. Earlier, in a cavernous hall carpeted incongruously with turf, they witness a thrilling collaboration between Waltz's company and radical chamber ensemble Zeitkratzer.

The improbable grassy arena comments ironically upon a suggestion from Berlin's politically conservative faction that the Palast should be demolished and replaced with a lawn plus a monument to a castle that once occupied the site. At present an organisation called Volks Palast rents the building to stage unorthodox cultural events in the vicinity of the State Opera House and the university. Nine members of Zeitkratzer, aligned along one edge of the pitch, perform Xenakis (A)live, composed by the group's founder, pianist Reinhold Friedl. Opposite them, Zeitkratzer's sound technician Ralf Meinz monitors and modifies the projection of the dense, edgy and exhilarating music. Waltz's dancers appear in the space between - singly, in small, entangled groups, at times approaching 20 in number. They move athletically, interact erotically, with startling filmic shifts from slow motion to flickering rapidity. At one point the company repeatedly criss-cross the arena at breakneck pace like agitated particles reacting to surging heat. Somehow they avoid collision. Japanese dancer Junko Wada, a guest for the occasion, threads serenely through their frenzy.

Zeitkratzer are engaged in parallel activity, the ceaseless scintillation of discrete events coalescing into rich clouds of sound. Towards the end of the 53 minute performance some members of the audience clamp hands to ears, dance enthusiasts unprepared for the unyielding force and intensity of this music.

Next day, over coffee on bustling Alexanderplatz, a short distance from the Palast, Reinhold Friedl remarks, "When I started Zeitkratzer I always said I needed musicians who look good on stage, so you have a visual quality, a body quality. You shouldn't avoid that. From the start I said I don't mind which instruments. I've just taken people that are interesting, so we have a strange combination of instruments. We always have our lighting man with us, Andreas Harder. You see music and hear bodies."

Friedl's sense of relevant action and situation within musical performance is expansive. Physical posture, appearance and activity of the players are integral elements of any concert. Accordingly he feels a special affinity with dancers. There have been earlier collaborations with Jutta Hell and Dieter Baumann's Dance Company Rubato and with choreographer Christoph Winkler. During the past year Friedl and members of Zeitkratzer have done improvisatory work with Waltz, performing at the Montpellier Festival and Berlin Dance Festival. When Waltz's partner Jochen Sandig heard a Berlin performance of Xenakis (A)live – a piece commissioned by the Vienna Festival – the music excited him and he initiated the current project. "Sasha did a kind of open, instant choreography, because she only had about two weeks to rehearse," Friedl observes. "The dancers didn't listen to the music then and were quite astonished when they heard it the first time."

Zeitkratzer have undergone personnel changes during the past five years. Major players such as trumpeter Axel Dorner and saxophonist Ulrich Krieger have departed. The current line-up seems fixed for the foreseeable future, a potent combination of immensely accomplished and boldly exploratory musicians. Friedl plays 'inside piano', his idiosyncratic sounding of the instrument's interior, along with percussionist Maurice De Martin, trumpeter Franz Hautzinger, cellist Anton Lukoszevieze, double bassist Uli Philipp, Melvyn Poore on tuba, Frank Gratkowski on bass clarinet, Marc Weiser's electronics and violinist Burkhard Schlothauer. Technician Ralf Meinz oversees their amalgamated sound.

An important aspect of Zeitkratzer's identity is that each member plays regularly in other contexts, bringing disparate experiences and perspectives that allow cross-fertilisation of ideas and methods. Weiser, for example, makes mischievous electronica as Rechenzentrum. Lukoszevieze founded the impressive contemporary music group Apartment House. Gratkowski's fabulous improvising has been heard with the likes of drummer Tony Oxley. Hautzinger, another remarkable free player, is a member of acclaimed contemporary music ensemble Klangforum Wien. Schlothauer was one of the founders of Edition Wandelweiser, a label specialising in music that grants an important structural role to silence. The benefits of this diversity, Friedl notes, are social too, adding, "On tour they have many things to discuss because they are from completely different musical worlds."

Friedl studied piano with improvisor Alex von Schlippenbach and had classical tuition from Alan Marks. Earlier, in Stuttgart, he had learnt about jazz from organist Paul Schwarz and acquired classical techniques from Renate Werner. Shaping the nature of Zeitkratzer, he has chosen musicians who share his commitment to extended techniques. This interest in redefinition of instrumental possibilities dates back to his mid-teens. "A friend of mine started to prepare his piano at home," he recalls. "He put huge nails in and played ragtime. I really loved it, so I started to prepare the piano. My mother let me do it. It was OK. Then I saw what Martin Theurer was doing. He's a German piano player - a radical improvisor. Our mothers had been at school together. Later on I met Mario Bertoncini. He's the one who really started 'inside piano' in the early 60s."

