Digital cables use twisted pair geometry. The conductor is solid OCC copper core with cotton insulation. Embedding in a cotton damper allows the relatively hard sheath to be kept at arm's length for mechanical integrity.
Pure aluminum foil covered with braided copper shielding layer ensures complete coverage, low resistance and good mechanical strength. Special care should be taken inside the connector so that the shielding layer is as circumferential as possible.
The shield of the unbalanced SPDIF is connected to the return conductor only at one end (a "symmetric pseudo-balanced" topology).
For balanced AES/EBU, we found the Furutech FP-601/602 Rhodium XLR superior to all other brands/types we tried. For non-balanced versions, WBT 0152 next-generation Silver RCA gets the best results. The length of digital cable starts from 1 meter and the step length is 50 cm.