Mofi, thanks for the mention.
The way I would put it is that the length that has the greatest chance of being optimal is a very short length, such as 6 to 8 inches, if that is practicable. If a longer length is necessary, the length having the greatest chance of being optimal is 1.5 meters, or a little more, such as 6 feet.
However it is still possible that there will be some systems in which 1 meter will be the best length. At least a few members here who have tried different lengths have reported that to be the case in their systems.
The reason for the uncertainty is that what length will be optimal depends on a complex set of generally unspecified, unknown, and unpredictable system-dependent variables. Including the exact input and output impedances of the components that are being connected, the risetimes and falltimes of the signal being sent into the cable (i.e., the amount of time the signal takes to transition between its higher voltage and lower voltage states and vice versa), the propagation velocity of the particular cable, the jitter rejection capability of the DAC, the susceptibility of the particular components to ground-loop issues, the AC power distribution scheme in the particular setup (which can affect susceptibility to ground-loop issues), and even the possibility that supposedly objectionable amounts of jitter in the timing of D/A conversion, that might result from a non-optimal length, might be subjectively preferable to the particular listener in the particular system, depending on its frequency spectrum and other variables.
As to why cable manufacturers sell 1 meter lengths, as I say it might be preferable, at least subjectively, in some systems. Or it might make no difference in many cases. And some manufacturers and listeners figure to be unaware or unconcerned about length-related effects. And a 1 meter length can be sold at a lower price than a 1.5 meter length, everything else being equal. Among other reasons that are conceivable.
Regards,
-- Al
