So, I got a couple of 5' Mogami W3080 + Neutrik connector cables for total ~$65 shipped to take the AES/EBU interface for a spin.
Setup: Transporter --> AES/EBU --> Behringer DEQ2496 (bypass mode) --> AES/EBU --> Transporter --> shielded 6' RCA (Tributaries brand) --> E-MU 0404USB --> AMD X4 laptop (Win8)
For the TosLink condition, I'm using 2x3' generic plastic TosLink cables instead of the AES/EBU.
Firstly, I wanted to make sure the analogue output remains good/unchanged:
The upper table contains 24/88 measurements - these days, it seems more hi-res is available in 24/88 (often SACD/DSD conversions), I figured it would be good to have a look to make sure it all measured well. The lower chart are the same conditions at 24/96. Note that these are the RCA output measurements so a little lower than with the XLR results posted before. Note that there is some inter-test variability compared to my previous results with the Transporter but generally we're talking <1dB difference.
As you can see, from the analogue perspective, there isn't any difference whether I'm measuring the Transporter DAC directly, or if it's running through a total of 6' TosLink or 10' AES/EBU through the Behringer DEQ2496.
Let us turn our attention to those pesky jitters. As usual, using the Dunn J-Test:
Transporter direct (16/44):
Transporter-DEQ2496 AES/EBU loopback (16/44):
Transporter-DEQ2496 TosLink loopback (16/44):
At 16/44, the Transporter is very clean for both direct and AES/EBU loopback. Most of these spikes are just J-Test modulation products from the 16-bit signal being recorded in 24-bits. As you can see, the TosLink loopback is considerably worse with more sidebands congregated around the primary 11kHz signal. You can also see that using the TosLink interface with the Transporter slightly raises the absolute noise floor in general.
The AES/EBU loopback does add a small amount of jitter to the graph but it really is quite insignificant!
In the 24-bit domain, here are the same conditions with the 24/48 J-Test:
Transporter direct (24/48):
Transporter-DEQ2496 AES/EBU loopback (24/48):
Transporter-DEQ2496 TosLink loopback (24/48):
These graphs look a bit different from the previous Transporter jitter measurements because I'm measuring RCA output (rather than the XLR from before which has a lower noise floor). Nonetheless, the results are the same in terms of jitter - TosLink is significantly worse.
Well, I think that's it for my suite of Transporter measurements... The practical side realizes that jitter at these levels even with TosLink doesn't make an iota of difference playing real music. However, the audiophile seeks for "perfection" in as much as it's possible. Using AES/EBU digital cables instead of TosLink in this loopback configuration with the Behringer DEQ2496 for room-EQ does not add any significant extra jitter within the resolution limits of my test equipment. Cool.