Digital cable optimal length?

just curious -- though i haven't read anything to this effect -- but would this 1.5M rule apply to standard interconnects as well? Or in general just the shorter the better?

No, all of this has no relevance whatsoever to interconnects carrying analog audio signals, or any other kinds of signals of comparably low frequency.

And in the case of digital interfaces, the 1.5 meter figure would only be applicable to situations where the signal risetimes/falltimes are similar to the values for typical transport-to-dac digital interfaces. Similar principles would apply to other digital and high speed interfaces, but the numbers would work out differently.

Regards,
-- Al

Thanks Al. Appreciate your knowledgeable posts.

Yes, Indeed Al. Thanks for elaborate clarification. It is much more clear now.
Thanks Roadmann99999 for the link. I had Canare once. $32 for 3 m digital cable is a surely a bargain. May be for me the solution would be to bring my other transport closer to my common DAC.

I am still itching to try out though. If it works, it will save me a lot of trouble. The real estate between two transports, a Pre, three TTs, two phonos is really crowded. All of my components have separate power supplies too, and that makes things even more jumbled.

The Canare idea sounds real good. I will report back. Later..

Nilthepill -- A further thought. While I haven't researched or particularly seen listener comments on going longer than 1.5m, it seems to me that you should be able to have a significantly longer run than that without running into the problem we have been discussing (that arises from going shorter than 1.5m).

Especially if you are using only redbook cd data rates (44.1kHz sampling, 16bit data for each of 2 channels). But perhaps even at 96kHz/24bit/2 channels, or more.

Given that the two-way propagation delay of a 1.5 meter cable gets the re-reflection past the middle area of the leading edge of the original waveform, adding the additional delay of a longer cable will not become a problem until it is large enough to place the re-reflection on the NEXT edge, the next edge being of the opposite polarity (e.g., negative-going instead of positive-going). (Even if only the positive-going edges of the waveform are used by the dac, distortion in the middle of a negative-going edge could conceivably cause it to be seen as a positive-going edge).

The clock rate is 2 times the bit rate for SPDIF and AES/EBU: http://www.epanorama.net/documents/audio/spdif.html

So for redbook data, the clock rate is around 2.8MHz, a half-period is around 180ns, which at 2ns/foot propagation speed (which is roughly typical for electrical cabling), or 4ns/foot factoring in the fact that a round-trip is involved, corresponds to a length of 45 feet, or about 14 meters. To allow some tolerance on what part of the edge is actually responded to by the dac, we should reduce that somewhat, say to 10 meters.

At the other extreme, if you were transmitting 192kHz 24bit samples, you are increasing the data rate by a factor of about 6.5 compared to redbook (192/44.1 x 24/16), so the 10 meters would be reduced to approximately the 1.5 meters we have been talking about. For 96kHz 24bit samples, the corresponding answer is right at the 3 meters you were hoping to use.

So the bottom line, it seems to me, is that if you will not be dealing with data rates that are above 96/24, you could very well see no degradation from a 3 meter cable. That assumes, of course, that the cable is good quality, so that other possible forms of degradation don't arise.

Hope that helps,
-- Al