Video & audio editing neophyte here, so please forgive any use of incorrect terminology.
I am using Shotcut to edit commercials out of OTA TV that has been DVR’d. This is US broadcast, so the source is mpeg2 video with 6 channel AC3 @ 384kbps or (rare & happy occasion) 448kbps. I’ve been editing under linux for a while, but I’ve also tried Shotcut under Windows and macOS. What I’ve noticed is that when I set my audio settings to 6 ch (Settings->Audio channels->6(5.1) and do my work, both linux and Windows will mix those 6 channels to stereo to play out my speakers. On macOS, I just hear channels 1 & 2. The exported audio is fine, it’s just that for monitoring purposes during editing I’d like to hear all the channels mixed down to 2, since my setup is stereo. I know I can set the audio channels for the project to 2(stereo), but I’d rather not, for two reasons: 1) I normally run a 2-pass audio normalization filter on my audio and I end up using different settings for stereo vs 5.1; and 2) I have to remember to pick 5.1 for the output.
This occurs in macOS on two different environments I’ve tested: 1) macOS 10.13 on mid-2011 iMac 27", and macOS 11.2 on M1 mini.
Is there a setting somewhere in Shotcut to have the audio mixed down to stereo for monitoring during editing, or perhaps a setting in macOS?
Shotcut tries to open your audio device with the number of channels configured in the project. If it can not do that, it retries to open with stereo. In that case, Shotcut only outputs the first two channels. Based on your description, I expect that your Windows and Linux installations are successfully opening 6 channels, but your Mac is only opening 2 channels. Alternately, your mac might be successfully opening 6 channels, but the mac itself is only outputting the first two channels.
You can view the application log to see how many channels were opened.
For example, when I configure a project for 6 channels on my windows computer:
[Info ] <MLT> Failed to open audio device: DirectSound CreateSoundBuffer: Invalid parameter
[Info ] <MLT> [sdl2] Try alternative driver: directsound
[Info ] <MLT> [sdl2] Open failed: DirectSound CreateSoundBuffer: Invalid parameter
[Info ] <MLT> [sdl2] Try alternative driver: winmm
[Info ] <MLT> [consumer sdl2_audio] Unable to output 6 channels. Change to 2
[Info ] <MLT> [consumer sdl2_audio] Audio Opened: driver=winmm channels=2 frequency=48000
Not other than the project setting which you described above.
If your mac is failing to open 6 channels, you should check if there is a setting to change it to support 6 channels. If it is successfully opening 6 channels, then look for a downmix setting in your Mac.
