so just buy a transport that is reliable and has the functionality you like and the Synchro-Mesh will make it sound no different than the most expensive and highly regarded transports?
You got it. It's that simple.
what creates jitter?
Anything that causes noise on the power or grounding systems or otherwise affects the timing of the bits. Slow risetimes in signals can cause jitter because the noise on the power supply changes the point in time that the gates detect the signal changing. In cables, its the bandwidth and dielectric absorption that can move edges and distort the signal.
dedicated transports seem to work better than CD players used as transports, does the DAC side of the CD player create jitter?
In dedicated transports, the manufacturer probably makes more effort to create a better S/PDIF output signal and reduces the path of the master clock to the output signal.
The D/A part of the CD player is certainly affected by the same jitter, but the jitter is usually less than what you see at the S/PDIF output. The jitter starts at the master clock in the CD player and is affected by the associated circuits, power supply, ground noise, decoupling caps used and implementation. By implementation, I mean what comprised the circuit. There are many ways to do the clock circuits and most are not optimum for minimizing jitter. The more stages, the more jitter. The slower the logic family used, the more jitter. Very few designers understand this. This is what I do for 42 years.
Also my Rega Dac-R claims to reclock the signal coming in, I never considered a reclock before because of this.
This means that a reclocker is already inside your DAC. This is fine, depending on the quality of the master clock used and the other ancillary circuits, as well as the implementation as described above. The problem is that this all goes out the window if they have not selected the best chip for the resampling. There is literally only one chip on the market that does a good job, and that's the one I use in the Synchro-Mesh.
I would predict that my Synchro-Mesh would even make your DAC sound better, even though it already has a reclocker inside.
If you change sources to your DAC or change coax cables and it changes the sound in any way, then the DAC is not immune to jitter. This is typical for DACs with reclockers inside.
Any downside to up sampling? there seems to be mixed opinions out there.
There are mixed opinions on practically everything. Ignore them
If a good chip or algorithm is used for upsampling, it is only beneficial. I used to believe that upsampling colored the sound, but that was because older chips were crap and older software was not great. With the advent of newer more optimum algorithms and chips, there is no downside. In fact, with many tracks it is difficult to tell the difference between the original and the upsampled providing the original has low jitter.
The main benefit of upsampling 16/44.1 to 24/96 is that it forces the D/A to select a more optimum digital filter that is well outside the audible range. This is not possible with 44.1.
BTW, my DAC has no upsampling on the input. The way I get around the 44.1 filter issue is that I can select the digital filter for 192 when playing a 44.1 track. Sounds great even without upsampling.