4-channel recording and surround

Is it possible to take a 4-channel 4-track recording, and convert it into a Dolby 5.1 AC3 file in Audacity? I’m getting a 4-Track digital recorder, and I’m interested in two things. Firstly, creating music recordings that give the space & depth of the old quadrophonic recordings of the 70s. Secondly, producing surround sound videos using the 4-track recorder for the sound source. I would be using 4 microphones, positioned appropriately for quadrophonic sound in both cases. Given that I’ll have 4 tracks, not 6, would I be able to process my 4 tracks in such a way as to make them work as 5.1 surround sound, using Audacity?

“Dolby” is a registered trademark, and they own patents for their encoding system, so the only way to achieve “true” Dolby 5.1, would be to use a licensed (non-free) Dolby encoder. However, you can create your own 5.1 encoding (“MJZ 5.1”), which could be compatible with standard 5.1 systems.

To do this, you would need to create a centre channel, which could be a simple mix of the front two channels. The Front Left / Right channels may need to be “widened” a bit, which you could do with this Channel Mixer plug-in: https://wiki.audacityteam.org/wiki/Nyquist_Effect_Plug-ins#Channel_Mixer

The LFE channel should be a mix of all 4 channels, mixed down to mono, and filtered quite steeply with a 120 Hz low pass filter. You could do that with the Equalization effect (https://manual.audacityteam.org/man/equalization.html). To prevent the bass from being overpowering, you would probably need to roll off the bass below 120 Hz from the other channels.

To export the 6 channel mix, you would need to use the “Advanced mixing options” (see: https://manual.audacityteam.org/man/advanced_mixing_options.html)

For AC3 encoding, you will need to have FFmpeg installed (see: https://manual.audacityteam.org/man/faq_installation_and_plug_ins.html#ffdown)

That’s the basic idea anyway.
See also: https://en.wikipedia.org/wiki/5.1_surround_sound