Jitter and 75ohm cable length

To really tell you would have to have the same cable in each length. I have not found any particular length to sound better; I have tried .5M, .75M, 1M, 1.2M, 1.5M and 2M but these were not all the same cable. The best one out of the 8 or so I currently have is a Clearer Audio pure silver with silver NextGen WBT RCAs; it is .5M. Would it sound better in 1M or 1.5M, your guess is as good as mine. Sounds very good as it is.

My experiment was via two Kimber Orchids(.5m and 1.5m) There was no contest, as the longer cable was much more open and liquid.

I am using a pure silver Neotech coax which is 2 feet long and sounds great. Not sure if 4 feet would sound better or not, suppose I could convince myself of almost anything....

I compared a 3' and 6' length of the same brand of a digital coax and liked the 3' better. The 6' was darker (mellower?) and might have been my choice if my system was on the bright side. They were extra cables I had laying around and was experimenting with the iPod/dock/DAC interface. They are copper and I ultimately went back to my 3' silver cable.

Like the fact that you used what you heard and not what you read to decide.

I came across a number of articles saying you should use at least 1.5M of cable to reduce reflections in the cable so as to not harm the clock signal, yet an RF engineer said this was a bunch of "Bunk" and 1M would be better, in fact he said the shorter the better.

I am an EE with multiple decades of experience designing high speed digital circuits, and also significant experience designing RF circuits (none of it for audio). I say it is definitely not "bunk." However, there are several factors that introduce a degree of system dependency and unpredictability into the issue, so 1.5 meters should be taken as a general guideline, which will not always be optimal. And in some cases the length won't make any difference.

Also, if a very short length is practicable, say 6 to 12 inches, that should be at least as good or even better. It is the intermediate lengths that are the concern.

The basis of the length concern is not to "reduce reflections." The magnitude of the reflections is determined mainly by the closeness of the impedance match between the cable and its connectors, the input impedance of the dac, and the output impedance of the transport.

The point to optimizing length involves the TIMING of the reflections, or more properly, the arrival time at the dac input of reflections of the original signal arrival at the dac input that have re-reflected from the transport output. What needs to be avoided is re-reflection arrival time at the dac input that coincides with the mid-point area of the risetimes and falltimes of the original signal arrival, which is where clocking occurs. If that were to occur, the resulting waveform distortion would be likely to cause a significant increase in jitter. All of that timing is directly dependent on the length of the cable.

Therefore the lengths that should be avoided are dependent on the risetime and falltime of the output signal of the transport, which are normally unspecified, and can be expected to vary significantly among different transports. This paper by Steve Nugent, which you've probably seen, is based on the assumption that those risetimes and falltimes are around 25 nanoseconds. I assume that is a good rule of thumb, but I would not expect it to be precisely consistent across different makes and models.

Also, the amount of time required for the signal to propagate from one end of the cable to the other will vary among different cable designs, because propagation velocity is dependent on the dielectric constant of the particular cable.

Also, jitter can be contributed to by noise caused by ground loop effects between the transport and dac, which, if present to a significant degree, can be expected to worsen as cable length is increased. Conceivably that effect could outweigh the timing consideration in many setups.

Also, different dac designs differ widely in their jitter suppression capability, with some of them (such as the Benchmark) being nearly completely immune to jitter on the incoming signal.

Finally, if the degree of mismatch among all of the impedances that are involved is insignificant, the whole issue becomes moot.

So as I see it the recommendation to use either a very short length, or a length of 1.5 meters or a little longer, has a sound technical basis, and while not always applicable, can be expected to be applicable more often than not.

Regards,
-- Al