To convert stereo soundtracks to 5.1, I use Pro Tools Ultimate with the Waves DTS neural upmix, Penteo Upmix, and Halo upmix plugins.
Upmixing stereo with Waves DTS neural upmix and Izotope RX loudness control in Pro Tools Ultimate
To normalize the sound to a loudness and dynamics level that works with Waves DTS neural upmix, I use Izotope RX loudness control.
To use the Waves DTS neural upmix for a Dolby surround or an LCR mix, I normalize the mix first. I try to avoid a true peak over -6 to avoid clipping when the DTS neural upmix plugin runs. A TV mix often has -23 integrated loudness with max true peak -1. In this example, the stereo mix has an integrated loudness of 21.7, and the true peak is 0.5. If I set the plugin to -28 integrated loudness, the true peak goes to -6.8, and it will not clip in the upmixing.
I choose Multichannel plugin – sound field – DTS neural upmix (stereo 5.1) insert.
For a standard upmix I use the auto setting but lower the LFE by -7. When doing an LCR mix, the Bass Management is turned off, and the Depth slider is moved all the way to the front.
To save the 5.1 mix, I use Bounce to disk.
Cinemas started with mono sound.
Then Dolby Stereo in the 70s used 4 channels extracted from the stereo track: Left, Right, Center, and Mono Surround.
In the 90s, compressed 5.1 Dolby Digital, DTS, and SDDS were used with discrete Left, Right, Center, LFE, Ls, and Rs channels.
Today digital cinema uses uncompressed 5.1, 7.1, and Dolby Atmos.
DCP specifications for film festivals often say that the sound must be 5.1 or specify that a film should have at least an LCR mix with dialogue in the center channel.
Some cinemas that screen archive DCPs with stereo sound can decode sound in Dolby Prologic II or similar. But a lot of cinemas only have the option to play 5.1. Without a center channel, the dialogue will sound like it comes from the left side if you sit on the left side of the auditorium, and the right side if you sit on the right side. If you sit in the center, you can get a phantom center, but only some seats will get this effect, and phase cancellations between the left and right speakers can cause an unpleasant effect. In 5.1 mixing, there is a best common practice to avoid using the same sound in two different channels at the same time to avoid these phase issues.