Cables

Curious Cables USB Review

Intro

Contents

Introduction

I received the Curious Cables USB a few months back and postponed this review until I was able to compare this venerable cable with at least a dozen other USB cables. We all know it’s a fantastic cable, but how does it fare against the giants of today?

So over the course of a few months, I was insane enough to interview over 20 audiophile USB cables, most of which I had on hand for direct comparisons. Each and every cable had their “flavor” and I believe most will be able to blind test with high accuracy, especially silver vs copper. Without exception, even the worse sounding audiophile USB cable sounded better than the generics I had on hand. Most generic USB cables are just peaky and disoriented. Once in a while, I’ll pop in a generic for a sanity check.

When you’re copying your bootleg movies from your PC to an external hard drive, it doesn’t matter if it takes 5 minutes or 5 hours, your bits are error checked and consistent. Barring any physical problems with your storage device, you’re guaranteed a consistent copy via retransmission of bad packets. For streaming media such as video from your webcam or audio to your DAC, an isochronous transfer protocol (USB has a few endpoint types) is used. What this means is that once the bits are sent from your PC, packets are never retransmitted regardless of CRC errors. If packets had to checked and resent, this will be clearly audible in your system.

Asynchronous & Isochronous Basics

Isochronous refers to the transfer type and asynchronous USB essentially refers to the location of the master clock (who controls the clock?). In the context of USB, isochronous and asynchronous are mutually exclusive notions.

USB initiates a transfer with a specific device with of one of four types: bulk, control, interrupt, and isochronous. Bulk transfers are not time-critical (USB hard drives) and frames will be retransmitted when an error is detected.

With isochronous transfers, a certain amount of bandwidth is allocated on the host to accommodate the frames/bits required per second by the audio stream. DACs could detect errors but frames will not be retransmitted. But what defines the notion of a “second”?

With synchronous USB, the clock resides on the host (your PC, server, etc). With asynchronous USB, the master clock typically resides on the DAC. The DAC tells the host when to send the packets which essentially defines what a “second” means. This way audio data is “packaged” and streamed properly and the DAC never misses a sample. This does not ensure the integrity of those incoming bits, however, it just guarantees a specific rate using the presumably more accurate/consistent master clock on the DAC.

USB Cable Design

To get a better idea of what goes into the design of a USB cable, let’s break it down.

We have the two data lines (D+ and D-), the 5V power leg, and the ground leg. USB works via differential signaling, much like balanced cables. What determines whether the signal is a logic “0” or “1” is the differential voltage differences. On the receiver end:

  • Differential Logic 1 – D+ is 200mV above the D- signal
  • Differential Logic 0 – D+ is 200mV below the D- signal

Put simply, on the transmitter end, when D+ is above 2.8V, it’s a “1” and if it’s below 0.3V it’s a “0.” D- is the inverse so when it’s below 0.3V, it’s logic “1.”

These signals are square waveforms and any slight slope, tilting, or mistimings are measurable and audible. With USB’s fast transfer rates (various voltages at 480 MHz+) and our noisy gigahertz speed PCs emitting electromagnetic radiation into the USB port, it doesn’t seem unlikely that ‘0’s are mistakenly represented as ‘1’s and vice versa. The digital signal is a bit more analog and fragile than you would think. Due to various environmental and internal factors, these voltages might be skewed one way or another. I’ve broken down some of these factors:

  • Crosstalk – The electrical interaction of adjacent conductors with one another. We want to isolate and prevent any “leakage” of energy to other conductors.
  • Capacitance – Electrons get temporarily “stuck” in the cable typically in the insulation/dielectric. When electrons are held and release, this nonlinear propagation attenuates the signal causing plenty of signal timing issues.
  • Inductance – As a signal is going across a wire, a magnetic field is created. This magnetic field will create eddy currents in the conductor. These eddy currents will generate an opposing magnetic field which will impede the propagation of the signal and could potentially convert some of that signal energy into heat. Typically, the larger the conductor, the stronger the opposing force.
  • Resistance – Impediment in voltage typically correlated to the length and gauge of a conductor. Generally speaking, the shorter the USB cable, the better. From my listening sessions, imaging and resolution seem to improve with shorter cables. Both of my Danacable TruStream USB cables are less than 2 ft.
  • Impedance Mismatch – USB cables are 90-ohm, a mismatch may cause reflections in the signal. Something like the UpTone ISO REGEN may help here.
  • Quality of Connectors & Termination
  • Geometry – Unsurprisingly, conductor layout is very important in all cable design. Shielding approaches need to be in place to prevent noise from affecting the signal path.
  • Clocks – This isn’t cable related but an accurate clock on both the PC and DAC side are imperative to a clean, jitter-free, and pure USB audio signal.
  • EMI/RFI – External factors could introduce noise into the signal. Proper shielding will help mitigate this.
  • Conductor – Higher quality conductors won’t degrade the signal over reasonable lengths

