QUICKIES: Black aluminum alloy footers. Equipment racks. Center channel in music. Underwhelming Spotify Lossless. More Paul McGowan/PS Audio USB cable & jitter foolishness.

Hey guys and gals, not much time this week to test stuff but to start I thought I'd show some pictures of the black version of the inexpensive aluminum footers that were discussed last week.

I got these black-colored ones (AliExpress) for my center channel, the Paradigm Signature C3 v.3 which is a 3-way, 4 speaker design that sits on my wood/glass component rack. (You can see other pictures of that component stand in my room article, or earlier when setting up the room.)

I picked the black ones since the center channel is sitting straight in front of me when watching movies, keeping it black will not add any color contrast to potentially distract from the show.

Do bit-perfect digital sources affect a USB DAC's sound quality? [2023 Edition - phone, Raspberry Pi, MiniPC, laptop]

In the image above, we see embedded some pictures of contemporary, expensive (>US$10k) digital audio devices. Each one of these are just streaming or server boxes that can take data over your network or internal storage and pass them along to a DAC with no actual conversion. Typically, these "high end" audiophile products (for example some featured here) are not asked to perform DSP, thus passing the data in a "bit-perfect" fashion. Furthermore, these expensive devices do not include an internal CD/DVD/SACD/BluRay mechanical reader. They're basically computers with hard drives or SSD storage and software to index and manage your music library. You'll typically be connecting them to your favourite external DAC most popularly through USB although proprietary interfaces (eg. Wadax's optical), ethernet, S/PDIF coax/optical, and i2S could be other options.

The other day, a family member was in need of a replacement phone so I gave them my Huawei P30 Pro which I've been using since 2019 and got a Google Pixel 8 Pro. After transferring the apps and data over, I figured I'd try to see if USB Audio Player PRO on the Pixel 8 Pro would work with the very high resolution Sabaj A20d 2022 DAC (ES9038PRO converter) which the Huawei phone previously did not. It worked, reminding me that Android USB hardware and drivers can result in different levels of USB DAC compatibility.

With the phone connected to the DAC, and given that it has been awhile since I've put together a "shootout", let's see if there's any evidence now in the 2020's of potentially audible differences between source devices playing to a very high-resolution USB DAC. Not that I have a $10k streamer lying around here, but a $1000 phone could be different from a sub-$100 Raspberry Pi, right? 😉

[FYI: years ago, I did a test similar to this using various mostly S/PDIF outputs connected to the same ASUS Essence One DAC.]

Let's look at some data.

RETRO-AUTOPSY: Museatex Melior BS-1 Bitstream S/PDIF DAC (circa 1995?). A few words about external clocks for audiophiles. And a speaker cable blind test?

The company Museatex was formed around 1990 by Ed Meitner and John Wright based in Calgary, Alberta, Canada. Over the years, the brand was assumed under A/D/S which eventually closed in 2001. There's still a web page for information on Museatex products here. Ed Meitner has moved on these days to EMM Labs since 1998 and given his work with SACD, his name has been associated with "bitstream" type technologies since (some more info/video).

By the end of the 1980's, 1-bit Philips "Bitstream" PDM DAC chips started to show up in the market with the Philips SAA 7321 chip as one of the 1st generation devices (see Stereophile reporting back in 1989). Some of these chips were incorporated in CD players like the Meridian 208 that came out in early 1990, well-reviewed by Paul Miller in High Fidelity.

By 1991, the "second generation" Philips SAA 7350 "bitstream" DAC became available, capable of taking up to 20-bit PCM (from 16-53kHz sample rate) and upsampling to 1-bit SDM applying 3rd order noise shaping (some more history here). It is this DAC chip that's at the heart of the Museatex Melior Bitstream DAC, I believe first released in 1992. Here are the specs from the company.

The exact history of this particular unit I had access to has been shrouded in the sands of time, but was probably purchased around 1995. The original suggested price on one of these was US$899 which would be about equivalent to over US$1500 in 2023. Even to this day, I've seen comments from audiophiles who have a special affinity to the sound of this vintage DAC. Claims to fame included anti-jitter input circuitry (supposedly reducing "glare and harshness"), a custom digital filter allowing for "more accurate signal conversion", and high quality power supply with line noise filtering.

