MEASUREMENTS: USB Cables for Audio DACs. [2013-06-18 UPDATE]

Up to this point, as you've seen in my measurements, I have been using standard generic shielded cables for analogue and digital. As you likely also know - if you've been involved in "high end" audio for awhile - there are few issues as contentious as whether expensive cables are necessary or if they even make a difference!

I've generally steered clear of this "debate" over the years because inevitably, these discussions with friends or forum threads on-line almost never really "go" anywhere...  Nobody leaves the discussion having really learned anything, and ultimately if the discussion turns into an argument, everyone just ends up feeling "pissed off".

The conclusion one usually can get agreement on is that the best cable is no cable. Cables are passive "components", and ideally should transmit signal without loss, and does not impart any kind of signature on the quality of the signal. This certainly makes sense especially in the analogue domain where noise can easily creep in, causing various distortions or raising the noise floor. But what of digital cables? When something as ubiquitous as a generic USB cable can easily transmit >20MB/sec data to a USB hard drive, how problematic can it be to get 150KB/sec across for 16/44, ~500KB/sec for 24/96 audio, or 'just' ~1.12MB/sec for 24/192 high-resolution audio!?

After all these months of testing and posting on various topics, I figure it's at least reasonable to explore the cable issue as best I can with (admittedly) the limited selection of choices I have available around here.

For the record, up to this point in time, the most expensive USB cable I have tried has been the AudioQuest Carbon (~$160 for 5') more than a year ago connected between the PC and a Benchmark DAC1 USB. I have since sold off both items. I could not clearly hear a difference with the AQ Carbon, but then again since the DAC1 uses ASRC, I really wasn't expecting much either. Since then, I've been satisfied with the selection of generic cables I've collected over the years as computer parts or 'freebies'. It is in this light that I selected among my assortment, 3 A/B USB 2.0 cables to test out:

USB Cable A:
- 3 feet long
- not gold plated
- ferrite core on the B end

USB Cable B:
- 5 feet long
- gold plated A & B ends
- no ferrite core

USB Cable C:
- 17-feet long! (6 feet Logitech extender, 5 feet Linksys extender, 6 feet generic cable)
- not gold plated
- possible ferrite core on the extender connections?

As you can see, Cable C was made by Dr. Frankenstein. It's unreasonably long (USB specs indicate a reasonable limit at ~16'), has 2 detachable connections in between, is not even gold plated, and who knows what kind of shielding is in that thing. Furthermore, the Logitech extender portion has the thinnest wire of all the cables. Any card-carrying "audiophile" would be laughed out of town for even suggesting they use something like this!

I. Cables tested with Asynchronous USB 2.0 CM6631A interface:

Setup:

Aspire Win8 laptop --> *test USB cable* --> CM6631A USB to SPDIF --> Generic TosLink --> AUNE X1 DAC --> silver braided RCA --> E-MU 0404USB --> Aspire Win8 laptop

Let us start with what has been previously demonstrated to be a very robust asynchronous USB interface; the CM6631A. Remember that the previous tests showed that despite different laptops used, the analogue and jitter spectra were almost identical.

With the different USB cables, RightMark DAC analogue analysis (24/96 used):

Frequency Response:

THD:

Stereo Crosstalk:

No difference.

Dunn Jitter Tests (16-bit & 24-bit tests):

Cable A:

Cable B:

Cable C:

No difference.

II. Cables tested with Adaptive Isochronous USB 1 AUNE X1 interface:

As shown previously, this interface can get noisy if I play and record with the Aspire laptop (see the laptop post). Therefore I'm going to use my ASUS Taichi to play the test audio while measuring with the Aspire as usual.

Setup:

ASUS Taichi --> *test USB cable* --> AUNE X1 USB1 --> silver braided RCA --> E-MU 0404USB --> Aspire Win8 laptop

RightMark DAC analogue analysis (16/44 only):

Frequency response:

THD:

No difference.

