Lesson 15

Understanding the Inputs and Outputs of an Audio Recording Interface

When I was starting out in audio recording, being able to decipher all the inputs and outputs of a recording interface is something that stumped me for a while. For instance, let’s take a look at this interface, it has 18 inputs and 24 outputs. Do you have an understanding of where all those inputs and outputs are? If not, then this video is for you.  I found a good exercise to help me understand all the connectivity of an interface was to look at pictures of interfaces and test myself to figure out all the inputs and outputs. We’ll do this test towards the end of this video.


It’s important to keep in mind that the main function of the interface is to route audio to and from the computer. There’s 2 types of audio that can be routed in and out of the interface, analog and digital.  Analog audio is an electrical current that represents a sound wave, digital audio is 0’s and 1’s that the computer can recognize and process.


Both analog and digital have several types of inputs and outputs.


Typical analog inputs are microphone inputs which are on an xlr connector, Line level inputs which are usually on a 1/4” balanced connector, but sometimes they’re on an XLR, and Guitar Direct Inject inputs which are optimized to accept the high impedance signal from a guitar. Ultimately, the goal of analog inputs is to send a line level signal to the AD conversion. A microphone has a very weak signal output and needs a preamp to be amplified to line level.  A guitar needs to go through a preamp too, but preamps have better sound quality with low impedance sources, and since a guitar has a high impedance output, it needs to go through circuitry to be converted to low impedance, then it can go to the preamp. Since preamps are expensive, most interfaces only provide preamps on some of the analog inputs channels, and the remaining channels will need to accept a line level input.


So here’s a schematic of the signal flow inside the interface when connecting a microphone. The signal goes from the microphone into the preamp, where it is boosted to line level, and then it goes to the AD convertor and to the computer.

If you plug a guitar in, the signal first goes through a transformer which lowers the impedance, then to the preamp, then to the AD conversion, then to the computer.


If you have a keyboard or an electronic instrument with line outputs, you would plug those straight into the line inputs, where it bypasses the preamp and goes straight to the AD conversion, and to the computer.


Same goes if you have an external preamp. Since it’s output is line level, You would plug it into a line input. Usually, a microphone input can also have the option of being used as a line input. Sometimes they will use a combo jack like this which can accept XLR or 1/4”


Now, it’s worth noting that when you plug a line level signal into the line input of an interface, most interfaces bypass the preamp, but there are some poorly designed ones on the market that do not bypass the preamp, they simply lower the volume of the line inputs and still route the audio through the preamp. In these rare cases, using an external preamp is completely pointless, because the audio still goes through the built in preamp where it loses fidelity.


There’s 1 more audio input that I didn’t mention, and that’s RCA. This is only used on consumer level sound cards, and you shouldn’t see it on a proper recording interface.


Now for the digital connectivity.   Since The most common types are ADAT, SPDIF, and AES/EBU.  With these, the interface is simply routing the digital information. To use them, you’ll need to purchase an external piece of hardware that does the conversion. 

Adat uses an optical toslink cable which can carry 8 channels of digital audio at a 1 times sample rate. If you don’t know what that means, then check out lesson 4 on digital audio.

SPDIF uses an RCA connector, and can carry 2 channels of digital audio per connector.

And AES/EBU uses an XLR connector and carries 2 channels of audio per connector. It can also use a DB-25 connector which will carry 16 channels of digital audio at any sample rate. 


Now headphone outputs are a bit of a wild card. Since a headphone output will require 2 channels of conversion, one for the left, one for the right, technically, it could count as 2 more output channels. Generally though, most interfaces don’t even count the headphone output in their channel output count, but some do, and some don’t. 


Now, let’s take a look at some interfaces, and go over the inputs and outputs.


Here’s a focusrite 18i20


For inputs, it has 8 combo jacks which can accept XLR for plugging in a microphone, or 1/4” for line inputs. These are the analog inputs, and all 8 have preamps.  The other inputs are digital inputs. On the far left is a SPDIF connector which is capable of 2 channels, so now we’re at a total of 10 inputs, and here’s the ADAT which adds 8 channels and brings our total number of inputs to 18.  and for outputs, we have 10 line outputs which are analog, and then 2 channels of SPDIF and 8 channels of ADAT, bringing the total to 20 outputs, hence the name 18i20.  This interface also has 2 headphone outputs on the front panel which they don’t include as part of the output count.


Now, let’s take a look at the Universal Audio Apollo 18x24


for inputs we have 8 analog  and 10 digital. Here’s the 8 analog inputs. 4 of these analog inputs have a preamp, and can have a microphone plugged in. Since they didn’t use combo jacks, this is where you would plug a microphone in to channels 1-4.  By having a separate input jack, it’s a clear indicator that the line inputs do not force the audio to go through preamp. That’s a good thing. And on the front panel is Hi-z also called high impedance inputs for channels 1 and 2 for direct injecting a guitar. Although channels 1 and 2 have separate connectors for hi-z inputs, microphone inputs, and line level inputs, you cannot use all 3 simultaneously. You would select through the software which one of the 3 is the active input.

And the digital connection has 1 Adat. Although there’s actually 2 Toslink connectors for the input, the second one is only used so there is not a loss of channel count at 2 times sample rates. I explain this in lesson 4 on digital audio. 

And there are 2 channels of SPDIF digital inputs. All for a total of 18 inputs.


For outputs, we have 10 analog outputs right here, 8 digital outputs through ADAT, 2 digital outputs through SPDIF, and 4 analog outputs through the 2 headphone ports on the front panel. And that makes up the 24 outputs.


and one more example is the metric halo ULN-8. This is a very simple design. It has 8 analog inputs and outputs, and 8 digital inputs and outputs. They use the DB-25 connectors, which connect to a breakout cable with 8 XLR’s. There are 8 preamps, so each channel of analog input has the option of using the preamp, or the line input. If you want to plug a microphone into the preamp, you would plug it into the break out cable that goes into here. If you want to use the line input, you would connect it to the breakout cable that plugs into here. Although you can connect both a microphone and a line input to the same channel, within the software you must select one or the other as the input.  Channels 1 and 2 have a hi-z input on the front panel for connecting a guitar. If you want to use the hi-z input, you must select it in the software, and that will disable the microphone input, and the line input for those channels.

and it has 8 channels of digital inputs and 8 channels of digital outputs through AES/EBU on this DB-25 connector.