Thanks again to my friend linnrd for digging this unit out from his audio device archives. While externally it looks to be in very good shape, unfortunately, it didn't quite power up normally for me which sadly resulted in this article being more of an "autopsy" rather than actual measurements. I guess having a look inside is still better than nothing for historical documentation if nothing else.

MEASUREMENTS / REVIEW: Topping D90SE DAC - Part I: The Hardware, Filters, MQA, and Jitter Performance

Today, let's start with what I'm aiming to be a 3-part review/discussion of the Topping D90SE DAC. I figure I'll take my time on this since things are busy around here and I hope not to purchase more hi-fi DACs going forward, so I might as well savor the moment. ;-) As a "more objective" audiophile who has a vision of aiming for "high fidelity" / "transparency", there is a clear target and end-point to what's needed from hardware performance especially with DACs.

Readers of this blog know that I've had a number of Topping devices reviewed here over the years. In fact, for any single brand of DACs, I think I've reviewed more Topping gear here than other brands. This, I believe, is a reflection of a brand that provides multiple products at price points with features that actually speak to me as a consumer interested in value which includes features, and price. I have certainly not been disappointed by overall quality to this point.

At a current retail price of around US$900, this is not an inexpensive model. As per most of my reviews, I bought this through usual retail channels so what I'm reporting on here is not any potentially specially-selected unit sent from the manufacturer.

Feature-wise, this is a DAC only but accepts multiple inputs - USB, HDMI-style I2S, S/PDIF Coax, S/PDIF TosLink, and wireless Bluetooth 5.0 (SBC, AAC, aptX [LL,HD], LDAC). While it has both unbalanced RCA and balanced XLR outputs, there is no headphone amp. For that, Topping recommends grabbing their top model the Topping A90 (~US$500).

What makes this DAC interesting in mid/late 2021 is that this is reputed to be the highest fidelity converter in the world (available to consumers) at this point in history, Topping's "flagship" device. 

Let's take a look...

MUSINGS: Noise, Jitter, Faith & The Autistic Fantasies of Digital Audiophilia ("For adults only?" Darko/Lavorgna conversation.)

Happy New Year dear audiophiles!

I hope 2021 brings with it many happy returns in the year ahead.

Note that this post will be of a rather critical tone... So if on an auspicious New Years Day you want something a little less confrontational, I suggest coming back another time :-).

Let's address something which I think many well-heeled digital audiophiles will probably have observed being discussed if they read typical "mainstream" audio magazines, online sites, or watch associated videos. It's the simple observation that when it comes to subjective-only reviewers discussing why something (often expensive!) can improve the sound of a digital system, it almost always comes down to 2 factors they want you to be concerned about:

1. Jitter

2. Noise ("electrical", RF/EMI, conducted/induced)

MUSINGS / DEMO: Why "Bits Are Bits". Let's not add unnecessary fear, uncertainty, and doubt.

[Available as audio podcast summary.]

Something I have noticed over the years commenting on the audiophile hobby has been how incessant and persistent various themes tend to be. Just like the apparently never-ending arguments of "digital vs. analogue/vinyl", or "CD vs. hi-res", or "subjective vs. objective", there has been this mostly friendly banter between those who feel that essentially "bits are bits" vs. those who think there is significantly more to digital transmission than bit-accuracy.

Seeing recently this article "Why the 'Bits is Bits' Argument Utterly Misses the Point" from Upscale Audio published compelled me to write this post to explore the topic further with a review of measurements and some demo tracks for readers to listen to themselves. I don't know how long the Upscale article has been on the site since there's no date or author listed, and was only made aware of it through the Darko.Audio Facebook page (it seems Mr. Darko felt the article was accurate, really?).

While the article claims that some people have "missed the point", let us examine their points and see if perhaps it might be the author(s) that are a bit too aggressive in making these arguments. After all, it is 2019 with decades of development in digital technology that impact our lives in more sophisticated ways than just audio reproduction. It's hard to imagine there are huge lacunae in our knowledge of digital communications and digital-to-analogue conversion of audio frequencies.

MEASUREMENTS: Intel i7 PC and Raspberry Pi 3 B+ Audio Streamer - XLR / RCA, Noise and Jitter. Do digital transports / streamers really make a difference? Do USB cables?