An audio signal needs to be heard in real-time. Anything less would introduce some level of artificialness of the music. The USB standards themselves don’t even take into consideration the inductance or capacitance of a cable, which really only affects an audio or video signal. Many of these electrical parameters could introduce jitter into the signal (timing errors), which all human ears are unequivocally sensitive to. It’s apparent the quality of the material and geometric design used for a USB cable is correlated to the level of signal deformation. Coming up with the right dielectric that matches well with the choice conductor material and figuring out how to arrange all of it is a big part in USB cable design.

This isn’t Rob Woodlands’s first rodeo in audio. He also manufactures the highly esteemed Bullet Plugs, and had few creations of his own: the Bayonet plug and Cable Pods.

So what was Curious Cable’s Rob Woodlands approach to USB cable design?

Curious USB construction:

  • Physically isolated 5V, fully shielded mini coax power leg. Most of the noise on a cable will come from this interaction. There’s no better way to get this noisy leg away from the data lines than moving it outside the cable. This is the only cable I’ve tested with a fully isolated power leg.
  • Shielded ground leg to prevent the transmission of noise to the data lines. The relationship between the data and ground lines are of utmost importance.
  • Unshielded data lines
  • Pure silver conductor. In my opinion, this accounts for the “dimensional” sound you get with the Curious USB.
  • Out of all the cables I’ve tested, the Curious USB seems the most laborious to construct. Kudos for not cutting corners here.

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Jay Luong

Mr. Audio Bacon himself. An open-minded electrical engineer and software developer by trade. I have an obsession with the enjoyment of all things media - specifically in the realm of music and film. So much heart and soul (and money) go into the creation of this artistry. My aim is to find out which products get me closer to what the musicians and directors intended.

View Comments

      • I own the Curious, Chord and Supra. I prefer by far Chord cheaper C-Line, even the Supra.

        Curious is a little more detailed but an unnatural sound. I don't know why i still belive in professional reviews, just my fault.
        Wasted money.

        • The Curious surely doesn't have a natural tone. Only resolution. I'll have to listen to the C-Line.

  • Nice review, and maybe I will get it but there is one cable I don't know if you ever saw it, its QED Reference USB they promote it with very low jitters 12 ps rms.

    Cheers

  • Very well written review, as someone whom uses the Audience AU 24 SE | Cabledyne Reference Silver | Neutral Reference I [ improved ] Silver USB cables, and was seriously thinking about adding the Lush......, your review has in fact made me realize I've seriously been overlooking the Curious USB cable for far to long. And it's nice to know that its constructed from silver, as most reviews never hinted at what the actual conductor was.

    Thanks for being so insightful on all of your reviews, it makes getting through this whole concept behind CA, all that more entertaining.

    Regards,
    O_o scar Johnson

    • The great thing about the Lush cable is its ability to layer the sound well, without sacrificing warmth or demolishing your wallet.

  • Thanks for the review! I'm wondering if you have any thoughts on the Curious cable versus the Nordost Blue Heaven USB cable or even the Audiowuest Carbon?

  • Good review, but your breakdown of how USB works is incorrect, pretty much all USB transmission is asynchronous now not isochronous, unless the DAC is really old. Also no need to worry about timing errors or any the waveform problems you wrote about that only applies to spdif not USB. USB is all packets and frames, all are CRC checked and packets discarded if there is an error. The good thing is that rarely happens even with the most grossly noisy and poorly made cables. I'm glad you liked the curious cable, others seem to as well.

    • Your knowledge of the USB protocol is obviously perfunctory but that's OK, some of the terms are confusing. Isochronous refers to the transfer type and the asynchronous approach essentially refers to the location of the master clock (who controls the clock). Isochronous and asynchronous are mutually exclusive notions.

      USB initiates a transfer with a specific device with of one of four types: bulk, control, interrupt, and isochronous. Bulk transfers are not time-critical (USB hard drives) and frames will be retransmitted when an error is detected.

      With isochronous transfers, a certain amount of bandwidth is allocated on the host to accommodate the frames/bits required per second by the audio stream. DACs could detect errors but frames will not be retransmitted. But what defines the notion of a "second"? With synchronous USB, the clock resides on the host. With asynchronous USB, the master clock typically resides on the DAC. The DAC tells the host when to send the packets which essentially defines what a "second" means. This way audio data is "packaged" and streamed properly and the DAC never misses a sample. This does not ensure the integrity of those incoming bits, however, it just guarantees a specific rate using the presumably more accurate/consistent master clock on the DAC. There are reasons why external master clocks exist (timing errors) and many have shown these malformed waveforms while measuring these USB cables.

