Which budget digital cable ?

Unless you are absolutely sure that your DAC maintains internal impedance at 75 ohms, get a cable that is 1.5m long. Often the difference in price over 1m is minimal. The extra length makes a huge difference in cleanness and coherence in most setups. This is because the extra half meter delays internal reflections just enough that they don't arrive at the DAC in time to cause jitter.

Gee, that's a very interesting point that I haven't seen stated before, and which does seem very conceivable technically.

If the input impedance of the dac and the impedance of the cable don't match precisely, a portion of the incident signal would be reflected back to the transport. A portion of that reflection would then re-reflect from the transport to the dac. The two-way reflection path, assuming propagation time of roughly 2 nanoseconds per foot, would be 12ns for the 1m cable, and 18ns for the 1.5m cable.

I don't know what the typical risetimes/edge rates are for transport outputs, but it does seem very conceivable that the extra 6ns could move the arrival time of the re-reflection sufficiently away from the middle area of the edge of the original incident waveform so that it would not be responded to by the digital receiver at the dac input.

Thanks for pointing that out!

Regards,
-- Al

Would it be the same issue for Toslink?

I'm not particularly knowledgeable about electro-optics, but I don't think so. What underlies the phenomenon Tobias referred to is that for electrical signals with spectral components at rf frequencies, such as the electrical digital outputs of cd transports, a mismatch between cable impedance and load impedance results in a fraction of the incident energy being reflected back towards the source (instead of being absorbed by the load). As you may realize this is referred to as a "vswr" effect ("voltage standing wave ratio"). I wouldn't think that is applicable to light waves being detected by a sensor, and even if it were the quantitative parameters (risetimes, etc.) would be different.

Also, multi-mode fiberoptic cable (which I believe is what Toslink and most inexpensive fiberoptic links are) is subject to other sorts of effects, such as light repeatedly reflecting off of the walls of the conductor, resulting in some of the energy arriving at the destination later than that part of the energy which has taken a direct path. Not sure if that is a significant effect over the short cable lengths and the data rates that characterize audio systems (it may very well not be), but it's another indication that transmission of optical signals and transmission of electrical digital signals are just different animals.

Regards,
-- Al

Regards,
-- Al