7 min read
Choosing an audio interface for film sound work
Converters, preamps and a clock in one box. What matters for post production sound, and what is marketing.
An audio interface is the bridge between analogue signals and your computer. It contains microphone preamps, analogue to digital converters, digital to analogue converters for monitoring, a clock and a driver. Everything else on the spec sheet is packaging.
What each part does
- Preamp: raises a microphone level signal to line level before conversion. Low noise matters more than gain range.
- AD converter: samples the analogue signal at your chosen sample rate and bit depth.
- DA converter: feeds your monitors and headphones.
- Clock: decides exactly when each sample is taken. Jitter here is measurable but rarely audible on modern units.
- Driver: how the operating system talks to the box, and the main source of latency complaints.
How many inputs do you actually need
For dialogue editing and mixing, two inputs are plenty; most of your work never touches an analogue input at all. For ADR you need one clean preamp and reliable talkback. For recording foley with several microphones, four to eight inputs starts to make sense.
Do not buy channel count you will not use. A quiet two channel unit beats a noisy eight channel one every time.
Latency, in plain terms
Latency is the delay between sound entering the interface and coming back out. It is set by the buffer size, the sample rate and the driver. At 48 kHz with a 128 sample buffer, round trip is typically around 7 to 10 ms, which is imperceptible for ADR. Raise the buffer for mixing sessions with heavy plugin counts and lower it for recording.
Sample rate and format discipline
Set the interface to the sample rate of your project, normally 48 kHz for picture. Mismatches produce resampling somewhere in the chain, and each conversion is a chance to introduce artefacts or, worse, an off speed playback. If you inherit files at 44.1 kHz or 96 kHz, convert them deliberately with a proper sample rate converter rather than letting a driver do it silently.
You can check the true sample rate and bit depth of any delivered file in the inspector on this site before it reaches your session.