This is a picture of the corner of the room where I performed the measurements for the post last week. Notice that Intel i7-3770K computer in the left corner I use for gaming? Within that box are 16GB DDR3 RAM, both a SanDisk Ultra II SSD and a recent 2TB Firecuda drive, plus a rather powerful nVidia GTX 1080 graphics processor which is what is driving the display on the 4K TV. Inside the case, it's all powered by a 6-year-old Antec 650W switching computer power supply these days. The computer is about 6 feet away from the TEAC UD-501 DAC.

Suppose I take a 16' USB cable and connected this computer to the DAC and compared the measurements with the low-power Raspberry Pi 3 B+ streamer... What do you think the result would be from the perspective of distortion and noise?

DEMO / MUSINGS: Let's listen to some jitter simulations with sideband distortions...

A couple weeks ago, we started getting into the topic of jitter and the concept of whether jitter is audible and at what level. As I had expressed at that time, my belief based on experience with the equipment I have looked at / listened to is that with almost any reasonable modern day digital audio device, the likelihood that one would hear sampling jitter effects is extremely unlikely. No need for crazy expensive cables that claim jitter improvements. No need for high-priced servers (like this), expensive streamers, or "de-jitter"/"reclock"/"regen" devices. But one of course does want to have a good DAC with excellent jitter rejection, and these days, competent asynchronous USB devices almost universally will achieve excellent results by reducing jitter from the interface (remember, the older S/PDIF digital interfaces typically perform worse than modern USB or ethernet even though newer devices like the Oppo UDP-205 perform excellently with any of the inputs).

The reason I say this comes not just from measurements and my own listening to devices with different severities of sampling jitter, but also experimentation over the years in simulating the distortions introduced by jitter.

As I mentioned last time, with Yamamoto2002-san's WWAudioFilter, we can easily use DSP to introduce fixed amounts of sinusoidal periodic jitter to "bake in" the kinds of sideband anomalies often found with devices that suffer from jitter. For today's post, what I want to do is provide some test files you can use to actually hear what severe jitter distortions with sideband anomalies sound like. Think of this post as similar to one years ago when I demonstrated what poor USB cables sound like.

Furthermore, coming out of this article, I hope the audiophile reader can appreciate the magnitude of jitter that is necessary in order for the effect to become audible. In so doing, I hope it will help us appreciate the results from the J-Test FFT's I publish here and also when you look at measurements elsewhere (like on Stereophile).

Summer Update! Huawei Matebook X Pro, Western Digital Gold HDD, and jitter simulation.

Well guys and gals, hope you're having a great summer (or winter as the case may be!). The weather here is certainly conducive to taking it easy and just enjoying the music instead of thinking much about the audiophile hobby apart from short comment/forum posts.

But tech things are happening in the background :-). Really exciting things as well which I'll discuss more in the days ahead. For today, I just wanted to discuss a couple changes / acquisitions this month...

MEASUREMENTS: Oppo UDP-205 Part 3: Jitter, some conclusions, discussions, and suggestions.

We've reached the last of the "trilogy" measurement posts I'm planning to make on the Oppo UDP-205 (at least for now!). We've already examined some "microscopic" measurements like the oscilloscope reading of square waves, talked about the filter settings and relative differences. Then we've looked at the RightMark measurements demonstrating excellent low noise level, low distortion readings, good frequency responses, and minimum crosstalk across the different outputs.

With this post, let's have a look at the J-Test results from the various outputs to make sure there are no issues, and of course in the process examine temporal stability with the various digital inputs including USB, ethernet, S/PDIF, HDMI, and of course as a CD player. We will then close off with some general thoughts, suggestions, and opinions around this device and broader about the features that might be worth pursuing in audiophilia.

MUSINGS: Evidence of digital player jitter with asynchronous USB DACs? Melco N1ZS20/2 review in HFN&RR...

The UK magazine Hi-Fi News & Record Review is interesting. Like Stereophile here in N. America, it includes objective measurements. Over the years, I've seen times when they have criticized questionable "hi-res" downloads showing nothing more than upsampled music. But they also seem to like "showing" rather meaningless measurements such as a few years ago with their USB cable roundup.

Recently, I believe in the May 2017 issue, they reviewed the very expensive "cost-no-object" digital server - the Melco N1ZS20/2. This device is the "mk2" of their top-of-the-line music server/player with the capability for data output through ethernet or USB connection to one's DAC.

