Later With Jools Holland has brought live music triumphantly back to Saturday night TV. If it looks convincingly like all the bands are in the studio simultaneously, there's a reason. They are. The VT editors may get a night off, but the sound engineers make up for it in overtime and Paracetamol. So sit back, tuck into the Indian take-away and join Phil Ward on the set...

"Channel four? You've got the wrong channel, mate. This is BBC2." Jools Holland is responding typically to an innocent enquiry regarding microphone channels. The words fly off the cuff, around the bewildering network of headphones and monitors in Television Centre's Studio One, and settle nowhere in particular. Everybody is too busy.

From early in the morning, the studio must be transformed from vacant hangar into a colourful and convivial backdrop to Mr Holland's programme, in which four entire pop groups and their assorted tackle must perform, live, cheek by jowl, in a single hour's frantic recording - later that same night. The insanity of the enterprise is its genius. Despite abundant edit suites to flip reality on its head at leisure and tell television's time-honoured lie, this programme asks 20-or-so highly sensitive and artistic personalities to drop their professional trousers not behind the screens but in front of them - and in front of each other. Somehow, it works.

In truth, there is a safety net. The show is recorded, as a continuous take, 24 hours before it is broadcast, allowing just enough time to powder any unacceptably shiny noses. But the contents of the single TDK D3 digital video tape, set before a last-ordered and curried nation only a day later, do two things: they capture the joy and tension of that hour in minute detail; and they belie the 13 hours of audio-visual skill and daring that preceded it - 13 hours, in fact, that go something like this...

10:30am

Aswad's crew rolls in the band's flightcases. The band have been asked to soundcheck first, in a regimented schedule that will stagger the four bands. In fact, the schedule sent out to all the bands says: "These call times are sacred - mess with them and you will be dealing with the Director, and she's horrific when she's mad." Aswad duly arrive an hour and a half late.

11:00am

Today, sound supervisor Mike Felton expects about 95 inputs to come from the studio floor into his control room. This calculation is based on information, often sketchy, that he has managed to extract from busy road crews over the last week. Crowded House are coming and they've got a Maori choir in tow. Or it may be some log drummers. Michael Nyman may do a piano duet with Jools. It hasn't been confirmed yet. Aswad are late. On the floor, John Sweeney has already begun miking up drums and backline, and Mike's deputy Dave Lee is engaged in endless talkback with him as he labels up channels.

During the show, all the sources are manipulated in real time from one band to the next. All these channels go to a stereo mix which is fed to the engineering department, who record the total audio output - and, of course, the total vision mix - onto a single five-track digital video tape. There is one video track and four audio tracks; two of the latter constitute the finished audio master of the show, printed as it's happening. One of the spare tracks can be used to spin in a back-up, if needed, captured on a Tascam DA88 which records eight sub-groups from the overall mix, just in case. However, Mike and his team keep this to themselves. Post-production tends to prompt bands to interfere with the final product. No-one is allowed that luxury on Later, in order to preserve the feel of a live show. Neat compromise.

1:00pm

All the bands' gear is in. Crowded House, The Auteurs, The Cranberries and Aswad. Four bands, four complete stages, one in each corner. Jools' piano fills a gap between Crowded House and The Auteurs. The centre of the room is dominated by the Technocrane, an articulated camera that swoops alarmingly into every cranny like the Alien in pursuit of Sigourney Weaver. It spins, twitches, and flexes in readiness.

Up in the gantry, lunch is spurned. There is one Calrec 72-channel mixing desk permanently installed in Television Centre Sound Control Room One. Every week, extra audio mixers have to be brought in. Today we have a Soundcraft 600 24:8 and an Amek BC-2 8:4. There is also a BBC-designed Glensound modular mixer feeding eight auxiliary effect sends to the reverbs, all of which come back on one group fader. There are in fact 116 inputs including the reverb returns.

Channels 1-49 in the Calrec desk have compressor/limiters built in. The faders are also assignable to different channels - in other words, any given fader can control any channel. Fader 20 can control channel 4, or indeed channels 4 and 5 as a stereo pair. According to Dave Lee, in this situation it's an essential feature:

"It gives you ultimate flexibility. You can lay things out however you like. You get to choose where best to use stereo pairs, the expanders, and the compressors. Given that up here inputs are being changed right up to the last minute, it also means that instead of having to go to the bays, repatch things and then change the settings, we can copy things to other channels and juggle them around."