Bertoncini – from 1965 a member along with trumpeter Ennio Morricone of pioneering free improvisation ensemble Gruppo Nuova Consonanza Di Roma – made huge advances on American composer Henry Cowell's tentative probes into the piano's interior. His piece Cifre (1964) also anticipated Friedl's instigations. "I'd developed all these techniques and he'd done it already," Friedl reflects. "He developed all the fundamental techniques like bowing the piano. I've gone a little further in some points, but he already had the idea to make the piano sing." As a member of the trio Piano Inside·Out, with Yun Kyung Lee and Michael Iber, Friedl has recorded a fascinating version of Cifre on Piano Inside-Out (Edition Zeitklang 1998).

Zeitkratzer recorded Bertoncini's orchestral work Sinfonia on SoundinX (Timescraper 1999), and in May 2000, using five grand pianos placed around the Chopin monument in Warsaw's Lazienki Park, they realised his installation Il Cimitero Oegli Elefanti (The Elephants' Graveyard). "The pianos were played with a little machine, a motor with a small wheel connected to the bass strings and if you get a harmonic point it can hurt your ears, it's so loud. There are incredible overtones. Very rich," says Friedl. Bertoncini's innovations dramatically shift the concert grand away from traditional piano repertoire towards the wild soundworld opened up by electronic music, a world that is Zeitkratzer's domain.

"It's such a multi-dimensional space, music," Friedl exclaims. "There are so many different things you can use. The whole sound of Zeitkratzer and the techniques of the players have been influenced by electronic music. I feel that strongly when I play 'inside piano'. 'Influence' doesn't mean I sit down at home and say I have to do some electronic sounds at the piano. Rather I find I love a sound and years later I realise it probably comes from electronic music." More than any broadly comparable ensemble, Zeitkratzer have transformed the values of chamber group playing to match current conditions of performance. "The main idea at the beginning was to have an amplified group," Friedl recalls. "Normal contemporary music groups don't know how to play with a microphone. They just think it's the same music, but amplified. That's not true. I met David Harrington from Kronos Quartet and I took some ideas from them. I could see how many technicians you need, what they have to do and what the musicians need to know. Kronos developed this kind of amplified playing very well. I had started already because of special sounds inside the piano that you can't hear when it's not amplified. Amplification alters the whole sound possibilities of the instrument. Different microphones have different sounds. The microphone is an electroacoustic transmission, but also a transposition. If you use a microphone, it's electronic music. A musician needs experience to know how to use a microphone. Sometimes we play acoustic concerts. It's really peculiar."

Xenakis (A)live pays homage to composer lannis Xenakis, who died in 2001. Friedl describes it as a "paraphrase, based on the sounds of Xenakis's Persepolis mostly, a little bit from his Concret PH, the last three minutes from La Legende O'Eer". Significantly it's primarily Xenakis's electromagnetic tape compositions that Friedl's tribute draws upon. "I really oriented myself on Persepolis, an eight channel composition. There are different recordings of it and very different versions yet it's a kind of modular materials thing - the shape is always the same, the evolution of the sounds. I listened to more than 20 Xenakis CDs, all the orchestra pieces, and found which of those sounds he had used in making this piece. He often has this orchestral sound. I had the idea we could have it also, via amplification. We used refined microphones for each instrument, closemiking, special positions. Xenakis (A)live is very sound based, rich sound textures. It's really about the mix of instrumentation in terms of merging sounds, mixing textures yet completely defined. You have to be very strict to keep it defined."

(A)live signals that Xenakis lives on in memory. It also indicates the readiness and ability of Zeitkratzer to find flesh and blood correlatives for what were once exclusively studio sounds. In 2002, in high profile concerts in Venice and Berlin, the group made audacious, clangorous realisations of Lou Reed's 1975 double album Metal Machine Music, a studio composition that Friedl feels has qualities in common with Xenakis's vividly energised work. Transcriptions making the recording into a playable score were made by the now departed Ulrich Krieger. For the concerts, Reed joined them on stage to add guitar to the final section. "When we made Metal Machine Music I said we should also do a Xenakis thing," Friedl explains. "I always said they are quite close to each other – the same orgiastic approach." In this music the experience of sound itself is sensuous and immediate, at times overwhelming as it offers access to levels of instinctual response where, emphatically, "you hear bodies".

Friedl recalls meeting Lou Reed for the first time, in a hotel room during the early hours of the morning following one of his gigs: "I told him we had violin, cello, bass, trumpet, saxophone, percussion, piano, plus accordion at that time, and we wanted to do Metal Machine Music. He said that was completely crazy. He had some other pieces out of a theatre project based on the works of Edgar Allan Poe he did with Robert Wilson and suggested we could do those. So we played them. And we made a short version – five minutes – of Metal Machine Music. When he heard that he said, 'Go and make it'. So we did. It was really fun to do, but it was hard work to convince him.