      • But the malformed waveforms that so many people reviewing cables like to point out rarely the cause of problems in USB communications. In almost all cases the voltages is enough to be distinguished as a 0 or 1 and assembled into the packet. If there was a problem with even one of the bits being flipped the packet would fail the CRC check and be discarded, but that almost never happens. My real point is just pointing out that when you explain to people the idea that a 0 or 1 could be slightly malformed and cause little errors due to the USB cable you are misleading them, because I am sure you are aware that via asynchronous USB the data is not sent as a bitstream, rather as chucks of data or packets. The only thing that matters is that the packet arrives to the DAC fast enough to fill the input buffer. The whole idea of using packets is so that things like time intervals in communication don't matter anymore, the protocol just needs to ensure the packets are delivered with perfect integrity (CRC check) and get there quick enough to fill the buffer. This idea of malformed waveforms has little to no place in USB discussions.

        • My point is that the transition between a zero and a one isn't clean due to the design of the cable. The digital signal has to be treated as more of an analog waveform. https://www.bluejeanscable.com/articles/all-ones-and-zeros.htm

          This eye pattern depicts a signal which is not terribly degraded, so that the difference between a "one" and a "zero" is still easy to read if your clock samples the bit at the right point in time -- but the values shown vary enormously for both ones and zeros,and as the effects of return loss and crosstalk and attenuation pile up (by making the cable longer, or of lower quality) this becomes more difficult, and eventually impossible, to do. As you can see, even in this very clean "eye," the value representing a "one" or a "zero" is quite variable, and the transition between them is not sharp and clean, but has a considerable (and variable) slope time.

          The clocks also have to sample at the "right" time but this is timing is difficult to achieve.

  • VERY NICE REVIEW. I HAVE OWNED AND TRIED SEVERAL CABLES- CURIOUS, NORDOST, WIREWORLD
    AUDIENCE AND THE LATEST CARDAS CLEAR (CLEAR HS USB)
    MY FAVORITE IS THE CARDAS IT HAS GOOD RYTHYM AND TIMING, SOUNDSTAGING, DEPTH AND HEIGHT AND A VERY NEUTRAL TONE
    WOULD SUGGEST YOU TRY TO REVIEW

    • I tried the Cardas vs Audioquest Carbon, Neutral Audio of Italy, and LAT international. The Cardas is easy to listen to. Audioquest definitely more dynamic and vivid, but a little hard on the ears. The LAT is about right between those two, more vivid than Cardas, easier to listen to than the Audioquest.
      Based on this review, I would like to try the Lush USB cable. I'm pretty sure the "winners" in these tests will vary somewhat with equipment, but not by much.

  • Thanks for the review. I have both the Lush and Curious USB and prefer the Lush. I found the Lush to be more detailed than the Curious but in a more even manner. The Curious is indeed more exciting and a bit lean.

  • Thanks for the revieuw.

    Please, could you tell me the difference between the TruStream and the Lush?
    Does these 2 cables compare to the Puist 30th ann?

    Thank you.

    • Lush has a richer and denser tone than the TruStream. TruStream is more dimensional and more tonally accurate IMO. Purist 30th doesn't have a deep or full low-end but is silky smooth with proper timbre.The Purist is more detailed and resolving than both the Lush and TruStream, but less warm sounding.

      • Ok thanks!

        Would you describe the Lush as a bit to much dense sounding (for a natural sound)?

        Both the TruStream & Lush will add body/weight to the sound, right?

        And the Lush would be superior to the Vovox?

        • Both Lush and Vovox have more body than both the Curious and TruStream trading off depth and resolution. There's a naturalness to the Vovox and most of their stuff just sounds "right" as in what the artist intended (as far as tone and timbre). I feel the Lush is the warmest cable I've ever tested and it colors the sound a bit more with a richer midrange. However, it does have better depth, separation, and layering than the Vovox.

  • My at home set up has included an iMac with Pure Music, an old but fine Kimber silver USB cable, Chord Hugo TT, Danacable Reference cable, Focal Utopias. I was totally happy then read your review. I auditioned the Curious Cable USB after about 150 hours of break in and am so pleased by the music transformation. All the adjectives other reviewers have used are seconded here. I've not heard the other high end cables you've mentioned but you have opened my ears about the role of the USB cable. Thank you.

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