MEASUREMENTS: ONKYO TX-NR1009 as HDMI / SPDIF DAC... Are AV Receivers any good?

The rat's nest.

Running separate components like multichannel processors/preamps to monoblock amplifiers are generally considered the ideal, "cost no object" approach to home theater. In the real world, cost and space are considerations and AV receivers become the "Jack-of-all trades" central device that most of us have in the home theater setup. But like the proverbial "Jack", it's useful to also consider the second part of that saying... Just how bad  is he also "the master of none"?

In the last installment, we looked at passing an analogue audio signal through the Onkyo and found that noise can be an issue. Today, I want to demonstrate the quality if we were to just use this device as a DAC - a look at the digital portion. Some natural questions arise - how well did the designers shield noise from getting in (especially in light of the high analogue noise measured previously)? Is the jitter through the use of HDMI "bad" (compared to TosLink and coaxial S/PDIF)? How does it compare to other stereo DACs?

Based on the Onkyo specs sheet, the TX-NR1009 uses the TI PCM1690 6-channel + PCM1789 2-channel DAC chips. Both are rated as 113dB SNR. These DAC chips are often found in consumer AV receivers and are lower spec'ed than most stand-alone DACs like the TEAC UD-501's PCM1795 with >120dB SNR. Of course, you cannot just look at this specification and judge the quality of a DAC. Much depends on the analogue circuitry around that DAC so the measured results are more useful than just looking at the components individually.

Setup:

As usual, for the sake of full disclosure and opportunity to repeat/verify, here is the setup for these measurements:

Win8 AMD A10 "Trinity" HTPC --> HDMI/TosLink/coaxial cable --> ONKYO TN-NR1009 front stereo "pre-out" --> shielded RCA --> E-MU 0404USB --> shielded USB --> Win8 laptop
CM6631A device used for asynchronous USB --> coaxial / TosLink conversion duties.

HDMI driver: default AMD WASAPI. I used JRiver for playback.

Since I want to check the performance in a more "naturalistic" fashion, I made sure the TV was connected and on as well as my Blu-ray player (Panasonic BMP-TD220). Remember that in my previous post, plugging in the HDMI TV cable added significant noise to the analogue pass-through. All results were made with the Onkyo in "Pure Audio" mode to defeat any audio DSP/bass management.

HDMI cable: A decent looking 6' length capable of high speed HDMI (officially rated as HDMI 1.3 but fine with my HDMI 1.4 3D TV), brand named "ION" that I purchased for something like $20 about 2 years back at a local computer/supplies store.

TosLink and coaxial SPDIF cables I'll be using for comparison are the "Acoustic Research" branded 6' lengths I measured previously (see links for details).

Results:

As usual, I ran the output through my digital oscilloscope first to have a look:

Here is a 0dBFS 1kHz square wave sent through the HDMI and measured off the front stereo "pre-out" RCAs. Not bad - good square waveforms with excellent channel balance (sorry about the pixellation, usually screenshot looks better than that). With the receiver volume set to the "reference" of 85 (there is a little popup on the front screen when you hit 85 that correlates to the 0dB THX reference level) and in "Pure Audio" mode, the peak voltage is around 2.3V.

Here's a 24/44 impulse response:

Good linear phase impulse response, nothing fancy here. Absolute polarity also maintained by the Onkyo.

I. RightMark:
As usual, I used RightMark to look at the measured dynamic range, noise level, distortion levels; here's the summary for HDMI at the various bit depths and sample rates:

As you can see, I've also included for comparison the results at 24/96 for the Squeezebox Transporter and TEAC UD-501 (unbalanced outputs) - two of the best measuring DACs I've tested here with the same hardware/software.

Clearly the Onkyo is capable of hi-res with >16-bit dynamic range. With 24-bit data, it can do ~109dB dynamic range which equates to just over 18-bits! Not as good as the dedicated audio units like the Transporter or TEAC but pretty darn good for an AV receiver! This result is about equivalent to the AUNE X1 and ASUS Essence One using unbalanced RCA output - however, those DACs had better distortion numbers.

Some graphs to review from the 24/96 dataset:

Frequency Response

Noise Level

THD

The Onkyo rolls off a bit more in the high end, a little more noisy, and notably more harmonic distortion.