1:30pm

Aswad are soundchecking. The kick drum thumps the control-room air and levels are set. There's something weird about the faders on the Calrec, though. They're all up, on maximum.

"No they're not," says Mike Felton. "They're all off. On broadcast desks the faders are reversed, so 0 is at the top and 10 is at the bottom. The faders you aren't using are away from you, out of the way. It's standard BBC practice."

"The bands are jamming, basically, even though they're playing a strictly arranged song. It's unpredictable and you have to be ready for anything"

On occasion, Dave has had to mix on the fly using a BBC desk and one of the extra mixers hired in for the overspill. This, of course, was a mixer with normal faders. Moving from the Calrec to the hired mixer, he has to remember not to pull the faders down and the volume with it. Nightmare.

2:30-4:30pm

The Auteurs and The Cranberries soundcheck, one after the other.

"We look at each band as a fresh work of art," says Dave. "They play and you make them sound as nice as you can within the time given. We maybe put a little bit more reverb on things than you would in the studio, and maybe avoid pushing instruments back as much, in case somebody plays a break. The bands are jamming, basically, even though they're playing a strictly arranged song. It's unpredictable and you have to be ready for anything."

This is in spite of the fact that filming the songs is planned in meticulous detail. Producer Mark Cooper establishes a running order in consultation with the artists and their management, so the instrumentation for each song is known well in advance. In fact, camera scripts are prepared which include the lyrics for every verse and chorus of every song in the show. This also means that Mike can get the list of expected instruments down to the bare essentials. It helps if not everything used on the tour is pulled off the truck and dumped on the studio floor.

5:30pm

Mark Cooper is chatting with Neil Finn, who has arrived with a small child in tow. The atmosphere is surprisingly calm. Earlier, Mark had explained over coffee how he and Jools had arrived at such an original, and logistically challenging, programme format.

"We wanted to have a lot of different music in the same studio, live, without stopping and changing sets. Once we overcame that problem by using the four walls of a studio, the idea came that Jools would be a sort of ringmaster and that we could exploit the Technocrane. In a way, the feel of the show reminds me a bit of late-'60s rock shows, like Monterrey Pop where you had Otis Redding next to Janis Joplin next to the Byrds...

"Contrast is the strongest thing in it. I don't think modern music listeners are as uniform as the media likes to make them out to be. Youth TV programmes sell identity all the time; we're selling music. The idea for contrast came out of making the multi-band format come alive. The format grew because Mike Felton kept not saying 'No' to me. I kept saying: 'We need a solo violinist, or another eight-piece band with horns'. And he kept being foolhardy enough not to say: 'You can't do this. I'm already at breaking point, you bastard, stop!' They're brilliant, those guys.

"We do have dance music, but the emphasis is on human performance. There really haven't been many bands of the techno, house or rave genres that have been about performance. Hands on keyboards are probably the least interesting thing on television when it comes to music - even though the world is now dominated by keyboard/vocalists. With guitars and acoustic instruments you can see what people are doing, so you can feel connected. Television is good at intimacy; it's good for getting a closer view than you can at a gig.

"So much about being a star is about being protected, and this show is about surrendering egos. You're being asked to be very close to other bands on the floor and get off on each other - and musicians generally seem to like that."

5:45pm

Crowded House's FOH engineer appears in the control room. As the band soundcheck, he offers a suggestion here and asks a question there. Curiously, he's the only tour engineer to visit the sound supervisors all day. Mike and Dave welcome the contact.

"What we're doing here is sort of midway between live sound and CD"

"They're on the road with them all the time," says Dave. "They can tell us when lead breaks are coming up and the overall objective of the band. We know what their album sounds like, but we don't really know what sound they want. There's never any problem. Sometimes they'll come in and just say: 'That's great'. And other times - it doesn't happen often - they're very concerned and have very strong opinions. We just have to explain clearly the restrictions we're working under."

6:00pm

Now that all the bands have soundchecked, Mike has a complete list of outboard effects required on each number. He converts this into a hand-scribbled matrix, dividing up the units and their patches into an at-a-glance runsheet. In the rack there is an Aphex Aural Exciter Type C, a Lexicon PCM70, AMS RMX16 reverb, Klark Teknik DN780 reverb, two Yamaha SPX90 MkIIs, a PCM60, another DN780, and a Sennheiser System EM1036. There's also a Neve 33609 compressor/limiter on top of the desk..

