ac3 channel doubling

[Thunderbolt8]

I want to change a mono 1.0 ac3 track into a mono 2.0 track by doubling the channel. so rather have L, R instead of just the center. how can I do that?

[sneaker_ger]

ffmpeg -i "input.ac3" -ac 2 -b:a 448k "output.ac3"

[Thunderbolt8]

why 448k? is this operation lossless?

[sneaker_ger]

448k is the bitrate. You can choose some other bitrate, it is just an example. The operation is not lossless.

[Thunderbolt8]

I need it to be lossless. simply like copying a channel. like wavewizard can do with wave files (I could convert the file to wave or flac, but I want to see if this can be achieved as well with smaller file size)

[nevcairiel]

This is not possible, you need to re-encode to change channels with pretty much any compressed format like ac3, and re-encoding a lossy codec is not lossless.
If you want it to be lossless above anything else, you need to convert to a lossless codec, and not ac3 anymore.