The follow was a post I made of the Squeezebox Forum a few weeks back to hopefully answer the question of: How well does the Touch function as a digital transport?
Setup:
All the results here are made with the Touch with Triode's EDO firmware installed. For the sake of clarity, when I indicate a test was run in "EDO mode", this means it's the "digital only" mode, otherwise I'm referring to the default "digital + analogue" mode. For those wondering, there was no difference in measurements between stock firmware and with the EDO firmware installed.
The Touch was connected by ethernet to the basement music server --> either TosLink or coax out --> ASUS Xonar Essence 1 DAC --> XLR cables --> E-MU 0404 USB for measurement.
As previously measured, the balanced XLR output from the Essence One has so far given me the best electrical noise suppression and lowest noise floor, the Touch was used as the digital transport for this system. I then played the RightMark calibration, and test tones off the Touch and measured the analogue out from the Essence 1.
About the digital cables:
TosLink - cheap "VITonet" labelled plastic fibre cable - no idea where I got this, if I bought it, would likely be <$10 at local supplies store. Pretty thin and flimsy looking but gives a good tight connection with the ends. You can see this cable in the picture above just to the right of the Touch.
Coaxial - this gave me an opportunity to try a simple unshielded 3' stereo audio RCA cable (zip chord that I never used supplied with an old DVD player! forget any impedance matching or electrical shielding) vs. an actual shielded 6' coaxial cable "Acoustic Research Pro Series" I bought 10 years ago for ~$20.
I unplugged the Essence 1's USB cable from the computer during these tests to avoid potential noise pollution. Also, when testing the TosLink, the coaxial SPDIF was not connected.
First off, 16/44:
Nothing to see here in terms of the Touch as transport! Essentially perfect measurements... The 1st column is just the audio played to the Essence 1 through USB 2 (note there's something wrong with the IM value here - I suspect it was a spurious error from this run). Whether I used TosLink, cheap RCA, actual coaxial to connect to the Essence 1 from the Touch did not matter.
Now, 24/96 (standard "digital + analogue" mode):
Hmmm, not unexpectedly, the "RCA as coaxial" is the stand out here. Slightly reduced noise floor (~1 dB), reduced dynamic range (~2 dB), mildly worse stereo crosstalk.
How about 24/96 in "EDO mode" with the analogue output turned off?
Essentially the same as the standard mode. It appears that turning off the analogue output circuitry (or at least silencing it) does not affect the test results in any meaningful way. Again, the RCA cable performance is inferior to a proper shielded coaxial cable. However one has to realize that at this level of performance, the difference is really so minor that it's unlikely anyone would be able to tell a difference from listening! To give you an idea, here's the THD plot, notice how little difference there is with the RCA cable (cyan) just rising over the others like around the 2-10kHz range.
Conclusion so far:
1. 16/44 performance is beyond reproach according to these tests.
2. 24/96 performance likewise is excellent. Starting to see the limitations of an unshielded cable connected to the coaxial SPDIF but even in such an extreme situation, the rise in noise floor is likely inaudible.
3. Turning off the analogue output does not appear to improve the quality of the digital transport.
Now it's time to talk 24/192 with the Touch with EDO plugin. Thank you Triode - amazing plugin/kernel!
Results:
What I find impressive here is the fact that TosLink to the Essence 1 worked at 24/192!!! In fact, that picture of the Touch in the previous post playing the 24/192 John Coltrane's "Blue Train" (Classic Records HDAD release from 2001) was through the TosLink (you can see the 192kHz LED lit up on the Essence 1). This is why I'm very impressed by the components used in the Touch; kudos to Logitech and ASUS! Over the years, this is the first time I've been able to play 24/192 for hours without obvious clicks/pops/disruptions even with an inexpensive plastic cable. To show the TosLink result above wasn't a "one-off", here's a series of 4 runs with the TosLink done over about 2 hours - notice the inter-test reliability.
Like with the 24/96 tests, the cheap RCA cable is showing its limits even more with measurably increased noise floor (10 dB worse!) from the lack of shielding and possibly data errors as the speed of data transfer doubles. Another interesting phenomenon is that even the shielded coaxial cable has a measurably higher noise floor compared to the TosLink by about 2dB. Would an expensive coaxial cable improve this? Possible I guess if the shielding is excellent, but even so, would anyone ever notice at around -110dB!?
Another observation with this test is that the noise level, dynamic range, and stereo crosstalk are all WORSE than 24/96. Every piece of equipment here from the E-Mu, to Essence 1, to Touch are running at max. specifications so the system is running at it's limit.
Here's the THD graph showing the increase in noise with the coaxial interface - el cheapo RCA (cyan) is looking "bad" here:
Conclusion:
1. Thanks again to Triode for the EDO plugin/kernel. It works beautifully and in my system with TosLink working, the Touch measures within 1-2 dB in terms of noise level, dynamic range, and stereo crosstalk compared to a direct USB connection to the DAC for 24/192 playback!
2. If you can get the TosLink to work, you've set yourself free from electrical noise with "galvanic isolation" of the Touch and DAC. Again, the fact that I could get TosLink 24/192 to work reliably between the Touch and Essence One really is impressive and speaks well of the equipment.
3. A recurring theme in these tests is that of ELECTRICAL NOISE. Coaxial SPDIF cables need good shielding at 24/192!
4. 24/192 does not measure as well in my system as 24/96. Writers like Lavry and xiph.org ("24/192 Music Downloads Make No Sense") have already eloquently documented their opinions against 24/192 and I guess I can echo their concerns with the gear I'm using for these tests... Firstly, between 24/96 and 24/192, the difference is ultrasonic; do we demand high-end SLR digital cameras to also capture ultraviolet light? (Sure, you might want to do this for specific scientific reasons.) Secondly, other than a handful of albums usually from smaller labels like 2L, Reference and Linn, I have rarely come across truly native 24/192 (or 24/176) recordings. IMO, it also makes no sense to buy stuff like DSD64 converted to 24/176 such as many of the HDTracks offers.
Addendum: Feb 27, 2013
Thanks to slimdevices forum member "tpaxadpom" who measured the digital output with the AP2722 unit:
SPDIF RCA 377.3 - 324.5 ps
Toslink 1.604 ns
RCA much better in terms of jitter measurements.
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