Different length digital cables all true 75 ohms?

The 75 ohm impedance rating of a digital cable does NOT mean that its resistance is 75 ohms. It's resistance for any reasonable length is likely to be well under one ohm, and insignificant in relation to the source and load impedances of the two components it is connecting (which presumably are 75 ohms). If you were to measure the resistance between the two ends of a digital cable with a multimeter (either between the center pins of the two rca connectors, or between the shield connections/ground shells of the two rca connectors), the multimeter would indicate that very low value.

What the 75 ohm cable impedance refers to is what is called "characteristic impedance" which is a concept that only comes into play at frequencies that are much higher than audio frequencies, such as radio frequencies, and the high frequency spectral components of digital audio signals.

The idea is that the characteristic impedance should match the source and load impedances, or else reflection effects will result at those high frequencies, resulting in waveform distortion, which in turn can result in increased jitter and even mis-clocking and data corruption or loss if severe enough.

The characteristic impedance of a cable, btw, is to a close approximation equal to the square root of (its inductance per unit length divided by its capacitance per unit length). Therefore it is independent of length.

Regards,
-- Al

Thank you Al for clearing that up for me. I guess I don't have to worry about it, unless I suppose it was some stupid long length cable.

I was thinking about your last statement regarding the square root of a cables inductance divided by the capacitance gives the impedance.

Many high and low end cable manufacturers will use the same cable that they use in their interconnects and speaker cables in their digital cables and are probably not 75 ohms. I wonder how many actually make the effort to alter the cables inductance capacitance or inductance to make it 75 ohms.

Also how many audiophile cable companies actually use near true 75 ohm RCA connectors. I understand some of the pedestrian cable companies actually use Canare RCA connectors which are regarded as near true 75 Ohm. However when I read the product description of many of the high end companies description of their digital cables, for some reason they don't describe the connector.

Yes, it makes one wonder.

I would expect that the significance of the connectors being close to 75 ohms, as they are with the Canare's, would be subtle, and would probably be insignificant if the dac has good jitter rejection capability.

If the cable had a characteristic impedance significantly different than 75 ohms, though, that would figure to be quite significant.

There are lots of system dependencies involved, though, which can obscure or overshadow the differences. Quoting from myself in another thread from a couple of months ago:

Digital cable performance is dependent, among other things, on the length of the cable; the accuracy of the impedance match between cable, connectors, transport output circuit, and dac input circuit; the data rate of the signal being transmitted (44.1 or 96 or 192 kHz, etc.); the jitter suppression capabilities, if any, of the dac; the integrity of the shielding in the cable; the risetime and falltime of the transport output signal; the ambient noise environment; and other factors.

Regards,
-- Al