Dunn Jitter test (16-bit only):

Cable A:

Cable B:

Cable C:

Well, the adaptive USB interface is noisier and more jittery than the asynchronous setup as we have seen before in previous posts. Depending on which cable, we can see noise showing up at certain frequencies. For example, there's a -120dB 11.7kHz spike on Cable A, 11.3kHz for Cable B, 10kHz for Cable C. But these are not the symmetrical sidebands associated with data jitter. In fact, the low-frequency jitter associated with widening of the base around the 11kHz signal looks essentially identical and perhaps most importantly, there's really nothing unusual here to suggest that Cable C is of unusual length and construction!

III. Cables tested with TEAC UD-501 Asynchronous USB 2.0 DAC: (2013-06-18 Update)

For the sake of completeness, I decided to run these same "cables" through to the direct asynchronous USB 2.0 interface of the TEAC UD-501 DAC to see if we can see any differences. Some folks asked if I could try a "name brand" cable for point of reference...  Suggestions included the AudioQuest Diamond, maybe some Crystal Cables... Well, I search high and low... And here's the Belkin Gold 6-ft (1.8m) which I managed to get on sale for <$10 locally :-)

That red stuff is just some electrical tape I put on it for identification on another project.

Here's the RightMark data for 24/96:

Frequency Response:

THD:

Stereo Crosstalk:

No difference.

Dunn Jitter Tests (16-bit & 24-bit tests):

Belkin Gold (6'):

Cable A (3'):

Cable B (6'):

Cable C (17' monstrosity):

As you can see, the jitter plots for the TEAC are quite clean. On the whole, the 24-bit noise floor is measurably lower than the AUNE X1 but the jitter plot does show a few more noise spikes (made more obvious because of that low noise floor). This is really quite academic because we're talking about noise down near -140dB!

No evidence of significant jitter differences between the Belkin Gold or any of the other cables including that terrible looking Cable C (17-footer).

Conclusion:

No evidence in these tests to suggest that the different USB cables used here with the asynchronous CM6631A USB-to-SPDIF converter, direct asynchronous TEAC UD-501 USB DAC, or adaptive isochronous USB setups should sound different (even though one would expect Cable C to be the worst). Subjectively, listening to music with Cable C through Sennheiser HD800's sounded fine. 

No evidence with the J-Test to suggest data-correlated jitter is significantly different between cables. By the way, for a good demonstration of how jitter improves with interface/cable change, look at my post on Transporter-to-Behringer connection with TosLink vs. AES/EBU. Also, remember that my Oppo BDP-105 tests were done with a single 15' USB cable - still better than Cable C in construction :-).

As usual, feel free to drop me a note if there's good data or controlled tests to suggest USB cables make a significant difference contrary to these findings.

Happy listening...

MUSINGS: About Those USB Cable Tests...

Back in April, I posted my USB cable tests (note this was updated recently with the TEAC DAC & Belkin Gold results). To recap, basically I could not find significant analogue output differences between a few cables despite differences in construction and lengths - including one of which consisting of 2 cable extenders and totaling 17 feet in length. The analogue output from the DAC did not show significant change in frequency response, distortion, noise levels, or jitter whether the USB cable was used to feed an asynchronous USB-to-SPDIF device (the CM6631A), directly to an older adaptive isochronous DAC (AUNE X1), or directly into a modern asynchronous USB device (TEAC UD-501). Since it is these analogue waveforms that get transduced to sound waves, it's not a stretch to conclude that USB cables make no audible difference. My subjective evaluation of USB cables is consistent with the measured results - no audible differences in controlled tests (my own blind tests).

These days, we all use USB cables of different lengths and varieties for high-speed devices like hard drives and generally either it works or it doesn't. USB protocol sends data in packets which consists of not just the audio data itself (up to 1024 bytes at a time for high speed mode), but CRC check to detect errors, flow control, and also address information to direct the data to the appropriate device (remember, you can attach up to 127 devices to each host controller).  The low-level details of this communication including timing are addressed by the hardware and generally outside of the purview of the software we install on the computer. Logically this means that fine timing issues (like data flow control and scheduling of the data packet delivery) would be outside of the effect of things like audio player software. Furthermore, with modern DACs, the machine will use its own internal memory buffer and very fine timing (like pico- and nanosecond jitter) will be derived from the accuracy of this internal clock.