"Zeitkratzer is a kind of sound fetishism idea," he continues. "That's what I told Lou. He also is totally crazy about sound. We told him how the guitar on the record was tuned, a special tuning, and he was really into that. He came and played with us and there was discussion beforehand with the audience. It was the first time he really spoke musically about it. In the short rehearsals we had with him he didn't say much. He doesn't know about our kind of instrumentation, but he knew what was going on musically. He's an American rock 'n' roll professional who knows that if it's a provocation everybody thinks that's really cool. If you say it's a musical thing everybody thinks you've gone crazy."

Reed's recording of Metal Machine Music baffled and infuriated reviewers upon its initial release. It was almost universally dismissed as a petulant taunt. Contexts of reception change and more widespread acclimatisation to an aesthetic of noise has prompted a re-evaluation of the work. Zeitkratzer had already performed pieces by Merzbow and by sound artist Zbigniew Karkowski – a student and admirer of Xenakis – which Friedl recognises as essential preliminaries for tackling Reed's prolonged Heavy Metal howl. Irrespective of Reed's original motivation, Zeitkratzer chose to approach Metal Machine Music as they would any other contemporary music composition.

[page break]

"When you play a piece by a composer, never mind what they tell you about it," Friedl observes. "I've experienced it myself with pieces I've written. People play them in another way and sometimes they have qualities you don't see when you write them. It's often like that. I didn't mind what Lou's intention was. I didn't mind at all if Metal Machine Music was a provocation. He was probably on heroin at the time and in a strange mood and he had the courage to do it."

Working from the inside of sound is a vital aspect of what Zeitkratzer do. Immersion can produce new forms of awareness. "It's very interesting to perform Metal Machine Music several times," Friedl comments. "You really get into it and at the end it's changed completely. For me it was like walking through this forest, where you have light effects. Because of its altered tuning, based on pure fifths, it's very light – when people listen to it the first time they don't realise. And there are all these kinds of texture moving through it, very different things going on in the different movements. It became a little bit like a Mozart piece, very relaxed. The sound, which you at first think is so compact, became really transparent - you could hear everything, like in some Mozart pieces."

Playing 'inside piano', performing inside Zeitkratzer, Friedl finds he now listens in new ways, discovering forms of clarity within ostensible complexity. This can produce revelations; it can also exhaust a work. "I just listened again to some Ligeti pieces and I thought it's so simple!" he contends. "I had this experience with his Ramifications. I heard it live with The Berlin Philharmonic Orchestra and I thought it's great! Wow! Then I listened again, ten years later, and I thought, This is a little bit boring, I can hear everything. It's the training I've had in the meantime."

His sense that an established repertoire is exhaustible in terms of its interest for performers and audience alike has encouraged Friedl to seek out encounters with musicians such as Reed, whose work can bring extra vitality to Zeitkratzer, and who in turn can make discoveries through involvement with such an idiosyncratic chamber ensemble.

Sonic Youth's Lee Ranaldo has supplied them with three compositions. Raster-Noton's Carsten Nicolai has been another recent collaborator, arriving with his laptop to supply looped samples of asymmetric rhythm as the basis for two minimalistic pieces.

Friedl speaks with particular enthusiasm of Zeitkratzer's work with transgendered electronica conceptualist Terre Thaemlitz – 2002's SuperSuperBonus LP (Comatonse). "I'm really a fan of his," he declares. "He sent us a project. We transcribed the rhythms and put some transparent sounds on it. He came to rehearsals for two or three days and was the best-prepared musician I have ever met in my life – a really good musician. I want to work with him again."

Elke Moltrecht, music curator at the Podewil, introduced Zeitkratzer to audio and visual artist John Duncan. "He came to us with a CD of his music," Friedl relates. "He went through it with the musicians, one by one, talking about the sounds and then we played the piece. He kept making interventions so I said, 'John, come on stage and conduct'. We tried it in rehearsal and he conducted it with his hands doing shapes and it worked well. In the end he loved conducting it. We immediately did a second piece." Recordings of Duncan's "Nav-Flex" and "Trinity", made in 2001, can be heard on Fresh (AliquestionsjX-Tract).

The Zeitkratzer repertoire extends sufficiently far from what Friedl identifies as the "noise orgiastic music connection" to take in even Bernhard Günter's threshold-of-audibility minimalism. In 1993 bassist Ulli Philip presented a copy of Günter's first CD, Un Peu De Neige Salie to Friedl, who was at that time writing about music. He reviewed it enthusiastically.

