Convert Stereo to 5.1 Without Upmixing

[koberulz]

I just want to map FL to FL, and FR to FR, and have the rest of the channels stay silent. Is there an easy way to do this? All I can find on Google is ways to upmix, which I don't want.

[johnmeyer]

It takes two seconds to do this in Vegas, if you have access to that NLE.

[ProWo]

Try ffmpeg -i "yourvideo.mkv" -filter_complex "[0:a]pan=5.1(side)|FL=FL|FR=FR[a]" -map 0 -map -0:a -map "[a]" -c:a copy -ac 6 -c:a ac3 -vn output.ac3

[koberulz]

I'm confused. You seem to be setting the codec twice?

[ProWo]

-ac 6 isn't the codec, it says to ffmpeg you wish 6 channels

[koberulz]

It's got both -c:a copy and -c:a ac3

[hello_hello]

The foobar2000 channel mixer DSP will do it. There's a portable version here:
https://forum.videohelp.com/threads/396860-foobar2000-portable-%28for-audio-encoding%29

I'm adding it to the playback chain in the screenshot below (as you can see by the output meters), but you can add it to the conversion chain. Under the General tab you select the number of output channels, then deselect the ones you don't want to contain audio.
Under the Upmix tab you can select "copy" or "surround", the former being what you want, as best as I can tell by listening to it.

The second screenshot is Audacity.
Import your stereo audio.
Under the Track menu add a new mono track. Then add another one.
Add a new stereo track.
Click on the area to the left of the original stereo track to highlight it, then Edit/Copy. Paste it into the second stereo track.

Under Edit/Preferences/Import-Export you need to check "use custom mix" otherwise Audacity will downmix and export it as stereo.

[koberulz]

I have absolutely no idea how to export anything with that foobar thing.

[hello_hello]

You don't export. You add DSPs to the conversion chain and convert. If that's too hard, do it with Audacity.

[koberulz]

What conversion chain? Convert how? Eventually I have to save an output file, right?

The Audacity method produces four channels of audio, doesn't it? And how do I know which channel is which when I'm exporting?

[hello_hello]

I think I misread your opening post. I thought you wanted to duplicate the stereo channels in the surround channels, without any sort of surround sound upmixing, but reading it again, I gather you want to keep the stereo channels and add four silent channels for 5.1.

Everything I posted earlier still applies though, but for foobar2000's channel mixer DSP you'd configure it for a 6ch output and disable up-mixing completely, add it to the conversion chain and convert your source file. There's no instructional video on the way, so if you've never had to convert and save a source file using a GUI before, maybe you should try Audacity instead. I showed you the correct channel layout. The only difference would be you don't need to copy and paste the stereo channels into the surround channels as I originally thought you were wanting to do.

[ProWo]

It's got both -c:a copy and -c:a ac3

You are right. Correct commandline is
ffmpeg -i "yourvideo.mkv" -filter_complex "[0:a]pan=5.1(side)|FL=FL|FR=FR[a]" -map 0 -map -0:a -map "[a]" -ac 6 -c:a ac3 -vn output.ac3