For fun, here's the spectrum off the Onkyo playing 24/192:

Yup, capable of 24/192 although the roll-off on the high end is obviously earlier than the TEAC UD-501 (Onkyo drops -3dB at 50kHz).

I was curious if the SPDIF (TosLink and coaxial) inputs measured just as well:

Yup. They all look pretty good. The graphs all look identical except for slightly more high end roll-off with the HDMI interface compared to the SPDIFs - not sure why.

II. The 1kHz -90.3dB "Microscope"

With the dynamic range >16-bits, this test should be no problem for the Onkyo (HDMI input).

That looks very nice given that many very expensive DACs are not even capable of this degree of resolution! Again, this is an AV receiver! As I previously posted, even recently released DACs like the Wadia 121 Decoding Computer is incapable of this resolution.

III. J-Test for Jitter

As usual for my DAC tests, let's have a look at the Dunn J-Test spectra for both 16-bit and 24-bit signals. Here is the summary using the 3 digital inputs - HDMI, coaxial SPDIF, and TosLink SPDIF:

As you can see, the Onkyo is quite jittery in general whether HDMI or the other SPDIF interfaces. Although quite similar, I am somewhat surprised that the sidebands were more pronounced for the coaxial digital input! For comparison, here's the Transporter and TEAC:

Although the scale and dimensions are a little different, one can certainly appreciate just how jittery the Onkyo is compared to the others especially with the 24-bit signal. From this data, we see that the Onkyo itself has more jitter as a whole; specifically it's not any worse with the HDMI interface. We'll talk about jitter again in a little bit...

IV. Does sending a 5.1 channel signal degrade the measured performance?

I thought this would be interesting to check out. I left the RightMark test signal as the two front channels and added some AC/DC "Thunderstruck" into the center, rear, and LFE channels played back in JRiver as a multichannel FLAC through HDMI.

Beautiful ain't it?! The idea is to see if driving 6 channels (5.1) at the same time through the HDMI cable into the Onkyo's DAC will change the audio quality... For example, doing this might increase the noise floor, or perhaps worsen channel crosstalk since we've tripled the number of audio channels being processed.

Frequency Response

Noise Level

As you can see, there's very little difference whether 2 channels are playing or 6 channels. Great to see! Essentially no frequency response or crosstalk difference. However, there is a very small increase in noise level when playing multichannel... IMO audibly insignificant but measurable.

Summary:

Here you go folks! That's how a higher-medium end "modern" AV receiver measures as a stereo DAC. Of course, each model will be a bit different, but I suspect similar tiered receivers from Pioneer, Denon, Integra, Yamaha, H/K, Anthem, etc... should be comparable (won't know unless someone tests it out). Note that most magazines like Sound & Vision will measure receivers but usually in the context of power output and flatness of frequency response rather than on the accuracy of the digital-to-analogue conversion as I did here.

In some ways I am impressed and in other ways the results were as expected.

I was impressed by the low noise and very good dynamic range for example. To achieve almost 110dB in the audible spectrum is quite something especially considering the complexity of an AV receiver with all the potential electrical noise sources inside the box! The accuracy of the 16-bit -90.3dB waveform looks excellent; something which only the better stereo DACs or CD players would have been able to accurately reproduce a decade back. Likewise, the fact that the measurements remained excellent even with 6 channels being processed concurrently and only measuring about 1dB difference in the noise floor again demonstrates the engineering quality. Given the results I found previously with HDMI noise polluting the analogue input, I'm guessing that Onkyo put more attention in optimizing the digital side which makes sense since most people will be connecting digital inputs for multichannel sound.

As for the more "expected" results, let's talk about jitter...

A few years ago in 2009 this message came over the 'Net which I remember made quite an impression on me around how "bad" HDMI is as an audio interface (supposedly from Hi-Fi News & Record Review / Miller Research):

(I didn't notice it at the time, but that Denon AVR-3803A was a typo - the 3803 has no HDMI. It's actually the 3808.)

Later, a more comprehensive message appeared:

Hmmm, it looks like HDMI jitter can be cleaned out after all (eg. Arcam, Classe, Pioneer)! It's about the implementation, not necessarily the interface itself. If you read around these posts, one also finds that the jitter value and subjective sound quality do not necessarily correlate.

