LtRt Matrix Decoder

[carl]

Out of interest, how are you choosing the settings when using this plugin? Are you listening and adjusting "in real time" or just setting them based on some intuition/guesswork?

Up until now there have been no controls for audio processors in DCP-o-matic: they just do what they do and you either like the results or not. I'm wondering if we'd need controls, or whether they would even be useful given that I'm guessing most users don't have a 5.1 monitoring system to listen on.

[IoannisSyrogiannis]

Hello Tom,
given the opportunity, during the last days, I was trying to use the plug-in.
I found two significant obstacles that I am yet to surpass:

My (very) small experience with Reaper, which makes it an endeavour to try and set a 5.1 setup on it, and then wondering „Do I need to have a track that is 5.1 for Lt Rt, or two different tracks, or a stereo one?“, „do I need to use a plug-in, in order to set the 5.1?“, „if yes, does the decoder goes before, or after?“ and so on, and so on.
The difference between your videos and what I see on my ten years' old Mac, on what I see on the plug-in window. I see nothing bellow the main menu, which is not shown on your videos. No X/Y axis, no parameters to set, except on another window, some trigger-release times and some info that are not even in the description.

Not being familiar with Reaper, the link for the plug-in interpretation wasn't too clear for me as a noob either.

Bottom line, is there some way I can read/figure out what I need to do from A to Z?
Where should I look for guidance?

[MPdecode]

Hi,
Please give me a couple of days. I’ll post a How-To video, which should be easy to follow.
Thanks,
Tom

[Kewl]

To Ioannis Syrogiannis:

Reading your reply, I'm not too sure what's not working, but, here's my two cents:

In Reaper, the Master must be six channels and the audio track must be also six channels. Insert the LtRt Matrix Decoder on the audio track, put your two channels audio file in the audio track. Testing with stereo pink noise seems to indicate that it's working.

Here's a question for the plug-in's developer. I seem to remember that the S channel must have +90° phase shift when sent to the Left Surround channel and a -90° phase shift to the Right Surround channel: are you applying such a processing?

Capture d’écran, le 2026-01-28 à 08.25.33.png

[IoannisSyrogiannis]

Thank you both!
I haven't yet, but I will try the Pink Noise test. It does seem like a good idea.
Yet. having downloaded the .jsfx file from the forementioned github project and having it copied in the Reaper folder, the head of the window reads:
JS: LtRt Matrix Decoder.jsfx - (track #) (name of wav file in quotemarks)
there is no "with Logic Steering"
@Kewl
the height of the window of the plugin ends just under the "Edit..." button. No handles or monitoring, except if I choose Param>Parameter Modulation, where I may choose to check Audio control Signal (sidechain), LFO, and Link from MIDI or FX parameter.

Reaper is up to date, but my Mac is still running Monterey 12.7.6
I tried to reboot, just in case something was off due to update. No change.

Edit: I hope I am not derailing the thread here.

Edit2: I tried to download again the plugin. It seems that something was wrong with the original download.
Sorry for all the irrelevant info...
The pink noice trick is creating a center channel

[Kewl]

Yes, the plug-in must be installed into:

Code: Select all

~/Library/Application Support/REAPER/Effects

[MPdecode]

Hi Kewl,

Interesting question about the 90° phase shifts!

I understand that +90° phase shift in LS and -90° in RS sometimes appear in home/broadcast contexts (e.g. optional in Dolby Digital encoders for LtRt downmix compatibility from 5.1 discrete mixes), but it's not a theatrical requirement AFAIK.

Of course, in Dolby Stereo, 90° phase shifting is used as part of the encoding process when matrixing the mono surround into LtRt (thus creating the 180° differential for extraction).

Dolby's own 5.1 Channel Production Guidelines recommend placing mono surround equally in LS and RS (which is exactly what the plugin does here, defaulting to -3 dB surround level). I didn't include extra phase processing to keep things lightweight, authentic to contemporary cinema processors, and CPU-friendly.