"The effects there cover whatever we need, and we never need all at once," says Mike. "We have this chart which keeps a record of which unit is being used on which song, which we fill in as we rehearse. Most of the units have factory presets up to 40, so 41 is the first one you can use, 42 is the second one, and so on.

"We have a system of numbers relating to the order of songs in the show. We just store them in the relevant preset number to match the running order. For instance, in Crowded House's first number, the drums are being fed to the SPX90, so that's in memory 41. You can do all this sort of thing with MIDI these days; most of these units have got MIDI capability. Ideally, if you were making a more long-term setup, you could build it so that it was more automated."

"Don't forget," points out Dave, "that this starts at the beginning of the day and it's gone by the end of the day. There's another programme in here tomorrow and there was one here yesterday, so we can't actually have the sort of system you might have on tour - using typical memory settings. We have to be very flexible."

Much of the day is spent preparing these patches - balance, gain structure and all. It's only when the band arrives and starts playing that a real picture begins to emerge. Mike is sanguine.

"We're not using all eight units on each item. On some you might only use three. Having all eight available just gives us the flexibility."

6:30pm

The studio floor clears and an eerie silence falls. Incredibly, the BBC crews have succeeded in keeping the dinner break bang on schedule. But Dave and Mike have their own arrangements. Dave is unpacking another reel of analogue tape. Throughout the day, a Studer 818 quarter-inch machine has been recording every sound issuing from the studio floor, in stereo. There's also a Tascam 122 cassette machine and a Fostex D20-B DAT running all the time.

"You could say that what we're doing here is sort of midway between live sound and CD," Mike reflects. "On the one hand, you haven't got the disadvantages of live pick-up, and on the other you get the advantages of a performance. It's not technically perfect, you're getting a raw performance here, potentially with better quality than you'd get from a gig. And if you're listening on a digital stereo TV through your hi-fi, you're hearing more or less what we hear in the studio."

7:30pm

A hubbub from the floor indicates the return of the crews. We're listening to all this on Rogers LS5/8 monitors. The sound crew continue their mission to ensure that every microphone is correctly placed and every line is open, with constant reference to Dave and Mike up in the gallery. Down there also is a separate monitor engineer, sandwiched between Crowded House and The Cranberries and charged with keeping all the bands aurally happy. His audience is smaller, but he has almost the same amount of work to get through in the same time.

John Sweeney from BBC Sound rushes into the control room for the umpteenth time. This time, he can grab a break for a chat.

"Youth programmes sell identity all the time; we're selling music"

"We provide the mics, the stands, the cables, and the split feeds to the monitor desks," he says, laden with enough comms devices to shame a spy ring. "When I come in, I get the input lists for each band from Mike. I'll then go to the director to make sure we're rehearsing in the right order. Then we speak to each band's sound engineer and make sure that what Mike has got on his sheet is what the band are intending to play. If you start putting the wrong mics and equipment in and Mike doesn't know about it...

"Things can change every half an hour and that just doesn't work in television, you can't allow that to happen, particularly in this show. If you've got something to do, you've got to get on with it and get it done when you can. There's no messing around like you would do in mixing just music.

"Normally, in television, video takes precedence and sound matches the picture. The producer and the director of this show have a completely different outlook which I've never come across before. They're very conscious about the sound content of the programme. I'm in a unique position because I know what problems might go on upstairs, I know what time constraints the director has, I know the problems that the monitor guys have, and at the same time I can see if the drummer's having problems with his kit."

7:45pm

There's a hum from the studio circuits, and everything is systematically shut down to trace it. It takes over an hour. John tells the bands to adjourn to hospitality.

"On other programmes, the director will just say: 'Get on with it, I don't care what your problems are, just sort it. I'm going to carry on'. In, say, a sitcom, they will shoot the picture without the sound if there's a sound problem. In that type of show, the sound people don't even get a rehearsal. Here, the soundchecks come first, then we let the cameras come in and have their rehearsal."

9:00pm

Camera rehearsals begin. Every channel/fader assignment is monitored on a computer screen. Mike can see instantly which channel - which microphone, which instrument - is available on which fader. Today there are four bass guitar rigs, four drum kits, eight (or is it nine?) guitar amps, four acoustic guitars, a string quartet, Maori log drummers (no choir, to Dave's and Mike's relief), a grand piano, percussion, and a harmonium (if Jools does the duet with Dolores O'Riordan as discussed). All the electronic keyboards are DI'd.

