This short post originates from a chat I had with a friend a few days ago, as I realized the reason he was struggling a bit was because he hadn't really a clear idea of what an audio interface is. Since I've seen quite a few similar misunderstanding in online forums, and it's a slow Sunday morning, it seems a good idea to make post out of it.
So, everybody knows what an audio interface is, right? Well, maybe. But let's make sure.
When people think of "audio interface", often they think of a device that allows them to connect a XLR or jack cable to a computer to get sound in.
And that's broadly correct: most interfaces sport nice XLR and jack sockets to which you can connect microphones or instruments, and the result is that the sound you make ends up in the computer.
However, it's worth knowing that a "regular" audio interface is usually a package of different devices, conveniently put together in a chain so that you don't have to think much about it. Each device does its thing, and the whole result is that, indeed, you can get your microphone or instrument or line signal into the computer (to be used, for example, by a DAW).
In strict terms, the "interface" bit is only the part that takes digital audio signals and translates them into USB, Thunderbolt or Firewire or younameit.
In other words, an interface's not gotta need having any preamp, any A/D converter, nothing.. only the ability to take a digital audio signal (S/PDIF, AES/EBU, ADAT, proprietary format etc) and convert from and back to USB, Thunderbolt etc or directly into the computer's motherboard I/O bus.
(Technically, the "digital audio signal" which is converted by the interface doesn't have to be standard at all - just a form of PCM sample stream that the translation circuitry understands)
A typical audio interface
The boxes we call "audio interface" simply package together the most common facilities needed to have something to translate.. stuff like preamps, line input circuitry and A/D converters in general, plus of course the physical sockets, knobs for gain and volume controls etc.
Below there's a sketch of how a typical interface is put together (from the "signal coming in" perspective).
It's easy to see that different boxes can put together slightly differently and with different things inside.
The one thing that all interfaces will have is the last bit - something to convert a digital audio sample stream into a USB/Thunderbolt/PCI data stream and vice versa.
For example when I record at home I rarely need more than two or three channels, for which I have and use external preamps; but I also enjoy using outboard, so I have a few signal processing boxes that I use when mixing... the result is that my main audio interface does not have preamps at all, only a lot of line inputs. It still has direct monitoring of course.
In the studio installation I use an external 8 channel converter box which outputs AES/EBU digital audio, so my audio interface there consists only of a card into a PC which accepts (guess!) AES/EBU and brings the data into the computer's PCI.
Know what you are controlling
Why all this is useful to know?
Many reasons. For one, because it allows you to understand what you doing when you're turning that gain knob (you are actually increasing the gain of the preamp device in your interface) and why with a typical interface you need to keep it down (because there's no gain control before the A/D converter, so a hot signal will stay hot and clip the converters.
It also helps to understand why there may be no gain control on an interface, or no preamps, and even decide if that lovely vintage analogue preamp may replace your interface (of course it can't, you will still need all the bits after "preamp").
Or why an analogue preamp with a digital card (say the nice ISA One that I'd loaned to my friend) cannot be plugged directly into the computer! Because "digital output" means it's got an onboard A/D converter which outputs AES/EBU, not necessarily that it has an USB output (you still need the interface bit for that).
As usual, the more stuff you understand, the easier life's for you.. so hopefully this will help.
Have great recordings!