My previous post was about using an adapter instead of a cable on computer side of Topping HS02 isolator. Of course, on device side I need a cable. This thread inspired me to search for more information about optimal digital cable lengths including some explanations and best also some calculation.
Minimal cable length is calculated for a given rise time of digital signal. Rise time is transition time from digital level 0 to 1. The first reflection should not arrive sooner as the rise time value. Given that condition, John Siau* calculated maximum rise time for a cable length of 1.5m: “If the rise time of the original signal is less than 11.9 ns the reflections will arrive after the transition has completed and the reflections will have no impact on the timing of the data transition”. Given that information, max. rise time for 0.75m cable then would be about 6ns.
* Thread title on ASR is “Technical Article: Digital Audio Cable Reflections and DACs”. Actually it is the last post of the thread.
I did not find more exact information what is rise time value for high speed USB transfer (480MBit/s), which is used for audio.
John Siau also did some computing of maximum cable lengths: “If 2 transits through the cable is equal to the period of data transitions, the third transit will arrive in sync with the next data transition.”. It means he points to a situation when the 1st reflection arrives just at the time when next data transition starts at computer side. He calculated that for 44.1k clocked source side (SPDIF) this corresponds to a max. cable length of 9.5m: “Using RG59, this unfortunate alignment happens when the cable length is 9.5 m long. Using the fastest coax, this problem would occur at a length of 14.9 meters. Divide these lengths by two for 88.2 kHz audio, and divide by two again for 176.4 kHz audio.”
It is now easy to recalculate this for 8kHz high speed USB audio:
max_length1 = 9.5 * 44.1 / 8 = 52m
max_length2 = 82m
Since USB cable length is restricted to 3m it means the first reflection arrives always much much sooner than the next data transmit. These calculations show that longer USB cable is safer than shorter – from reflections point of view. Longer cables allow longer signal transition rise time without being influenced by the 1st reflection.
Given these calculations, I decided to compare my 0.75m Supra cable with 1.5m Belkin Gold Series. I repeated the comparison more times and I’m sure my decision is not result of bias after reading the above calculations. I prefer the longer Belkin cable so I will continue to use it instead of Supra cable. Without HS02 USB isolator I did not find Belkin as better cable. Supra has separately shielded power and data wires. With HS02 isolator it seems to bring no benefit anymore.
Always do a listening test and trust your ears more than my opinion!
