A USB audio interface is a handy method of transporting audio signals from an audio device to a computer. The actual USB interface may be inside a microphone, or inside a mixer, or it may be a separate, stand-alone device. Let’s look at some assorted configurations, and sort out where each one is most useful.
A microphone with USB built in may be fine for recording a simple podcast, with only one person speaking. Most recent USB mics include a headphone amplifier (this is important). If you were to connect your headphone to the computer, you will hear a delayed version of your voice (very distracting.) Monitor directly from the headphone jack at the mic for best results.
If your have no plans to ever expand your podcast (it’s just you, and that’s the way it will stay), then a USB microphone may be perfect. But if you plan to expand…
A mixer gives you places to connect assorted microphones and other audio devices, including CD players, telephone interfaces (for interviews and listener interaction), portable recorders, etc. When using a mixer, the audio devices that you connect to it will have regular audio connectors, not USB. The microphones will have XLR plugs, while the other devices may have either miniplug, RCA, TRS, or XLR. Those devices connect with cables that ultimately terminate in 1/4″ TRS plugs to finally plug into the mixer.
All of the sounds will be blended and controlled by the mixer. To record the resulting mix, the audio can be sent to the computer by a USB interface. There are several stand-alone USB interfaces available. They simply take the 2 audio channels from your mixer, convert them to a digital format, and deliver them to your computer by a USB cable. Many mixers have the USB interface built right in. (If you already have a mixer, just add a stand-alone USB interface. If you are starting from scratch, you may find a mixer with USB that suits your needs.)
Early versions of USB audio interfaces were limited to 2 channels of record, and 2 channels of playback. This is fine for many applications. But starting with USB 2.0, you could move many more channels of audio into your computer at one time.
For example, if you have a round-table discussion with several microphones, each mic can be recorded separately on its own track in your editing software. This is in addition to the regular 2-channel mix, which is always available. So, if your mixer (USB 2.0 or above) has 8 inputs, it could send to the recording PC all 8 of those inputs on record channels 1 through 8, and the traditional stereo mix would record on channels 9 and 10.
This allows you the flexibility to send the 2-channel mix directly to the internet as a “live” feed, while giving you the ability to come back later to edit the overall recording, with full access to each sound individually.
Most of the mixers that send multiple channels to the recording, are still limited to only 2-channel playback from the computer. Be sure to check the specs carefully if that is an important feature to you.
There are stand-alone USB 2.0+ devices that give multichannel conversion in both directions.
USB comes in several flavors, and can provide a lot of flexibility to your setup. Decide what you need to accomplish, then choose the gear that best fits the need.