How to record system sound without video?

Question

I installed Soundflower with brew cask install soundflower and I see it listed:

$ ffmpeg -f avfoundation -list_devices true -i ""
[AVFoundation input device @ 0x7f9a32515e80] AVFoundation video devices:
[AVFoundation input device @ 0x7f9a32515e80] [0] FaceTime HD Camera
[AVFoundation input device @ 0x7f9a32515e80] [1] Capture screen 0
[AVFoundation input device @ 0x7f9a32515e80] AVFoundation audio devices:
[AVFoundation input device @ 0x7f9a32515e80] [0] Soundflower (2ch)
[AVFoundation input device @ 0x7f9a32515e80] [1] Built-in Microphone
[AVFoundation input device @ 0x7f9a32515e80] [2] Soundflower (64ch)

I found several threads on recording the screen without system sound (one, two, three) and one about recording audio on Linux with ALSA. I could not find how to record audio only, without video.

I also tried this command to record the audio along with the screen ffmpeg -f avfoundation -i "1:0" ~/Downloads/Screen.mov, and I got an error Conversion failed!.

How can I record system sound to a file with FFMPEG?

Answer 1

With 1:0 you're including the video input from FaceTime HD Camera. Try:

ffmpeg -f avfoundation -i ":0" output.wav

This only selects audio device 0 which is Soundflower (2ch).

See avfoundation input device documentation for more info.