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.
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.