USB data transfer rate for audio is much less demanding than something like a hard drive. Whereas we can easily transfer >200Mbit/sec to the hard drive using a simple generic USB2 cable, sending standard 16/44 stereo audio to a DAC is only ~1.5Mb/s, 24/96 only needs  ~4.6Mb/s, and 24/192 ~9.2Mb/s. If we go all out with the TEAC UD-501 DAC with DSD128 or 32/384, even that "only" takes up to ~25Mb/sec.

Not unexpectedly, some message forums got a bit heated when the USB cable test post was published since obviously the results deviated from expectations or experiences of subjective evaluation or maybe the person is trying to sell something related. I have seen no good evidence from controlled tests demonstrating otherwise.

Shortly after that post, this set of measurements came out from one of the principles of Empirical Audio (I thought he promised to measure the AudioQuest Diamond USB cable?). Nice to see he measured the down-to-earth Belkin Gold anyhow. I will let you, fine readers, examine the data and determine if you think this correlates with audio quality, or just demonstrating minute differences between lengths of copper with little correlation to digital data transfer much less the analogue output from a DAC! (Note that he's measuring 5m [~16.5 feet] lengths of USB cable to get those picosecond numbers.)

For this installment of MUSINGS, I wanted to have a look at a recent article from the July 2013 issue of Hi-Fi News where they did a "Group Test" looking at USB cables. Since I do not subscribe to this magazine, I want to thank "Mushroom_3" and "Julf" off the Slim Devices/Squeezebox forum for bringing up this article.

As you can see on the cover - "USB cable sound - Fact or fantasy? p43" was advertised. So from page 43-51 (with 2 pages of ads), they tested 10 cables.

Here's the first 2 paragraphs of the article (as usual, I reproduce only small portions out of fair use for the purpose of discussion, education, and critique):

That's a pretty gross generalization and assumption to start off with isn't it? The only thing I think "every seasoned audiophile" knows is that without cables, there can be no sound - and that's assuming your system isn't a boombox :-). That's about the extent of discussion as to whether USB cable sound could be "fantasy" (I think the better word would be 'inaudible'). From there on, it's all about oscilloscope measurements (they give data risetime measurements for the cables and use this and the eye pattern as their objective data), and minimal detail was provided as to how they conducted the "blind" subjective test - we do not know how many subjects participated, how they were selected, in what way was the 'blinding' accomplished, duration between cable swaps, etc. Furthermore, we do not know how the participants scored the cables or what the statistical outcome is - it may be just a "blind" solicitation of subjective opinions on those 10 cables (a huge number to test properly!).

I appreciate that at least Paul Miller the editor spent a page talking about the technical bits of USB cabling on page 98. At least the physical cabling characteristics were described in detail but strangely nothing on the packetized nature or how they would correlate audible differences in such a system of data transfer since IMO this would be much more educational.

The comments below will hopefully make sense to those who have not read the article.

1. From what they wrote, the test setup went like this:
         Laptop --> Test USB Cable --> Musical Fidelity M1 S-DAC --> coaxial cable --> Devialet D-Premier --> speakers
Why was the decision made to use the Musical Fidelity M1 S-DAC as a USB-to-S/PDIF converter? Seriously, you're testing USB cables but introducing a coaxial S/PDIF interface into the middle of the signal path for no clear reason when the M1 is already a fine DAC?! We know that a good USB asynchronous interface has lower jitter in general than most S/PDIF, so why not use the analogue output of the M1 directly to a preamp/amp? I would not be surprised if the Musical Fidelity is a better DAC technically than the Devialet (see below). There was no mention of what coaxial cable was used - surely this must be important since this is the digital cable directly connected to the DAC and these guys believe S/PDIF cables make a difference as per the first paragraph quoted above. How is it possible to test USB digital cables if there's potentially an even more jittery (yes, the dreaded jitter bogeyman yet again) digital coaxial cable/interface in the audio chain?

