Audio Interface, Focusrite Scarlett 2i4
Audio Interface, Focusrite Scarlett 2i4

Why use an audio interface?

An audio interface gives a cleaner signal than a computer microphone. It can eliminate room noise. Musicians and podcasters use them with external instruments and microphones. Also an audio interface may offer a way to send audio to a Raspberry Pi.

Application- tv_commercial_silencer

tv_commercial_silencer monitors live audio, detects and automatically mutes commercials It compares live audio input to a database of “acoustic fingerprints” generated from previously recorded commercials.

recording and monitoring television audio

tv recording system
Television stereo audio line level outputs to Focusrite Scarlett 2i4 audio interface.

Configure macOS to use audio interface input

Go to System Preferences / Sound / Input. Select the interface.

Configuring Raspberry Pi Raspbian to use audio interface input

I’ve seen online reports of people doing this. I haven’t done it yet.

Digital Audio Workstation (DAW)

In Ableton Live Lite I set the input as 1/2 for stereo. Then I recorded audio, split and trimmed into individual clips, one per commercial.

mp3 files

I exported each clip as a mp3 file. mp3 maintains separate stereo channels. In tv_commercial_silencer I stored the files locally in directory data/commercial_mp3. The project .gitignore ignores the data directory and the recordings aren’t commited to the repository.

Appendix- audio interface configuration

input jacks

These accept 1/4 inch (6.35 mm) diameter phone plugs. Can use TR (tip ring, 2 contacts) or TRS (tip ring sleeve, 3 contacts)

unbalanced input

Ok for short cables < 20 foot long. If you connect TR plug, connection is unbalanced mono.

balanced input

Good noise rejection over long cable runs. Uses 1/4 inch phono plug TRS (tip ring sleeve 3 contacts) One wire carries -1 * signal in another wire, third wire is ground. If you connect a TRS plug, the interface attempts to process it as a mono “balanced” audio connection.

how to connect line level stereo input

NOTE: 3 wire stereo output from television must be split out to 2 mono TR plugs, then each plugged in to channel 1 and 2. I put TIP (gray) on channel 1 and RING (red) on channel 2.

input gain knob

Turn up as long as ring light is green. If ring light turns red, amp is clipping. Reduce gain. I set both channel 1 and channel 2 to approximately 1 o’clock position.

input switch

For recording line input use LINE not INSTRUMENT.


Attenuates input to avoid clipping. Don’t use unless necessary. I didn’t use it.

Direct monitor knob

Mixes between live input with no delay and playback which may have post-processing latency.

Direct monitor switch


source switch

For headphones. Can configure DAW so 1-2 monitors live input, 3-4 monitors playback.


Audio interface, Focusrite Scarlett 2i4

Support download files including User Guide scarlett2i42ndgenugenv1.pdf

Guitar Center

I bought from Guitar Center in Redmond WA and got excellent in-person advice and technical support from salesperson Alex.

Digital Audio Workstation DAW, Ableton Live Lite

bundled with Focusrite audio interface

Python Data Analysis to Automatically Detect and Mute Television Commercials


Recording multiple microphones in python

Discusses using PyAudio and Scarlett audio interface