"Often we'll have every channel set up, and at the last minute we'll need to move something," explains Dave. "Normally, you'd re-patch in the bay and manually change all the settings on the new channel. But I'll copy the settings to a spare channel - assuming I can find one - move the channel it's plugged into to another fader, and copy the settings back."

"We actually need two guest mics today," Mike notes, "and because of the pressure of channels, I've got them switched between mic and line on the one channel. I can switch between them when it looks like each guest is about to speak." He says this with breathtaking confidence. "Things just pan out by serendipity, usually. At the end of the show you find that every channel is full, and it's just as well there wasn't one more thing required..."

Dave looks up from the computer and announces: "We're on our last channel. We've got one left."

11:30pm

The studio audience is ushered in and spreads naturally round the periphery of the set. They're mostly friends of friends of the crew, the bands, anyone with a mate at the BBC. For this series, they hear the show through a simple PA, balanced against what the microphones are sending to the broadcast.

"In a sitcom, they'll shoot the picture without the sound if there's a sound problem... Here the soundchecks come first, then we let the cameras come in and have their rehearsal"

"We use it as reinforcement," explains John. "When you're in the studio you can obviously hear the drums, bass guitar, and lead guitar. What you can't really hear are the quieter instruments and the vocals, so we generally try and feed a bit of the vocals and some of the acoustic instruments to the PA. The radio mics come down one group fader which we feed gently to the speakers around the studio. From the same desk, we will also provide a feed to the monitor engineer which we call 'chat'.

This is the PA group from upstairs, the VT, and the hand mic. Through the monitors we can then say: 'Cue Crowded House', and they can take their cue. In the old days, we didn't do that. You could see it was a problem when the Floor Manager would say: 'OK, cue Crowded House', and nothing would happen. It's much easier to let people take their own cues if they can hear.

"The trouble is the amount of level you actually get out of these speakers is minimal. It's almost nothing. It's just enough so the people who are standing right in front of one can hear it, but you have to actually concentrate on it to know what's going on."

"Audience applause in our mix has to be grabbed very carefully," adds Mike. "The end of a number isn't too bad, but the start of the next one can be a bit awkward because the applause is still going. You've got to get rid of the audience before the snare goes boom! You're tailing the audience away and watching the drummer. The PA sound is totally different from the sound set up in here."

"That's the theory," says Dave, sardonically. "What usually happens is the audience is still fading and the drums go boom..."

12:00 midnight

At the beginning of the show, Jools plays a simple riff on the piano. The idea is that all the bands join in and jam together for a few glorious bars. The Technocrane pans 360° around the studio and comes back to Jools. Now you know why it never works.

12:05am-1:15am

During the actual recording, Mike and Dave mix frantically. Between each band Dave adjusts the reverbs and other outboard settings on the one rack. Meanwhile Mike re-routes the sends for the next tune, while Jools does the links. "How we actually achieve it in the small space of time it happens I don't really know," admits Dave afterwards. "We usually get about 30 seconds."

Also in this gap, Mike re-assigns the next band to the group of faders immediately in front of him. During songs, the eagle-eyed engineers look out for potential soloists, scheduled or unscheduled.

The Cranberries play a very quiet set with a string quartet. Earlier, Dave had highlighted the sheer dynamic range thrown at them by Jools' and Mark's eclectic tastes.

"The overall level of each band must be constant, whatever they do. I think tonight we might find that one of the strings in the quartet will play a lead line, so we'll watch for that particular string. But as it's an all-acoustic set, we'll just open everything up. On all the tight-mic stuff, we gate all the channels so we don't get too much spill from other areas. But when we've got instruments like violins and percussion we have to have more gain to get all the sound in the studio coming in. Then if you've got a rock band playing along with acoustic instruments, all the foldback spills into the acoustic instrument mics. When we rehearse, I'll be very selective about which of the acoustic instruments I'll have open. I won't have all of them open because it would just make the rest sound like a bloody mess."

1:20am

After the tapes have stopped rolling, Crowded House celebrate a glitch-free show with an impromptu rendition of 'Weather With You'. The small audience in the studio is delighted. So too are Dave and Mike. The elation is a mixture of released tension and genuine pride. One more, as they say, in the can. Oh, all right, cassette.

1:45am

Hospitality is awash with congratulations. The Auteurs are here, some of The Cranberries and most of the crew. Crowded House have already been bundled back onto the tour schedule and Aswad have no doubt reconvened in Notting Hill. Curiously, the affable star of the show is conspicuously absent. It really is later, now. Much, much later. But this time, without Jools Holland.