In 2001 Friedl was performing in a duet with Michael Vorfeld, who plays percussion and self-built stringed instruments. "Michael plays little sounds that are quite close to inside-piano sounds", Friedl observes. "I said to him, let's record. It's piano and percussion but it sounded really electronic so I sent it to Bernhard Günter and said would he be interested in releasing it on his Trente Oiseaux imprint. He said, 'For sure. There's perhaps too much information for listeners to my label, but I'll do it.' I was really proud because Defaut De Silence is the first CD using acoustic instruments released on this electronic label."

Last autumn Günter proposed that they perform as a trio, incorporating his electric cellotar, an amplified five-string guitar played with cello bow. After a concert at Podewil they got together for a recording session. The result is Message Urgent (Trente Oiseaux), a remarkable blending of the three musicians' unorthodox instrumental resources. "We got to that point where we didn't know where the sound was coming from," Friedl comments. A collaborative CD by Günter and Zeitkratzer is imminent on Podewil's XTract label. "He gave us a piece, a text of one page. We made a score out of that, quite open but very defined in terms of texture, the same note but with little microtonal things and we had an electronic piece of his called "Insects" to go with it. We played it together in Geneva this spring."

Zeitkratzer have given stunning interpretations of complex scores by Helmut Lachenmann and Karlheinz Stockhausen. A minimalist emphasis upon duration and slow change, as experienced in their performances of pieces by Phill Niblock or early Philip Glass, has also been significant to the development of the group's identity. "You have some pieces where quantity turns into quality," Friedl reflects. "You have to go through the first ten minutes and perhaps it's really boring but after that you get into it."

He finds the music of James Tenney particularly absorbing and revelatory; playing becomes listening to the intrinsic characteristics of scrupulously chosen tones and the relationships between them. A Tenney programme is planned for Zeitkratzer and Friedl is looking for other ways to enable listeners to share in that kind of engagement with acoustic material. ''I'm trying to set up installation concerts where you play for four hours and don't have a beginning or ending and the audience can decide for itself to walk around or to listen. Then you have only the texture."

Friedl enjoys the quirky minimalism of John White and other English experimental musicians associated initially with Cornelius Cardew and The Scratch Orchestra. He's preparing an entire programme of their music. "What I'm very enthusiastic about is when Fluxus turned into minimal music, especially that English scene," he says. "They did performance pieces where they wrote motoric scores and theatre became music. You can have great music and a strange visual situation." His interest in this scene was fired when, as a young musician, he encountered Michael Nyman's book Experimental Music: Cage And Beyond. Friedl relishes the openmindedness and humour that characterised a great deal of musical exploration during the 1960s, aspects which were reflected in Nyman's account.

Zeitkratzer have on several occasions performed White's Drinking And Hooting Machine, which invites performers to sip, swig, gulp and blow across the top of a bottle, performing sounds and actions which "are fed like raw materials into a machine or process and emerge as a pattern unique to the occasion", as White himself has put it. "He has written in a motoric way and music comes out," Friedl comments. "I love this idea. You have a kind of random function out of the body, random limits that the body gives. Some of these techniques I had in mind when I did Xenakis (A)live because Xenakis used kinds of random function that are nonetheless always very defined."

Theatricality and emphasis upon moving bodies are integral to the Zeitkratzer approach. During 2003 they staged a series of performances of Dry Clean Show, a presentation featuring fashion models wearing clothes by Lisa D, an Austrian designer now working successfully in Berlin. The entire project is pervaded by irony, Lisa D's luxurious and expensive handmade clothes being displayed in conjunction with a series of barbed messages about thinness, hunger, illness and the economics of production. Friedl, Schlothauer and Weiser wrote the accompanying music and songs.

"Lisa D and I have founded an enterprise called Global Concern, which provides politically correct clothes, but people have to pay more," adds Friedl. They are now collaborating on a project called Boat People, based on a story they heard about exploitation of clothing workers on board ships where employment laws don't apply. "There are some spaces here in Berlin where you can put water on the floor," says Friedl, envisaging the staging. "I'll probably write music for nine amplified sewing machines. They have very different sounds and very different rhythms. Each Zeitkratzer musician will also play sewing machine."

Another project currently taking shape for performance in Vienna and Berlin next summer pays homage to Arnold Schoenberg. "It's something I've always wanted to do," Friedl grins. "It will include freely instrumented cover versions of the Piano Pieces Op 19 and some songs from Pierrot Lunaire. We'll be singing. It'll be fun. We did cover versions of Deicide's song "Satan's Spawn" and Throbbing Gristle's "Hamburger Lady". I suggested we also should do a CD of covers of contemporary music. That would be fun – cover versions of Lachenmann! So we are starting with Schoenberg."

Fun is a recurring word in Friedl's conversation. Zeitkratzer administer massive jolts to routine listening habits while taking evident pleasure in the unpredictability of performance. "We always try to get to the point where you want to be surprised, you want to hear something you haven't heard before, you want to go a little bit more in that direction," concludes Friedl. "We always try to do this."