Let's think about the J-Test and what was found in measuring the Onkyo for a moment. The Dunn J-Test is a synthetic test of data jitter first published by the late Julian Dunn around 1994 which (in the 24-bit 48kHz version) superimposes an undithered LSB 250Hz square wave over a primary 12kHz -3.01dBFS sine wave which is of course an exact 1/4 of the sampling rate. This superimposition stimulates the effect of subtle timing inaccuracies (jitter) which can be demonstrated as accentuation of the sidebands measured in the spectral graphs.

Remember that this test is synthetic and stimulative. What you see measured is not something you're probably ever going to "hear" in real music! The noise floor is not going to be down to the last bit in 16-bit audio and essentially impossible with recorded 24-bit audio (unless it's purely computer synthesized music). Also, jitter is more pronounced in the higher frequencies (11kHz and 12kHz are used as the primary signals in the J-Test). Realize that the human hearing sensitivity is well on its way down by 5kHz (as can be seen by the Fletcher-Munson Curves). Furthermore, if we specifically look at the Onkyo's J-test spectrum, the most pronounced side bands are about -90dB below the primary signal. To make matter even less worrisome is that the tall sidebands are all +/-250Hz around the primary signal and the audibility would be masked even if one did have awesome auditory acuity at 11/12kHz and could hear a signal 90dB down! This is also why I feel adding up all those sideband peaks and calling it a number (whatever picosecond or nanosecond) is really not all that useful when it comes to audibility.

What I'm trying to say is this... Tests like the J-test can demonstrate that jitter is a real phenomenon. Engineers should pay attention to it when designing hi-fi equipment. A discerning audiophile should be aware of it and if able to, can measure it themselves and decide if the engineer did a good enough job. However, IMO, to say that jitter is somehow audible at these kinds of levels I think would be impossible. In fact, unless the jitter were ridiculously high (like Track 26 from Stereophile Test CD 2, where an insanely simulated 10ns sideband is inserted +/-4KHz around the 11kHz primary - again totally synthetic), the concept of jitter significantly deteriorating sound quality I believe is utter nonsense in the real world. That some companies would even consider using jitter as a reason for putative significant audible differences between passive "components" like cables is just not credible!

I had a listen to the Onkyo's output over the last few nights with some familiar music - Ella Fitzgerald "Sings The George & Ira Gershwin Song Book", Grateful Dead "American Beauty", and Keb' Mo' "Just Like You". Also had a listen to Sting's new album "The Last Ship". They all sounded nicely rendered as they should with a good DAC. Great details with my older Paradigm Reference Studio 80 v2's which will be my rear speakers in the new sound room. The wife and I both enjoyed Sting's "The Night The Pugilist Learned How To Dance" - cute.

So, even though the AV receiver might be the "Jack-of-all-trades", at least in this specific instance with the Onkyo TX-NR1009 as an HDMI DAC, he might not be a "master" at it, but I'd say he's a pretty decent tradesman :-).

Well, unless I dig up something else to report, I'm likely "going dark" for the next month as I head off overseas for some work and then the big move to the new home. I'll be sure to post some pictures in time... Enjoy the music everyone!

MEASUREMENTS: [UPDATE] Adaptive AUNE X1, Asynchronous "Breeze Audio" CM6631A USB, and Jitter...

In late February, I received an interesting USB2 --> SPDIF box from Asia. It's based on the C-Media CM6631A chipset (it says "Breeze Audio" on the metal case and "CM6631-V1.4" on the PCB) which communicates with an asynchronous USB2 protocol (recall that the Asus Essence One uses the CM6631).

Total price ~$50USD shipped.

Although it's said to be plug-and-play compatible with Mac OS X, I've run into a few snags which I won't discuss here - just be aware if you're planning to use this on the Mac (SEE ADDENDUM - ISSUE FIXED!)...  However, I'll restrict these measurements to the Windows 8 environment only where the custom drivers work well.

The idea I wanted to explore was whether using an asynchronous converter like this connected to the AUNE X1 would be better in terms of jitter than the adaptive USB port of the X1 itself. Also, how sensitive is jitter to computer load?

Note that beyond jitter, there are some other obvious reasons to do this... Firstly, using the CM6631A box allows hi-res TosLink/coaxial output up to 24/192 if you don't have a USB2 DAC. Secondly, it might be better to keep peripherals all running at high speed if you're going to be plugging this into a USB2 hub.

