Update (19.02.2011): I've also added a slower set of scratches which highlight the sound differences between them. This really makes clear that my vinyl is battered, my CD deck has ground issues and BabyScratch introduces a lot of digital distortion.
This was in response to my earlier post which considered the realms of scratching music sources on an iDevice.
From the few I've looked at so far, I would recommend to any developers introducing scratching to a DJ app;
- Ensure the best sound quality possible. Currently TapDJ and DJay offer the best, but this could be improved. Especially at slower speeds, this is where any digital artefacts in the sound really do ring out (listen to the slower BabyScratch app test as an example).
- Ensure the area for scratching (whether a deck representation or a waveform) is really suitable for it - ie offering plenty of space for the fingers
- Ensure the track responds in accordance with the users movements and what you'd expect to happen with a turntable (ie not too slow and not too fast when starting the audio again from a finger-tip held position)
- Definitely offer split audio output...this allows the user to make full use of their own or club mixer and the physical cross-fader which will react much quicker than a virtual cross-fader within the app.