1.5m for 75ohm digital coax?

How necessary it is will depend on the jitter reduction capabilities of the dac (if any), the risetime and falltime of the output signal from the upsampler or transport, the degree of impedance mismatch between cable, connectors, and interface circuits at both ends, and other factors. As Rcrerar suggested, the only way to tell for sure is to try it both ways.

BTW, it's not a matter of preventing signal reflections, it's a matter of affecting the timing of their propagation back and forth along the cable such that they don't arrive coincident with the critical parts of the original signal (the critical parts being their high-to-low or low-to high transitions).

Also, I would seriously question whether changing from copper to silver in the case of a short run of a digital signal will really accomplish anything.

Regards,
-- Al

There are quite a lot of very good digital cables that don't cost very much more in a 1.5m length than they do for a meter. Atlas Opus and Compass, VH Audio cryo Pulsar, Apogee Wyde Eye are examples. All these are at or near the top of their price class.

Mike, you are talking about digital cable here. For digital cable, impedance is important but length is not, unless you are talking about very long length in which signal degradation becomes a factor. Signal reflection is caused by impedance mismatch, not length. You shouldn't have to spend more than $30 to $40 for a good digital cable, silver or copper makes no difference. Don't spend too much on it.

Sidssp - Signal reflections are caused by length. For very short distance there is no transmission line effect. We have transmission line at about <8x ratio between signal travel time and rise/fall time. Signal travels 0.5m at about 2ns and rise/fall times are often in range of 20ns - we don't even have transmission line. If cable in question would be 1m or slew rate would be higher (good transport) then go with 1.5m to make impedance boundaries reflections (if any) come after transition.

Mike - at these frequencies signal travels on the surface only (skin effect) and many copper cables are already silver plated for that reason. I would pay more attention to shielding (braid and foil) and connectors. If you build cable yourself and one side has RCA connector then find one that is 75 ohm.