2. Why was the Devialet D-Premier used? I suspect why they used the USB-to-S/PDIF interface is because the Devialet internally resamples analogue to digital as per the discussion in the Stereophile review, plus it doesn't have a USB interface. Furthermore, the measurements for the D-Premier isn't all that remarkable as a DAC. It achieves a respectable 18-bit dynamic range (remember, the SB Touch already can do 17.5 bits) but like John Atkinson say - it's "not quite up to the standard set by the best-measuring standalone processors, such as the Bricasti M1, MSB Diamond DAC IV, NAD M51, or Weiss DAC202". I don't know about the M1 S-DAC specifically, but the Musical Fidelity M1 appears to outperform the D-Premier already (Stereophile data) and "offers performance that is close to the state of the art."

3. Why was the "grotty giveaway USB cable" measured on p.98 not part of the blind listening test? I wish they included a picture of this "grotty giveaway" - what makes it "grotty" anyways? Why not also tell us what that cable's data risetime is while you're at it? Doesn't this often seem like the case with these audiophile "shootouts"? They have all these high priced options (well, at least they included a £18 QED Performance Graphite) but neglect to include the real competition - something that is essentially free and works!  Another idea - how about testing a reasonable quality "non-audiophile" USB cable like the Belkin Gold for <£5?

4. USB data risetime & eye pattern: is it even relevant to the analogue waveform coming out of the DAC? Are they measuring something which actually COULD have sonic impact? In terms of this article with the risetime derived from the eye pattern as the prime objective measure, what evidence do we have that this actually correlates with the subjective impression? So what if the £18 QED scores 12.8ns, £60 Kimber B BUS Cu scored 12.4ns, £139 Audioquest Carbon scored 11.9ns (hey folks where's the Diamond?), and the insanely priced £6,500 Crystal Absolute Dream scored 11.0ns? What does this risetime have to do with the final DAC analogue output anyways given the nature of packet digital transmission, asynchronous protocol, and a cable that can provide much more bandwidth than required for audio - especially since you're passing the bits off through a digital S/PDIF coaxial in the audio chain?! I question if taking "a good few months" (p.98) to develop this test was time well spent!

5. Why does there appear to be so little correlation between sample rate and audio quality through the USB cables? Surely, a poor USB cable with "slow" risetime should sound worse with 24/96 or higher bitrate music, right? Yet the majority of the subjective complaints focused on 'Hotel California' off Hell Freezes Over CD or Oscar Peterson's We Get Requests FIM K2 CD. Heck, even The Beatles' Abbey Road 24/44 barely taxes the USB interface. The only really "hi-res" track was the 24/192 Helge Lien Trio Natsukashi which was mentioned 3 times total in the whole subjective write-up. Surely, to get a sense of how well USB cables work, we need to grab some DXD and DSD128 material, right? If timing/jitter were that important, there could be big issues with DSD sampled in the MHz range, don't you think?*

6. In the subjective analysis conclusion, you see the following comments: the QED is described as "almost sounds louder", Transparent Performance's cable had "rich and warm sound proving a little too luxuriant at times", Wireworld's Starlight 7 had the "woomph of air that would normally accompany the deepest bass was subjectively filtered away", or the Crystal Cable's "greater extension" - why not measure those things? Surely you can easily detect amplitude changes, differences to frequency extension, tonal changes to add "warmth" in the DAC output, right? Yet, for the objective test all we get is a risetime measurement of a piece of wire and the subjective testing done through a second digital interface of unclear quality and unclear blind testing protocol. Therefore neither the subjective or objective results appear all that convincing.

Assuming we do acknowledge this article as having validity and not just an example of a majorly flawed study (needless to say, the setup using a digital coaxial cable is a BIG problem IMO), there is one thing we learned in this piece for sure that could be of value and a good reminder. Don't buy the £70 Vertere Pulse D-Fi USB cable - apparently it's constructed out of "twin coaxial cables" (err, impedance matching anyone?), has bad rise time (27.6ns), and sounds questionable with "softening of its extreme treble and loss of atmosphere". I wonder if connecting this cable to a hard drive might show slower transfer speed due to high data error rate. Evidence that you can spend more and get significantly less quality than the 'freebie'?

Assuming we don't believe the listening tests are valid because of the methodological issues, then perhaps this article serves as an example of the unreliability of subjective listening. Isn't it possible they're all just listening to the same sound with the "jittery" coaxial digital cable and interface and coming up with different impressions? Since we have no concrete idea of how the "blind test" was conducted (especially what level of statistical confidence we can expect), it's quite possible the term 'fantasy' would be appropriate to describe the results (but verboten).