Lets take a look at the adaptive USB jitter spectrum directly from the AUNE X1 (remember, only up to 16 bits, so I'll be using the 16/44 Dunn J-Test signal) [UPDATED April 1, 2013 - SEE BELOW]:
(Setup: i7 computer --> shielded USB --> AUNE X1 --> shielded RCA --> E-MU 0404USB)

Numerous sidebands with spurious noise. I guess this is pretty typical of a very pedestrian adaptive USB1 interface. It doesn't "look" good but for the most part, these spikes are down below 100dB from the 11kHz primary tone.

Not only is adaptive USB said to be bad for jitter, but some believe it's especially prone to timing errors if you're multitasking...  Lets see what happens when the CPU is under 100% load (Prime 95, 8 threads), and to make it worse, lets also run the GPU (nVidia GTX 570) 100% with FurMark:

Essentially no difference to the spectrum whether the computer is fully loaded or not! You see a bit of noise showing up between 8-9kHz but it's all very low amplitude (again >100dB below the 11kHz peak).

Let's now put the CM6631A asynchronous USB --> SPDIF in the chain. Here's the setup now starting with a proper coaxial SPDIF 'digital' cable (Acoustic Research brand):
i7 computer --> shielded USB --> CM6631A --> shielded digital coaxial --> AUNE X1 --> shielded RCA --> E-MU 0404USB

This is a very nice 16/44 J-Test graph with minimal data correlated sidebands and most of the peaks are just due to the 16-bit J-Test modulation tone.

What happens with the CPU and GPU running at 100%?

No difference!

What about we use the TosLink output from the CM6631A instead?
(Setup: i7 computer --> shielded USB --> CM6631A --> decent plastic TosLink --> AUNE X1 --> shielded RCA --> E-MU 0404USB)

In this setup, the TosLink interface is almost exactly the same as coaxial (I'm actually impressed by this since TosLink often tends to be significantly worse than coaxial).

What about CPU & GPU running full tilt at 100% using TosLink?

Again - no difference! (Ignore that FurMark banner... Just got in the way of the screen capture.)

OK. We've also heard that a poor coaxial cable could be bad for jitter...  That is, if there is severe impedance mismatch between connectors (supposed to be 75ohms), the theory goes that there could be reflections which could damage the digital signal transitions. Furthermore, it is said that a short 3' cable is worse than a longer cable due to the transition time for these reflections.

What then would jitter look like if I replaced a proper shielded coaxial with cheap "freebie" 3' RCA connector that came with an old Pioneer DVD player (I used the red connector)?

Eh? That's all? Really no different than with the "proper" coaxial cable!

How about with CPU and GPU running 100% using the cheap RCA cable?

Again, that's all!?

Realize that in the analogue domain, I can measurably prove that the cheapo RCA is worse than the shielded proper coaxial digital cable based on noise floor differences. But in terms of jitter (digital phenomenon) which we're stimulating here with the Dunn J-Test, in this "real world" setup, I cannot show any significant difference despite the likely poor impedance match and 3' length! (Note that I'm not saying a cheap RCA cable is as good as proper coaxial in general, just that in this inexpensive setup, it works fine... I have measured in other situations like with my ASUS Essence One paired up to a Squeezebox Touch where a cheap RCA cable actually resulted in very severe jitter issues.)

Summary:

1. The asynchronous USB2 interface works to lower jitter! Having said this, I think we have to also recognize that apart from some noise spikes, the actual jitter sidebands with the adaptive USB interface wasn't terrible and likely not audible given how low in amplitude they were.

2. Whether the computer was running full-tilt at 100% or not, I could not demonstrate any change in the jitter characteristic! This applied to adaptive USB as well as asynchronous USB --> SPDIF. IMO, this makes me question the belief in "software-induced" jitter. I suppose if your computer is totally bogged down such that the audio buffer cannot be filled on time, you'll hear severe audio degradation. However, some people believe even light multitasking worsens sound quality, or the importance of shutting down background OS processes or running "stripped down" OS'es. By extension, some people swear by various playback programs having an effect on the audio quality (ie. Decibel vs. Amarra vs. JPlay vs. Audirvana vs. iTunes vs. foobar...) even if all that's promised is "bit perfect" output. It almost always comes back to "jitter" as the explanation some how. Really? Is there any objective proof for such beliefs or even a testable hypothesis?

