What makes a Digital Interconnect

Good quote from Charles Lamb, Stan. I'll have to remember that one!

For the op's info, 110 ohm digital cables are typically used with xlr connectors for transmitting balanced aes/ebu digital signals. 75 ohm cables are typically used for unbalanced spdif digital signals, via rca or bnc connectors.

In the case of digital or other high speed signals, it IS important that cable impedance match the output impedance of the transport or other source, and the input impedance of the dac or other load. Otherwise increased jitter or even mis-clocking and data corruption can result.

Good point about 1.5 meters (and not less) being the optimal length. The reasons are explained here: http://www.positive-feedback.com/Issue14/spdif.htm.

The importance of all of this is, of course, highly dependent on the jitter rejection capabilities of the particular dac design. There are several unquantifiable variables that enter into the picture as well, including the risetime and falltime of the output signal of the particular transport; the ambient electrical noise environment; ground offsets between the two components; the value of the logic threshold for the digital receiver chip at the input of the dac; the clock rate of the data (redbook or high res), etc. So Stan is right -- give it a try, and if possible compare with a few other cables in your particular system.

Best regards,
-- Al

Almarg

Al wrote
Columbia (BSEE); RPI (MSEE). However, my engineering career (now retired) was in defense electronics, not audio.

For a moment there I felt quite safe... knowing I was out and you were in.... then I read the retired part.

I'm feeling less secure Now.

I posed my OP to the manufacturer. Here's the response edited to anonymise, the cable in the OP is X:

"The X is an excellent digital cable, the Y is the digital version, and essentially just has more shielding. Both are 75 Ohm cables, with killer bandwidth and excellent group delay. If you are using X [instead of] Y, I would save your cash, spend it on music instead. "

Here are the specs on the dedicated digital cable "Y":

3.3’(1.0m) [RCA > RCA]
Geometry: coax
Bandwidth: > DC - 1 GHz | Linear Phase
Cp: 65 pF (pin / ground)
Ls: center pin 2.6 μH
ground 0.5 μH
Bend Radius: 3.5” (90mm)
Cable Diameter: 0.35” (4.8mm)
Shielding: Shielding shunted to ground at both ends of cable

Well, that's more like it, in terms of the shield inductance being less than the inductance of the center conductor. However, 2.6uH and 65pf computes to a characteristic impedance of 200 ohms. So I'm at a loss to explain how it could be a 75 ohm cable.

Regards,
-- Al

My curiosity piqued, I did a few minutes of detective work and figured out which cables are being referred to.

I notice that their higher-end digital cable (let's call it cable Z), which costs a bit more than four times as much as cable Y, has much higher capacitance, with slightly lower inductance (Ls = 1.5uH center pin; Cp = 270pf). That calculates to almost exactly 75 ohms!

It appears that cables X and Y, and possibly cable Z as well, use an unusual construction technique of having the conductive path for the center pin being a deposited layer surrounding a heavy gauge non-conductive core. The claimed advantages of that approach appear to essentially be mechanical in nature (durability and perhaps easier and more consistent manufacturing).

But I suspect that approach is the underlying reason for the relatively high inductance, which results in the cables having very high characteristic impedance unless the high inductance is offset by high capacitance (which is the case with cable Z).

The bottom line, imo, is that I would not recommend cables X and Y for use as digital interconnects.

Regards,
-- Al