Enjoy the music folks... IMO, there is still no good evidence that USB cables make a difference to sound (well, at least with decent modern gear!).

My musical selection tonight: C.C. Colletti Bring It On Home (HDTracks 24/96 Binaural recording). Wonderfully spatial sonics and details from the ASUS Essence One + Sennheiser HD800! Have a listen to that title track.


* For the record, I do not believe there is any reason to think DSD would be affected by USB cable jitter to an audible degree with good modern DACs. Just wanted to throw out a thought which the jitter-fearing-audiophile might bring up one day.

MEASUREMENTS: Computer USB port noise, USB hubs and the 8kHz PHY Microframe Packet Noise

Block diagram of the USB 'PHYsical' chipset. UTMI stands for USB 2.0 Transceiver Macrocell Interface, a standard bus interface for the USB chipsets out there...

One could see the "glass half full" when we run into imperfections in the audio system. For example, it was serendipitous that the Emotiva XSP-1 pre-amp had such a noise-sensitive unbalanced "HT Bypass" input that I was able to detect noise and measure it previously. If it were not for this issue, I probably would never have thought about using the Corning USB 3 optical cable or consider how to isolate the noise originating from the computer USB connection to the DAC simply because in my system using balanced interconnects throughout, I had never had a problem (subjectively audible or objectively verifiable).

MEASUREMENTS: USB Cable Extension with Ethernet Cables - does it worsen jitter?

I mentioned in my "look at the sound room" post the other day that in the Emotiva XSP-1's multichannel "HT Bypass" mode, I'm dealing with some noise when the USB DAC (TEAC UD-501) is connected. In an attempt to fix the problem, I've tried a number of ways to minimize the problem - different USB ports on the motherboard, USB3 vs. USB2 ports, different USB cables, ferrite cores attached to the USB cables, even put in a USB3 PCI-E card... I've even dissected an old USB cable to disconnect the 5V line but it looks like the TEAC needs this present. All to no avail - the high pitched whine and buzz continued.

I then began looking at alternate options; here are a couple I found. The Firestone GreenKey looks interesting. I'm just not sure if it supports high-speed USB2 however which is important since the TEAC needs high speed for the 24/192+ PCM and DSD/DoP sample rates. There are other galvanic isolators like this one but they're just "full speed". Another option is with a better USB card like this Sonore SOtM PCI/PCI-E to USB which is often talked about on sites like Computer Audiophile. However, I really don't see myself spending $400 (with taxes and shipping) for a single port USB card; and I'm not even sure this will do the job I need! (Anyone with one of these cards can comment? A review like this one is useless for my purpose when there's really no discussion of whether they tried it in a noisy system and if it reduced audible background distortion.)

Hypothetically, I thought it might be interesting to try a different angle with this... Knowing that the ethernet system uses isolation transformers, what about the USB extenders using ethernet cables? Already, a cheap one was taken apart on Project Gus and it looks like these devices do indeed use standard LAN filters like the LFE8423. [My mistake, that link was for a USB-ethernet adaptor, not a USB extender like this one.]

With a quick search of eBay (here's a current auction), I decided to take a chance on one of these (also found listing on Amazon):

It includes a transmitter (computer end) and receiver (TEAC DAC end). The transmitter side will use the computer's 5V rail and a standard 5V power supply in the plastic bag is used on the DAC side - this of course isolates the power from the computer to the DAC. It also comes with a short USB A-A male cable for the USB connection from the computer to the "transmission" unit.

Notice the 5V power connector on the unit to the left for the receiving device end.

Total cost was ~$60 for the set including shipping from Asia. The unit is capable of 100m (~300ft) transmission distance and specifically rated up to the 480Mbps high speed spec. Unfortunately I don't see an easy way to crack open the little boxes to see what's inside and I figure I didn't want to risk breaking them at this point. Set up was easy and intuitive since there's barely any English in the instruction pamphlet. Plug 'n' play, no drivers to mess around with at all.

So... Did they work for my purpose? Yeah... To some extent.