As I have hinted in previous posts, even though I (obviously!) believe that jitter exists and is measurable, I do not believe it is audible with music unless extreme like maybe >10ns. It's also worth remembering that research into the threshold of jitter audibility decreases with higher frequencies; but this also correlates with human hearing losing sensitivity the higher up we go - especially as we get older (therefore, sensitivity to jitter should decrease with age). These days, well engineered "high fidelity" gear should not be even close to showing 10ns jitter. Likewise, software player programs should be capable of "bit-perfect" output without difficulty or cause timing errors.

Over the years, I have also tried many software players like Amarra, Decibel, JPlay, etc... Ultimately I've never been convinced of significant differences in sound between them (so long as EQ and plugins not active). Foobar + ASIO driver sounds great to me on the PC side and I use Decibel for the Mac simply because it's inexpensive and works well to switch sampling rates.

Bottom Line: Don't worry about jitter! It's more than likely inaudible in a modern computer system and with decent (not necessarily expensive) audio gear. I see no evidence that high CPU/GPU load makes any difference to jitter. Isolating your DAC from electrical noise polluting the analogue output seems much more important.

ADDEDDUM (April 1, 2013):

I found out why there was the 15kHz peak in the adaptive USB1 tests with the AUNE X1. Had to do with some old ASIO4All drivers I had installed on the i7 computer...  A good reminder of how easy it can be to mess up settings when using a PC for audio playback!

Anyhow, for completeness, here are some updated graphs of the X1 using adaptive USB measured off my Win8 AMD Phenom X4 laptop. Note that even though the graphs look different, the conclusions are the same - asynchronous USB2 is less jittery, and when the computer is at high load, the noise floor worsens for the adaptive USB case only.

Adaptive USB1, AUNE X1 using WASAPI for 16/44 J-Test:

Now with 4 threads running Prime95:

Notice that slight noise floor elevation from 8-9kHz. Not as extreme as my i7 testbed with the powerful GPU running.

Asynchronous CM6631A --> shielded coaxial --> AUNE X1, WASAPI, 16/44 J-Test:

Nice and clean...  Looks just like with the i7 testbed. What happens with Prime95 running?

No change.

Asynchronous CM6631A --> TosLink --> AUNE X1, WASAPI, 16/44 J-Test:

Now again with Prim95 running on all 4 cores:

No difference...

OK. So I have now shown that the jitter results with the CM6631A device is reproducible on 2 platforms (i7 & AMD Phenom X4) as being low for both coaxial and TosLink. Also, the CM6631A device seems quite immune to noise from the main computer (compared to the adaptive USB interface). Both ASIO and WASAPI work well for the CM6631A. Still no evidence that the jitter pattern itself changes with increased CPU load.

ADDENDUM (March 31, 2013):

Noticed this very useful post on diyaudio.com from 'bvs':

I've recently found a solution for issue that caused when CM6631A module is connected to any Mac USB 2.0 ports. When you connect module to Mac it is detected as USB Audio Class 1.0 device and you have no ability to use anything more than 48/16. Module has a reset chip LM810M3-2.93 (http://www.nscrus.ru/content/catalog/pdf/LM810.pdf), but as we can see from the CM6631/32 datasheet, the CM6631A has a power-on self-reset. So, reset chip has been removed from a board and now the module is always correctly detected as USB Audio Class 2.0 device on any Mac USB port. This method has been tested on MacBook Pro, Mac Mini and this CM6631A module: New CM6631 USB Module Assembled Board for DAC3 AD1955 DAC7 WM8741 by Weiliang | eBay LM810M3-2.93 is a 3-pin chip located on the top of CM6631A.
So, with a needle-nosed plier, I went to work on removing that little 3-pin chip (labelled "SA B" on the board)...  Here's the result (chip removed): The unit works as expected on my MacBook Pro's now with full playback up to 24/192, no need for an external power supply, and no need for custom drivers. Remember, this modification is for the CM6631A only. Earth to Microsoft - isn't it about time we got native UAC2 driver support in the OS!? Especially considering that it's been available for OS X and Linux since 2010!