The Absolute Sound "review" of USB cable reads suspiciously like a press releaste


I mean c'mon—can't you even throw in a few according-to-the-manufacturer's?

http://www.theabsolutesound.com/articles/nordost-purple-flare-usb-cable/?mc_cid=1f41b2b3b4&mc_ei...
john_g
Actually it’s the same argument. The CD argument is that digital data encoded on the CD cannot be changed and that laser servo mechanism + error correction together ensure that the 1s and 0s maintain their integrity all the way to the DAC. It’s called the 1s and 0s argument. It’s the same argument used to “prove” that there’s no difference among digital cables, that 1s and 0s are being transmitted by the cable so aren’t affected by rf or vibration like analog signals are.
Perhaps you view it that way. I do not. CD readback is imperfect because there are mechanical and optical imperfections in the recording and playback processes that result in timing issues and there also is stochastic jitter. These processes are intrinsically more complex than the rather simple USB interface impairments.
The attached CD data recovery circuitry is tasked with the restoration of the near ideal bit sequence that then is applied to the DAC. 
There's plenty of room for the introduction of timing errors/jitter in that process even if the result is ultimately bit perfect.
Ideally the DAC will retime the input data to restore a pristine, low jitter, bit perfect,  data sequence for conversion to analog.
Incidentally, the same kinds of circuits/techniques are used to recover data for high speed data links such as is used on the backbone of the internet. The jitter requirements for those applications are orders of magnitude lower than the needs of audio and although the noise bandwidths are different the issues remain the same, particularly as modulation of the phase and/or modulation of the amplitude is used to increase the data capacity.
“There’s plenty of room for the introduction of timing errors/jitter in that process even if the result is ultimately bit perfect.

“Ideally the DAC will retime the input data to restore a pristine, low jitter, bit perfect, data sequence for conversion to analog.”

>>>In my view the damage is done immediately and irrevocably as soon as the laser reads the data. The primary reasons why the CD playback process is imperfect is due to scattered laser light getting into the detector and vibration of the CD player and CD itself. The nanoscale geometry involved - laser beam width, pit depth distance from laser to the CD metal layer - makes the job of the servo mechanism, especially with all the vibration that is occurring, impossible. You get what you get.

The USB cable argument is probably more similar to the digital cable argument. Or so it would appear.


One other thing that needs clarification- the concepts of bit perfect and low jitter are NOT homomorphic.
Bit perfect simply means that the recovered output data sequence is the same as the original input sequence- which in turn means that the recovered output changes in state are within the correct timing windows.
It tells you nothing about the stochastic and deterministic components of the jitter other than the combination of both in any given timing window does not cause the data to fall outside the recovery region.