They did definitely filter out the "computer noise". I can no longer hear the beeps and buzzes when the computer accesses the hard drive or when the CPU is busy processing. Unfortunately the high-pitched whine is still audible when I stand against the speakers; but much reduced - maybe 25% of previous and almost inaudible at my listening position about 9-10' away (ambient background noise is very low in my basement sound room). I tried different Cat5e and USB cables but this didn't make much different. The next time I go to the computer store, I'll see about getting Cat6a STP (Shielded Twisted Pair) cables and maybe shorter 3ft USBs to minimize any potential to pick up interference... I doubt this will make a difference however because I suspect the noise is embedded in the signal from the computer itself rather than picked up due to lack of shielding.

However, the opportunity presented in buying this item is that I can measure what happens to the DAC audio output through this USB signal conversion. If one believes that a digital cable makes a huge difference, then doing this should really create some nasty effects compared to having just a straight computer --> USB DAC cable. We could see diminution of dynamic range if doing this adds more noise to the DAC and furthermore, let's see if the dreaded jitter gets even worse! (Surely it must be awful, right?)

Setup

As usual, here's the setup used to measure the TEAC DAC output when I'm using the USB extender:

Win8 AMD A10 HTPC --> stock 3' USB (supplied in box) --> USB Extender Box --> 50 feet Cat5e cable --> USB Extender Box --> 6' Belkin Gold USB cable --> TEAC UD-501 DAC --> generic shielded RCA --> Emu 0404USB --> shielded generic USB --> measurement Win8 computer

For the direct USB situation, I used a 12' generic shielded USB cable (similar in build to the Belkin Gold USB cable) direct from the Win8 HTPC --> TEAC UD-501.

Notice how rather "unfair" this setup is for my needs... In real life I only use a 6' run of Cat5e cable, but to exacerbate any issues, I'm using a 50' run of just generic ethernet cable.

 

Results

Here's the RightMark summary results. I tested 16/44, 24/96, and 24/192 for your consideration:

Notice how there's no difference whether it's a direct USB cable or the much more complicated USB-Cat5e extender setup. 16/44 as usual measures perfectly and there's not even a hint of loss in dynamic range, worsened noise, or excess distortion using the Cat5e hardware!

Frequency response at 24/192 (16/44 and 24/96 look to be exactly the same as well [not shown]):

Noise level at 16/44 and 24/96 (24/192 looks no different as well [not shown]):

No difference!

How about the dreaded jitter? As usual, let's fire up the spectrum analyzer and have a look at the Dunn J-Test output off the TEAC DAC.

Yet again - essentially no difference.

Summary

Okay, short and sweet - using a USB cable extender with a Cat5e system like this one does not degrade the audio output from the asynchronous TEAC UD-501 DAC. Even measured at a length of 50 feet (much more than my needs), there's no problem.

Subjectively I hear no degradation in the sonic output either and as noted above, it does reduce the noise with the analogue multichannel home theatre bypass on the Emotiva XSP-1 so it does seem to filter out some noise originating from the USB port... Not perfectly silent yet in my system but I'm slowly getting there :-). (This noise does not affect my stereo playback at all off the Squeezebox Transporter or TEAC UD-501, just when I listen to multichannel.)

I had a listen last night with some hi-res stereo 24/192 material - John Coltrane's Blue Train Classic Records 2001 HDAD, Neil Young Harvest off the 2002 DVD-A. Also had a listen to some DSD128 - samples from 2L's website. As usual they sounded very good off the TEAC. All the precision I had come to expect from the TEAC with standard USB cabling was there with no hint of resolution loss or evidence of digital error.

Basically, I'm stumped when I read comments on places like Audio Asylum by audiophile "audio engineer" folks who claim that even adding a ferrite core to a USB cable will do terrible things like worsen jitter (supposedly audible!). As far as I can tell, these folks also never seem to throw up a few measurements or describe what method they use to come to such a conclusion... Even if doing this worsened the sound quality, why do they blame jitter as the problem?

