Thanks for your response! However, I don't think we're on the same page. Let me try explaining with an example.
Imagine a MIDI file containing a piano sound on channel 1 and a guitar sound on channel 2. My understanding is that setting a single-channel MIDI receiver to "Omni On" means it will effectively go "even though I'm getting messages on Channel 1 and Channel 2, treat them as if they were on the same channel". So if you send that MIDI file's messages to that Omni On MIDI receiver, you will hear all the notes from the MIDI file with the
same instrument sound. (For example, a guitar sound, if the Channel 2 Program Change message was sent last.) But Synthesia's "Omni" (All Channels) output setting is intended to do the exact opposite thing: It is intended to let you hear
different instrument sounds for each channel.
Here is
a description of the MIDI modes that helped me understand the concept of "Omni treats all channels as if they are one channel".
Here is
a video from 1991 that demonstrates a device in "Omni mode" (messages from Channels 1 to 6 are output with one channel's instrument sound) and then later in "Multi mode" (messages from Channel 1 to 6 are output with their own individual instrument sounds).
(The key word used to describe equipment that can produce multiple instrument sounds simultaneously seems to be "multi-timbral". I've seen the term "multi mode" used to describe MIDI devices that can receive multiple channels and use a separate instrument sound for each channel. However, as I understand it, a "multi mode" doesn't necessarily mean all 16 channels will be available. An 8-channel device might have a "multi mode", but wouldn't be able to handle all 16 channels, only 8.)
I don't have first-hand experience with Omni devices, just a strong interest in MIDI and understanding the channel modes described in the MIDI standards. I can understand my request to remove the word "Omni" from Synthesia is pedantic. But the way Synthesia uses the term "Omni" means the opposite of its intended meaning as I understand it. And Omni seems to mainly be a receiver setting, it doesn't really make sense as a sender setting. So my suggestion is it might be clearer to remove the word "Omni" from the name of the Synthesia output setting and just call it "All Channels".