I have shown that bit-perfect data can be transferred and played back without any concern off a USB asynchronous interface (eg. the various Mac and Windows software players and laptops). While I have the Squeezebox Receiver still on hand, let us have a look at the effect of using different transport devices with S/PDIF interfaces objectively.
Remember to keep in mind that the S/PDIF interface, unlike packetized asynchronous USB (or ethernet) conducts its data transfer in a unidirectional serial fashion formalized around 1985 when the AES3 (AES/EBU) standard was also laid down. As I think many of us have read, S/PDIF combines the data and clock signals using "biphasic mark code" and it is this "feature" of the interface that has resulted in many an audiophile nightmare regarding timing issues - jitter concerns especially well publicized. This is to a large part the basis for the well known paper by Dunn and Hawksford in 1992 asking "Is The AESEBU/SPDIF Digital Audio Interface Flawed?". (Of course the topic of jitter can be very complex as described in the paper going far beyond what we need to concern ourselves with here.)
With that said, let's be practical and see what the results looks like with the different devices using TosLink and coaxial interfaces (with comparison to asynchronous USB)...
Setup:For this round of testing, I decided to take a break from the TEAC UD-501 DAC and go back to the ASUS XONAR Essence One for a bit. Although I have been listening to the TEAC a lot in the last couple months, on a daily basis, I still use the ASUS Essence One at my computer workstation and it was just more practical to run these tests there. Despite my concerns around the upsampling feature of the ASUS, it measures well and sounds excellent.
Here's the hook-up:
* Transport device * -> Coaxial/Toslink/USB cable -> ASUS Essence One -> Shielded 3' RCA -> E-MU 0404USB -> 6' Belkin Gold USB -> Win8 laptop
Coaxial cable = 6' Acoustic Research
TosLink cable = 6' Acoustic Research
USB cable = 6' Belkin Gold
Transport devices tested:
1. Squeezebox 3 -> Coax / TosLink
2. Transporter -> Coax / TosLink
3. Receiver -> Coax / TosLink
4. Touch -> Coax / TosLink
5. Laptop -> CM6631A Async USB to S/PDIF -> Coax / TosLink
6. Laptop -> Async USB direct to ASUS Essence One
As you can see I've got the host of Squeezebox devices on the test bench along with the usual two ways to connect the computer's USB port to DACs (direct or through USB-S/PDIF converter).
I. RightMark Analysis:Since there are so many devices/combinations, I'll show the results a few at a time to demonstrate what was found. Let us start with the results of the four Squeezebox devices. I decided to "max out" the capabilities of the SB3 and Receiver by using 24/48 sampling rate:
Numerically, not much difference... From my subjective listening during the tests, I would agree with these numbers in saying that "it sounds like the Essence One DAC"; there are more similarities in the sound than subjective differences.
Let's have a look at the frequency response graph because there does appear to be some difference - here's coaxial interface only:
Hmmm, interesting! Small differences at the top end. Let's zoom into that top end and have a good look:
Notice the shape of the curves suggest slightly earlier roll-off with some devices. The flattest, most extended frequency response (and possibly most "accurate") is the Transporter, followed by SB3, then Touch, and Receiver. Remember, we are talking only about 0.15dB difference between the Transporter and Receiver at 20kHz; not perceptible IMO but since we're looking for evidence of a difference, useful to note.
Let's now include the TosLink measurements:
Even though we ran out of colors, it doesn't matter because there are still only 4 curves. There is no difference between coaxial and TosLink; they overlay on top of each other essentially perfectly.
Let us now add the computer-USB interfaces (take away the TosLink since no difference):
As you can see, the flattest response curves come from USB direct and Transporter. Here's the ranking: Transporter, ASUS USB direct, SB3, Touch, CM6631A, and Receiver. We'll talk more about this later, just keep in mind then that frequency responses are *slightly* different between transports despite bit-perfect settings...
The rest of the RightMark graphs - no significant difference:
II. Jitter Analysis:
A. Squeezebox 3 (16-bit / 24-bit):
B. Transporter (16-bit / 24-bit):
C. Receiver (16-bit / 24-bit):
D. Touch (16-bit / 24-bit):
E. Laptop -> CM6631A USB to coaxial (16-bit / 24-bit):
F. Laptop -> USB direct (16-bit / 24-bit):
Objectively, it looks like the CM6631A USB-to-S/PDIF and USB direct 24-bit graphs are cleaner, and of the Squeezebox devices, the Transporter on the whole seems to have the least data-correlated jitter. Even at its worst, the sidebands for the SB3 around the primary signal is down around -120dB. Is this a problem? I doubt it since auditory masking will easily make this inaudible (assuming one could even hear down that low around 12kHz pitch). Furthermore, in theory, the J-Test should create a "worst case scenario" for jitter which is unrealistic in real music.
Here's the difference between coaxial vs. TosLink:
A. Squeezebox 3 24-bit, Coaxial vs. TosLink:
B. Transporter 24-bit, Coaxial vs. TosLink:
C. Receiver 24-bit, Coaxial vs. TosLink:
D. Touch 24-bit, Coaxial vs. TosLink:
E. CM6631A USB to S/PDIF 24-bit, Coaxial vs. TosLink:
Remember that all of these jitter graphs are indicative of the interface between the transport device connected to the Essence One DAC. The graphs could be different with another DAC since much of the result will depend on the accuracy of the DAC in extracting the clock information and what other steps it might take (eg. data buffering, reclocking) to further stabilize the timing - it's not just about the transport device.
III. DMAC Protocol
BTW: If you're not aware, the Squeezebox devices by nature are asynchronous since they receive the data through WiFi or ethernet from Logitech Media Server and buffered with a decent amount of internal memory. You can see that the TosLink and coaxial connections have worse jitter than what's measured directly off the analogue outputs (eg. look at SB3, Touch, Transporter, Receiver jitter measurements).
Musical selection this evening:
Philippe Jaroussky - Carestini (The Story of a Castrato) (Virgin Classics, 2007) - amazing vocals and fascinating musical history. ("It's a man, baby!" -- Austin Powers)