As far as I can tell, bits are bits; at least with a decent asynchronous DAC (I suppose there could be problems with old adaptive isochronous units). Asynchronous USB protocols like the one used with the TEAC work as they should to clean up any stray timing errors so long as the buffer isn't over run and the data is "bit perfect" (easily verifiable with a USB hard drive attached and looking for data corruption). Hard to imagine than anyone needs more than a decent (inexpensive) USB cable for quality sound if doing this kind of cable extension doesn't lead to sonic degradation. I do believe that there are many mysteries in this universe outside of the science we understand today, but digital reproduction in the audible spectrum doesn't need to be one of these! As usual, feel free to provide a link to some results if there's evidence to suggest I'm incorrect.

As we wind down the last days of 2013, I want to wish you all a Happy New Year. And of course a healthy and prosperous 2014 ahead...

Enjoy the tunes.

[2013/12/31 Update]

So, I headed off to the local computer store earlier this morning and bought some 50ft Cat6a STP (shielded) ethernet cables to try. I figure if this doesn't make a difference, the higher quality cable can be used for longer runs around the house.

Surprise! It worked... The whine is now gone. The analogue "HT Bypass" is still a bit noisier than straight balanced cable into the Emotiva XSP-1 preamp but at least there's absolutely no high-pitched noise audible with ears right up to the speakers now.

Again, subjectively, everything sounds great comparing straight USB to the USB-Ethernet extender through the TEAC DAC. Objectively, no evidence of deterioration and looks the same as previous results above:

At least for me, it looks like the use of one of these high-speed ethernet cable extenders is a good option to try for those suffering from USB-related noise in the audio system with no evidence of signal degradation (including jitter with an asynchronous DAC).

Happy New Year and stay safe with those late night parties! :-)

DEMO & MEASUREMENTS: What does a bad USB (or other digital audio) cable sound like?

Okay, so the other day I was installing my new BenQ BL2710PT monitor (reasonable monitor for a decent price) and as I was rummaging through my old cables, came across a very old USB 6' cable that I probably got free when I purchased an old Samsung laser printer back in 2001 during the transition from USB 1.1 to early USB 2.0.

This cable is the thinnest, most flexible, likely most poorly shielded USB cable I have; in other words, about as "bad" as it gets when connected to a quality USB DAC which expects to operate in high speed USB 2.0 mode without completely failing... Behold the "Bad Cable":

Plugging this cable into my desktop ASUS Essence One provided the opportunity to demonstrate just what a poor USB cable does to the sound... I'm sure this is "old hat" to those who have experience with digital audio, but for those who haven't, have a listen...

I recorded 1 minute of a freely available track from Jason Shaw called "Pioneers" from here off the Essence One fed into my EMU 0404USB to the usual Win8 laptop using a good quality cable versus the flimsy one above.

Good USB 2.0 cable - well shielded 12', ferrite core on both ends of this specific cable:


"Bad" USB cable as pictured above - poorly shielded against interference and incapable of transmitting at bit-perfect high-speed data rate to the ASUS Essence One:


Even though SoundCloud recompresses the uploaded FLAC audio, I'm sure you can appreciate the obvious errors in the "Bad Cable" sample. (You can press play back & forth between the two samples to A-B them if you want.)

What you're hearing is what happens with digital error (ie. not bit-perfect), similar to watching digital TV with the occasional data error leading to macroblocking and bad pixellation as in this sample found off Google (notice the blue stripe due to digital error):

It's worth noting a few characteristics of this poor cable as it pertains to sound:

1. Poor digital cables leading to digital errors sound like brief pops or occasional static (assuming they do not completely malfunction). They're similar to the errors you get when ripping a CD without something like EAC or equivalent. Sometimes, you'll hear very brief dropouts. Depending on the data packet disrupted, occasionally they will occur in only one channel but not possible for this to happen consistently in a single channel. Remember that although asynchronous DACs have the capability to buffer, hence improve timing and lower jitter, they do not (at least not in the case of the Essence One with the CM6631 USB interface as far as I can tell) necessarily error correct or ask for a packet resend. The more data error, the less the amount of "normal sounding" music will be heard. Obviously if the data error occurs every few minutes, it might be difficult to detect, but if it happens frequently, it's not subtle.

2. A poor digital cable does not result in overall level changes in the song... This is not like analogue distortion that can consistently alter the volume level or change the dynamic range uniformly or periodically.

