Where three worlds meet: documentary film, apps, and classical music

We are standing in Hampstead Town Hall where we'll be shooting the video for our app: John the Camera, Chris the Sound, Laura from the orchestra, the location manager and me. Well, we aren't actually in the hall, because that's occupied by the touring cast of Singin' in the Rain, who are paranoid about security. We're perched on a balcony twenty feet above ground.

And for the same reason we can't take any test footage here either. We're thinking about the light – we can't afford to light a whole orchestra, so we're delighted at the big windows that flood the place with daylight. 'Which way would the orchestra normally face?' I ask. 'The doors' , Laura replies. 'Umm – could they turn round so their faces are lit from the windows, perhaps?' Worried looks. 'I don't know about that – they need natural light on their parts to play properly.'

As it turns out, they decide it's okay, but this is just one of dozens of logistical questions we've encountered setting up this project. The musicians are passionate about the music, but also about the authenticity of their performing style. Parts are photocopied from early 19th century originals in the British Library. The drum heads are made from calf and goat vellum. And the wind instruments have no valves or stoppers, so they're almost impossible to tune – at least to modern tastes. But then Beethoven's tuning wasn't the same.

We have entered a world where a timpanist has different drums for every period of musical history, and twenty sets of drum sticks. Where oboeists make their own instruments, and every player is an authority on some arcane aspect of musical history.

And we're trying – delicately, tactfully, without showing our ignorance or offending sensibilities – to marry these considerations with what an unschooled iPad owner will find engrossing.

The developers are inventing imaginary Personae to do imaginary user testing. The DP is trying to source something like Errol Morris' Interrotron, so the interviewees can look at me when they do their demos, but appear to the user to be looking straight to camera. The binaural sound recording headphones have been subjected to the rigorous scrutiny of the orchestra's approval system (and passed – hooray!). The GoPro camera may or may not be able to shoot at 100 fps for slow motion without distorting the image – tests so far inconclusive.

It's endless, but it's useful and it's all part of what it takes to bring together three very different worlds: documentary film, apps, and classical music. I'm just gagging to get to the bit where we actually capture their magic – and pray that it is magic, and they're not over prepared, under prepared, or just generally freaked out by the process.

Still, they've all done outreach, sat in rooms full of yelling Primary 2s and disaffected Key Stage Threes. They can do this. We hope.