Digital cable optimal length?

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

Excellent info Al. Now it is much clear as to what is happening. So then, is it possible to precisely calculate optimum( high success rate) length for 44.1/16 bit data transmittal? I am getting greedy here. Is 4 m better than 3 m. I am sure I won't need longer than 4 m. Is AES/EBU automatically better either at 3m or 4m or SPDIF exceeds AEU/EBU for this lengths?

Thanks,

Nil

Nil -- If you are just using 44.1/16, based on my analysis above you should be equally good at 4m or 3m. It would probably be a good idea to try to cross-check that against actual user experiences, if you can find relevant information via search (here or via Google).

AES/EBU and SPDIF do not differ significantly in terms of protocol, bit rates, or clock rates, and therefore do not differ significantly in terms of the reflection effects we have been discussing. They do differ in terms of amplitude, use of balanced interfaces, etc., which would seem to favor AES/EBU (balanced connections, higher amplitude) for longer runs. Just as any longer run will benefit, in terms of noise immunity, etc., from being balanced and having higher amplitude (everything else being equal). But at only 4m, for a digital signal, I would suspect that the difference would not be particularly significant.

See the following, re the differences between AES/EBU and SPDIF:

http://en.wikipedia.org/wiki/Spdif

Regards,
-- Al