3. Similar to the above point, poor digital cables are not capable of changing the overall tone of the sound. There is no such thing as a digital cable capable of acting as a "tone control", making certain sounds "brighter" or "warmer". A passive digital cable is not capable of acting with some kind of frequency filtering mechanism.

4. Poor digital cables do not consistently do anything to the soundstage. A poor digital cable cannot make a voice or instrument sound "distant" or move it "forward", or pan the soundstage to the left or right as a whole or in relation to other components of the music.

5. Bad cables cannot cause speeding up or slowing down of the data transfer. Poor digital cables therefore cannot cause sporadic or consistent timing issues like warble (speed up/slow down pitch changes), "pacing", or rhythm problems.

6. The concept of cable "break in" makes no sense with digital audio cables. If it carries data accurately when plugged in then the only problem that can happen in time is corrosion at contact points or reactions such as oxidation of the metal over time. This can only lead to transmission errors as demonstrated above, not some magical improvement due to "break-in".

7. I was reminded here the other day about the measurements with a poor RCA cable I used as coaxial SPDIF last year. Indeed, if you use a very poor, unshielded RCA cable paying no attention to the expected 75-ohm impedance specification with an SPDIF digital interface that's not galvanically isolated (eg. coaxial SPDIF of the ASUS Essence One in that case), noise can be introduced into the system. However it does not take extravagantly priced cables to make things right (an inexpensive 6' <$20 decent shielded cable from a reputable company will do). As always, noise can be introduced into the analogue domain with any electrical connection (or just being careless like putting your DAC right on top of a noisy desktop computer), so it's not really an issue with the digital system itself.

You might be curious how the 2 USB cables measure in terms of jitter...

Surprised? As you can see - not much difference at all! If you monitor the realtime FFT for the J-Test, you will see errors "popping up" with the bad cable due to bad data transfer, but in between, the jitter plots are essentially indistinguishable! This is expected... For an asynchronous DAC like the ASUS Essence One, jitter rejection is handled very well by design and there's nothing the passive cable can do about that.

Now I'm sure there will be a number of folks who disagree and hear various effects in the list above (see here, here and here for some interesting perceptual accounts and/or creative writing). The thing is, where is there decent evidence to show that passive digital cables (and I'm talking here not just of USB but also the SPDIF variants like coaxial or TosLink) sound different if they're error free (a.k.a. bit-perfect) and built to specifications (assuming no issue with analogue noise as in item 7 above)? I've never seen manufacturers come up with anything of substance... Or hobbyists/DIY guys show/demonstrate verifiable claims... As usual, I'm happy to change my mind if some kind of objective evidence exists since I personally have not subjectively heard a problem except as demonstrated above with digital errors.

(Digital cable summary from a number of months back for those who might have missed it. Recent post on EETimes blog on this.)

Recommended album:
- Have a listen to Babatunde Olatunji's posthumous 2005 album Circle Of Drums. This is one of the best Chesky albums I have heard. The drums sound fantastic with wonderful tonality and sense of 'space' around the instruments; a lovely exploration of African drums and rhythm. Unless you believe you can hear the difference between 16-bit noise floor and that provided by SACD, IMO there's no need to buy the SACD because it appears to be a 44kHz PCM upsample (here's the Master List). There is a multichannel mix on the SACD which sounds OK but derived from post-processing. An impressive sounding and quite enjoyable record for those interested in world music nonetheless!

Relax and enjoy the music!

----
Addendum (January 20, 2014):
For the sake of completeness in answering Frans' comment below, here is the J-Test result with the TEAC UD-501 using the poor USB cable vs. good one:

The TEAC uses quite a different asynchronous USB implementation than the Essence One. It appears to utilize a TMS320 DSP processor rather than "off-the-shelf" USB interface like the CM6631(A). I'm wondering if this interface is actually more robust since the digital errors were even less obviously audible.

In any case, using a different DAC, the jitter test remains unchanged; two examples now of how an obviously poor USB cable does not appear to affect the jitter from asynchronous DACs in terms of the analogue output (which IMO is the only important measure since that's what we hear!).