If it's crucial for you to charge your iPad while recording, you need to use a Made For iPad (MFi) interface. These are more sophisticated interfaces that connect directly to your iPad's Lightning connector and are able to transfer power and digital audio data simultaneously. At the moment, the only MFi models available are those by Apogee and Avid; only Apogee models charge your device.
As picking the right interface for your project can be a daunting task, I suggest checking out the excellent information page found on the Auria app's website, where you can find a complete list of audio interface (both USB and MFi) that are compatible with the iPad.
For Ella's song I built my portable studio around Focusrite's Scarlett 18i8 ($449). It provides excellent value for the money and is extremely flexible with its four mic pre-amplifiers, four analog line inputs (for extra high quality mic-preamps), one digital input in ADAT optical format, MIDI in and out, and SPDIF in and out.
The number of total inputs and mic-preamplifiers your interface should have depends on the type of recording session you are planning to do. There are two basic types of sessions in which you will record with your iPad: Studio sessions with overdubs and live sessions in which you record all the instruments simultaneously without overdubs. In general, a studio session requires fewer inputs, as you will be tracking/overdubbing one or two instruments at the same time.
For the "You'll Return" session, the Scarlett 18i8 allowed me not only to use the four built-in mic-preamplifiers, but also to add four external mic preamps. In this case, I used high quality preamplifiers configured in a portable API "Lunchbox" 500 format.
When it comes to instruments, drums usually require the highest number of simultaneous inputs — anywhere from a minimum of three (overhead, bass drum, and snare/high-hat) all the way to seven or eight (two overheads, bass drum, snare, high-hat, and toms). For a live session, since you have to capture the entire band without overdubs, eight inputs is really the minimum needed for a small band with drums, bass, piano and vocal.
If you plan to record more than eight inputs at the same time, you need an audio interface that features one or two ADAT digital connectors. Each will allow you to connect an additional unit of analog to digital converters with an extra eight inputs each.
Tune up your software: Last but not least, you're going to need the right software to record and mix your recording. The App Store features several decent applications that can serve as your basic DAW for the iPad; Auria and Cubasis are the only two apps that I feel confident recommending if you want to get close to a desktop-based recording experience, however. While both are capable of handling multi-track recording, mixing and audio editing, I usually use Cubasis for the pre-production phases of a project, while I prefer Auria for recording live instruments, editing, and mixing.
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