Do I still need a DDC in between?

@moonwatcher The 38fs is the jitter rms level in the femto CO (crystal oscillator).  The jitter level at outputs can not be that low.  Here are some measurements taken from coaxial output of various DDCs by Goldensound for their RMS electrical noise and jitter noise at 48khz.

                                  Jitter(rms)   Electrical(rms)   Jitter(peak)  Electrical(peak)

Holo Red                     73ps           2.97mV                213ps           12.9mV

Singxer SU-6              75ps           3.3mV                  225ps            13.7mV

Singxer SU-2 KTE      85ps           3.2mV                247ps            13mV

Gustard U18             453ps           5.2mV                1133ps            22mV

You could see U18 jitter and electrical noise performance is lagging.  That is why I mentioned I could pick up better DDC like SU-6 or micro DDC later on if needed.

@mattw73  Thank you for the recommendation. I like that the cable is affordable, and Cat 6a is more than sufficient for audio applications. I’ll gladly take a $15 cable as long as it works. For the same length, a DH Labs Ethernet cable would cost ten times more (around $300). I’ll definitely give it a try.

@lanx0003 ah, that Singxer SU-6 looks a lot better...Thanks. Just wonder what numbers the OP would end up with going directly from his Volumio Rivo Plus to the LAIV Harmony DAC via I2S? Would they be helped by the Singxer SU-6? 

Thanks for the input. It is telling that often specs that are advertised aren't always necessarily the ones to be basing purchasing decisions upon. 

@moonwatcher You are welcome.  It is interesting that you are curious about the Rivo+ jitter performance.  There is no measurement yet since it is just released but I found the measurement for Motivo, which is basically Rivio+ plus the DAC and headphone amp.  Their streaming circuit and performance is neck to neck according to iiwi but the Rivo+ has a slight edge.  See below.

The average (Lt / Rt channel combined) RMS (or so-called 1-sigma) jitter is around 8.2ps (I am not sure which output this measurement is taken from though). See yellow highlighted.  Quite impressive.  

So instead of calling internal clock in dac I2S clock, it is most commonly or rightly called the master clock, this clock positioned adjacent to digital to analog conversion circuitry. Only a ddc or streamer with I2S clock can replace the internal master clock in dac. Using usb or any other input cannot defeat or bypass the internal master clock. The only reason external I2S clock can replace this dac master clock is it's use of I2S protocol.