Thanks for your patience, then, but I did not fully grasp the response, hence I give a wrap-up in case it help others:
I know that SPDIF is 2ch with PCM up to 192-24, and that 5.1 can be sent within a PCM frame as "non PCM data". This is not the problem. (BTW, one of my DACs sounds clearly better and more refined when fed 192-24 from RME Digiface usb, than it does from its own USB connection).
I go back to my previous contribution. My assumption was correct. The RME Digiface gets a SPDIF signal containing both PCM data and 5.1 data, multiplexed, out of which the existence in the stream of 5.1 is verified by connecting it to another AVR receiver. It is a common way of doing it, and the transmitter says "here are two options for you, choose the one you can handle". Therefore RME grabs the PCM part and shows it as a 2channel PCM reception. It was this multiplexing, as I call it, that created confusion for me as I was unaware of it.
This cause a new question: What happens with the 5.1 part of the transmission when the RME accepts the PCM part? Is it passed through or thrown away? If passed through, then e.g. a Dolby decoder positioned as a VST plugin in some DAW may be able to detect it, hence my question.
Background information on the "Multiplexing":
1. The TV service provider:
I asked a friend in NRK, the national (and main) broadcaster of Norway. NRK uses this format: "HE-AACv1, 64 kbps for stereo plus an AC3 carrier at 448 kbps. In the AC3 carrier there is multiplexing of stereo and 5.1."
This format is found on the optical out on the TV decoder, and yes, creates chopped noise when connected to the RME unit. BTW, this is the only TV channel in my menu creating this effect, so looks like NRK is alone in this market with an AC3 wrapper.
2. The (Samsung in my case) TV:
When this same signal format instead is fed the TV via HDMI, the TV outpours on its optical out a format that is probably stripped of AC3, in that now once of a sudden it is readable as stereo PCM by the RME unit. The RME units picks the PCM part of the (assumed still) multiplexed signal. Therefore, it is indeed correct to state that the TV outpours PCM on the SPDIF, even if it is only 50% of the total transmitted.
3. This multiplexed type of signal comes again in the AC3Filter software package mentioned in my last contribution. The user manual states: "Normally when Use SPDIF option is enabled, the filter publishes two output formats: SPDIF and PCM at the same time". See here: http://www.ac3filter.net/wiki/AC3Filter_%26_SPDIF. As there is an option to uncheck "send PCM as well", it was verified that the behaviour of RME Totalmix was as stated above.
BTW, again, (not in any way related to RME, but for completeness), one problem I had a little earlier on, was that I could not get hold of the Center/LFE channels for downmix into Front Left and Right. This aim has a reason; that these speakers are a much higher quality system. Searching for a solution, I found the decoder has a switch between STEREO or DOLBY. It seems clear that the decoder itself does not downmix, it just use either the stereo or the 5.1 part of the multiplexed signal. Thus, we discovered that many main tv channels in Norway, among them Eurosport that sends all the soccer games for the time being, DOES NOT downmix center/lfe into the STEREO signal delivered to Cable Tv companies. And since it is the Center channel that most often contains most of the "soccer speaker" commenting the match, therefore, those Cable tv subscribers either selecting STEREO in the decoder, or that do not have surround engine in their TV, for these customers, "the speaker" is almost inaudible, or at least, audible in a most unsatisfactory manner. So I am not alone having trouble grasping it all, it seems.
