Does the 1.5m 75ohm rule apply to 110ohm?

mike, thanks for your useless input on this subject.
Please don’t post to my threads if you intend to be negative.

1. I’ve never heard it from a cable vendor.
2. There was at least one scientific paper pointed out by Al Marg concerning this issue.
His insights were frequently useful, always lucid. I believe he was an EE and audiophile so he knew his stuff.

oldears, I agree that bnc connections are probably better than rca because they are generally 75ohm although they come in other values as well.

@georgehifi, search the digital cable thread for almarg’s comments on the spdif question. The thread lasted a week or more at the time, 2yrs ago maybe. ??

1.5m rule is based on transition and propagation times. Reflection from RCA connector might be stronger, but there will be reflection on any impedance boundary, unless DAC’s digital input impedance is perfectly match to cable’s characteristic impedance (that can be a little different than 110 ohm). Transition time is unknown (only estimated) as well as propagation time (that depends on dielectric). 1.5m is good place to start (based on 25ns transition and 5ns/m propagation). Very short cables (less than 1ft) might also work, but are not very practical.

What happened to the post with the links? It appear to have been deleted?


I did check one of the linked articles. Conceptually it was okay, but the math was questionable. They considered the rise time to determine a mid-point, but did not consider at 0 time (start of rise), there is no signal to reflect and what you need to consider is both the amplitude of the initial arrival plus the amplitude of the reflection which is signal and mismatch dependent. With poor matching, that 1.5m could be longer (or shorter) and there will be one length that will be approximately the worst, and that is probably near 1.5m though it could be anywhere between 1-2 meters. 


One of the articles talks about speeding up the edges, but the best thing to do is just to use proper impedance matching.