You are correct
@KMO , it is already a multichannel PCM signal that does not need anything but PLIIx applied to it to get audio to the surround back speakers if it is a multichannel PCM 5.1 signal. Yes, the vertical row indicates sound modes. They are also indicative of the input signal. Multichannel PCM will display as MULTI CH IN. If it is a 5.1 signal, PLIIx can be applied to it so that sound is output from the surround back speakers. Matrix mode could also be applied to it. Denon has since removed their non matrix mode that simply duplicated the surround signal to the surround back channels. I liked it. Notice the " + " when applying PLIIx to DD 4,5, 5.1 or DTS 5.1 signals. Anyway, not the issue
@Dougey_Jones is actually having here and the RZ50 has the new Dolby Surround up mixer.
I also have this issue with my Onkyo and Mac. If my Mac is configured for multichannel output using the Audio MIDI Setup, 2.0 content will be sent out as multichannel PCM 7.1, if set to 8 channel output, and sound will be output only from the front channels, though bass management works outside of Pure Direct and Direct mode. I think sound from the surrounds was mentioned by
@Dougey_Jones but I'd be willing to bet it was in Dolby Surround mode and it bled into other channels as usual. These machines do not work like disc players when sending two channel or multichannel signals. Put a DD 2.0 DVD into a disc player and it will output a DD 2.0 bitstream, or a PCM 2.0 signal if converted, over HDMI. Put that same disc in my Mac configured for multichannel output and sound will be heard only from the front channels when applying Dolby Surround or DTS Neural:X/Virtual:X. "All Channel Stereo" and "Mono Music" will be the only modes that will produce sound from ALL speakers. If I change the output from 8 channels to 2 channels and set the speaker configuration to Stereo in the Audio MIDI Setup in my Mac, the receiver will recognize and display a PCM 2.0 signal and the up mixers will actually work. Configure for Stereo but forget to change from 8 channel output to 2 channel output and the Dolby and DTS up mixers will still not work when applied. Yet, regardless of the settings, AC-3 Passthrough can be selected in the Apple TV app for Mac and the Dolby and DTS up mixers can be applied and actually work with DD 5.1 signals but not PCM 2.0 signals. There are many other ugly audio characteristics concerning Macs but that is for another thread.
@Dougey_Jones is not configuring his Onkyo for 2.0 so the Denon X4700 issues don't really apply here. Multichannel PCM signals from a disc player or certain streamers will not have this issue with the RZ50's up mixers and a 5.1, 5.1.2, 5.1.4, 7.1, or 7.1.2 configuration. I haven't used Windows since 7 so can't comment on 10. The Dolby Access app for Windows and XBOX makes Atmos available using Dolby MAT and also contains a Dolby Surround up mixer, or, at least it does on the XBOX. So, when I use it on my XBOX and it up mixes a non Atmos track, my Onkyo will actually display Dolby Surround as the input signal as well as the sound mode if I select the Dolby Surround up mixer in the receiver. This concerns games and streaming service app output has changed in the past with updates. I just don't use them in the XBOX anymore. Those far more familiar with Windows 10 PCs and the Dolby Access app may be more helpful concerning output settings. But, I do know how frustrating it is to try and up mix a two channel signal using Dolby or DTS and only be able to get sound from all channels using All Channel Stereo or Mono Music. I have to set my Mac to 2 channel output and Stereo for up mixers to work properly but that raises others issues. But, again, those issues are for another thread but it is annoying to sit on the couch and browse forums on the TV while at the same time watching Youtube videos and not be able to steer the dialogue to the center channel with an up mixer if 8 channel output is set in the Mac. Best of luck to
@Dougey_Jones and anybody else with similar issues.