We can deduce from the prior page that there are quite a few areas where the intended square wave transmission can get messed up, the more so once we realize that contrary to USB data, the signal travels in real time for USB audio. The USB receiver cannot ask the sender "yo, retransmit the second to last packet again, I didn’t catch that". In USB data use like sending data to a printer, said printer can request a packet again. In audio, the DAC must mask the lost or corrupt packet by making one up. And then there’s the 5V line nicely tucked in with the data leads. That five volts is created by the computer and computer power supplies are not the quietest lot. So a noise-ridden power lead happily interferes with our precious music data. The same is true for the ground line which connects computer and DAC. A lot of garbage can travel down that conductor. That equates to jitter galore if not handled well by shielding. A thing we don’t have to worry over is bandwidth however. The USB 2.0 standard of 480Mb/s is way beyond the 1.4572 Mb/s required for 16/44.1 audio. Even 8 times oversampled to 352.8kHz leaves plenty of headroom.

No, with USB cables we only worry about power line interference from the 5V line, noise migrating down the ground, external EMI interference and cable-induced internal reflections. In audiophile environments, the 5V issue can easily be overcome. Just get rid of the 5V as it won’t be needed with a converter whose USB transceiver is powered by the DAC. There’s only one little problem. The initial USB handshake between two devices does rely on a power link. If not available, the computer’s driver won’t see the DAC. The power line therefore should be detachable to enable the first contact, then remove itself from the equation.

If the DAC is designed so it wants to see the 5V regardless, a separate detachable power line could be connected to a 5V battery. This setup guarantees clean power [that’s the thinking behind the Bakoon 24/7 battery power supply and others of its kind – Ed.] External electromagnetic interferences can be mitigated by proper shielding. The problem of reflections of the signal inside the cable can be mitigated by using the right quality of wires and obeying the USB standard’s impedance. Here we say no to copper-clad steel as used in $0.50 USB cables or silver-plated copper. The skin effect with these types of wire works against the desired sound quality. Using the right type of wire for the job is vital for the transfer of high speed digital signals. Braiding those wires in the right way can counter EMI influences.

The above observations were triggered by an email we received from India. Denny Kaippillil asked if we were interested to review a dual-headed USB cable his company Audiocadabra offers. Denny had been looking for the right cables first as a hobbyist for years and now in a professional manner. The right cables would not only offer the best possible sound, they’d be affordable and within reach of many. Exotic products with concomitant price tags are only for the happy few. At Audiocadabra they have both feet on terra firma and combine good ears with the availability of quality labour at hourly rates not achievable in the West. When we confirmed interest in reviewing their cable, a package from India arrived after dealing with the usual paperwork when receiving anything valued over €20 from a non-EU country. The package enclosed a lockable airtight plastic container not unlike you will find in most kitchens to keep food fresh - clever. In the container sat a 2-meter Audiocadabra Optimus Dual-Headed USB cable.